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LITERATURE STUDY: SHOREBIRDS AND THEIR ABIOTIC ENVIRONMENT Relation between Shoal Morphology and Shorebirds in the Westerschelde Estuary Vanermen, N. De Meulenaer, B. Stienen, E.W.M. Research Institute for Nature and Forest Ministry of the Flemish Government Kliniekstraat 25 B-1070 Brussels Report INBO.A.169 November 2006 1 2 INTRODUCTION.............................................................................................................................. 1 SHOREBIRD DISTRIBUTION: PREDICTING VARIABLES ............................................................ 3 2.1 Benthos ...................................................................................................................................... 3 2.2 Sediment .................................................................................................................................... 4 2.3 Emersion time............................................................................................................................. 5 2.3.1 Low tide counting bias ....................................................................................................... 5 2.3.2 Emersion time as a predictor in bird distribution................................................................ 6 2.4 Drainage channels...................................................................................................................... 7 2.5 Salinity ........................................................................................................................................ 7 2.6 Distance to high water roosts ..................................................................................................... 8 2.7 Predators .................................................................................................................................... 8 2.8 Prey availability........................................................................................................................... 9 2.9 Shoal morphology..................................................................................................................... 10 3 SPECIES: HABITAT DEMANDS ................................................................................................... 13 3.1 Oystercatcher (Haematopus ostralegus).................................................................................. 13 3.2 Avocet (Recurvirostra avosetta) ............................................................................................... 15 3.3 Ringed plover (Charadrius hiaticula) ........................................................................................ 17 3.4 Kentish plover (Charadrius alexandrinus) ................................................................................ 19 3.5 Grey plover (Pluvialis squatarola) ............................................................................................ 21 3.6 Knot (Calidris canutus) ............................................................................................................. 23 3.7 Sanderling (Calidris alba) ......................................................................................................... 25 3.8 Dunlin (Calidris alpina) ............................................................................................................. 27 3.9 Bar-tailed godwit (Limosa lapponica) ....................................................................................... 29 3.10 Curlew (Numenius arquata) ................................................................................................. 31 3.11 Spotted redshank (Tringa erythropus) ................................................................................. 33 3.12 Redshank (Tringa totanus)................................................................................................... 35 3.13 Turnstone (Arenaria interpres)............................................................................................. 37 4 EXPERT INQUIRY ......................................................................................................................... 39 4.1 Introduction............................................................................................................................... 39 4.2 Sediment .................................................................................................................................. 39 4.3 Emersion time........................................................................................................................... 41 4.4 Food choice .............................................................................................................................. 42 1 INTRODUCTION To improve the accessibility of the economically important port of Antwerp, a widening of the shipping channel is being planned. However, the Westerschelde has large ecological significance as well, and the area is therefore protected under the Birds and Habitat Directive (VHR) and the Water Framework Directive (KRW). These European Directives have made requirements in terms of substantiation, effect forecasting, mitigation or compensation during the planning stages of system alterations more strict. The project ZEEKENNIS aimed at ensuring appropriate responses to future area management issues, and both available water system expertise and forecasting method accuracy have been upgraded. The final report of the project ZEEKENNIS describes the expertise developed in the five foregoing years and now there exists better insight in the working of the Westerschelde ecosystem, which will allow better analysis and forecasting of alteration-effect relations. Yet questions concerning area management and policy remain, which cannot be answered completely in the absence of certain elements of expertise and insight or methods (Kater, 2005). In project plan LTV O&M of December 12th 2005, under Objective 2 – estuary restoration – the following problem was formulated: “To achieve the aims as stated in Streefbeeld (Target definition) 2030 VHR and KRW, simply being able to determine the impact of human intervention in the Westerschelde ecosystem (e.g. broadening of the waterway) on the habitat acreage will not be sufficient; it will also be necessary to determine which habitat factors are of real importance to the various organisms. To this end, reliable information and insight must be made available regarding the relationship between physical, biological and chemical processes and the habitats on the one hand, and the relationship between the habitats and the organisms on the other. At present this information is largely unavailable.” The relevant policy and management questions are therefore: what are the ecological effects resulting from physical changes in the Westerschelde estuary caused by human or other interventions? In the project plan, two research issues have been identified: • • Which habitat factors are of true importance; where and how much of each type of habitat is desirable and what is the response of the acreage and its users (e.g. water birds) to certain interventions? How can the effects of changes in turbidity on primary production and higher trophic levels be charted? One activity geared to answer these questions is an investigation into the relationship between shoal morphology and waders. The first step in any such an investigation is to acquire an overview of worldwide available knowledge with regard to the relationship between waders and shoal morphology (comparable with the situation in the estuary of the Westerschelde). The investigation is to restrict itself to those shorebird species designated in Natura 2000 (European Bird guidelines). Shorebirds named in this guideline are as follows: oystercatcher (Haematopus ostralegus), avocet (Recurvirostra avosetta), ringed plover (Charadrius hiaticula), Kentish plover (Charadrius alexandrinus), grey plover (Pluvialis squatarola), knot (Calidris canutus), sanderling (Calidris alba), dunlin (Calidris alpina), bartailed godwit (Limosa lapponica), curlew (Numenius arquata), spotted redshank (Tringa erythropus), redshank (Tringa totanus) and turnstone (Arenaria interpres). 1 2 2 SHOREBIRD DISTRIBUTION: PREDICTING VARIABLES In this chapter an overview is given of the variables influencing shorebird distribution. These include biotic as well as abiotic variables. It is discussed which of the mentioned parameters might be affected by a future channel widening. The density of benthic prey has to be sufficiently high to meet the daily energy requirements of staging shorebirds. However, prey densities alone are insufficient to predict shorebird densities since availability of the prey may be equally important. Availability of benthic prey is affected by several abiotic and biotic factors, such as tidal stage, sediment characteristics, morphology of the intertidal flat, exposure period or distance to high water roosts. Apart from benthos some other biotic factors play a major role in wader distribution too, such as the shorebird density itself, or the risk of being predated by birds of prey. 2.1 Benthos As a rule, there is a positive correlation between densities of shorebirds and their prey as found by, for example, Goss-Custard (1970), Zwarts (1981), Esselink (1982), Roukema (1984), Meire (1993), Kalejta & Hockey (1993) and Yates et al. (1993). However, the strength of these relations varies among studies, and studies conducted at fine spatial scales often result in weak correlations. Yates et al. (2003) quote that strong correlations between prey density and shorebird abundance is most obvious when their diet is well known and that predictability increases with increasing diet specialisation of the concerned shorebird species. According to Kalejta & Hockey (1993), densities of visual foragers like grey plovers are predicted better by using prey biomass rather than by using prey densities. Visually foraging waders seem to be able to assess prey size and profitability prior to handling the prey (Kelsey & Hassal, 1989; Kalejta & Hockey, 1993). Thompson (1986) and Colwell & Landrum (1993) also state that prey density alone is insufficient to account for the distribution of birds because often, the mean size of individual prey items at high densities is smaller and prey is thus less profitable than at intermediate densities. It is in fact not easy to link shorebird distribution with prey densities. The shorebird’s diet consists of more than one species and the total food availability can not be determined by simply adding the total flesh mass of all these species. Individual birds decide where to forage based on their expected food intake rate rather than based on food density. As reviewed by Goss-Custard (1985), many studies show that attractiveness of an area to shorebirds largely depends on the rate at which they can feed there. The food intake depends on several factors like prey density, prey size, calorific content, digestibility, burying depth and surface activity, which all partly determine the harvestable food availability (e.g. Zwarts & Blomert, 1992; Zwarts et al., 1992). Furthermore, intake rate is influenced by interference between foraging birds, and hence depends on the bird density itself. Interference results from prey depletion, disturbance, restriction of the field view and kleptoparasitic pressure. As already mentioned and as summarized in Figure 1, only a small fraction of the total prey population is harvestable to a specific shorebird species. Five requirements have to be taken into account. First of all, the prey must be accessible; some for example live out of reach of the bill of the bird, or are protected by a hard shell. There is evidence that the bill length exactly determines which prey is accessible and which is not (Zwarts & Wanink, 1984; Zwarts et al., 1996). Secondly, the prey must be detectable, and this primarily depends on prey activity (visual foragers) or sediment characteristics (tactile foragers). Thirdly, the birds have to be able to swallow the prey. Knots for example are able to swallow Baltic tellin (Macoma balthica) of 16mm long, but spit out any larger ones (Zwarts & Blomert, 1992). Furthermore, the prey must be digestible. Wader species that swallow shellfish whole (e.g. the knot) need to spend energy crushing it in their stomach. Other shorebirds open their prey to extract the well digestible flesh (e.g. the oystercatcher), however, they need longer handling times. Last of all, prey must be profitable. Some prey is refused because their handling efficiency (intake per unit of time handling the prey) is too small. In general, handling efficiency increases with increasing prey size, and prey that is too small can be ignored. For example, knots ignore clams (Mya arenaria) that are smaller than 4mm, and curlews only eat Mya arenaria that are longer than 25mm (Zwarts & Wanink, 1984). This lower limit can be explained by the fact that handling efficiency is lower than intake rate in eating this small prey. 3 Total prey population Accessible fraction Burried too deep Invisible … Not ingestable Not gaping Available fraction Not profitable Harvestable fraction Figure 1. Different fractions of the total prey population and the fraction that is harvestable to the predator (after Meire, 1993) The densities of benthic invertebrates show a strong seasonal and year-to-year variation, which may have major effects on shorebird foraging and activity patterns. Also spatial patterns in benthic prey can vary in time and species composition may change rapidly (e.g. Puttick, 1984; Beukema et al.,1993; Zwarts & Wanink, 1993). Finally, a major practical problem in using food availability to predict shorebird densities or shorebird distribution is that the quantification of invertebrate populations in large intertidal areas can only be achieved at very high costs and is very time consuming (Ens et al., 2005; Stillman et al., 2005). 2.2 Sediment There are several ways to describe sediment type, unfortunately these methods do not always give comparable results. A measure often used in the Netherlands is the lutum-content. Lutum-particles are defined as particles being smaller than 2 µm. Another frequently used measure is the median grain size. In the Wadden Sea, there is a strong correlation between lutum content and the median grain size, however, the exact nature of the relationship may differ between sites. Furthermore the amount of organic matter, calcium, surface pooling, presence of ripples and the depth of the oxygenated soil layer are all strongly correlated with the lutum-content. As the proportion of lutum increases, the waterholding capacity of the tidal flat also increases. Nowadays advanced techniques using laser-particle sizers are increasingly used to characterise the sediment. However, direct comparison with more classic methods is not possible because calibration studies have not yet been conducted (van de Kam et al., 1999; Blomert, 2002). There is a strong relationship between biomass of benthic animals and the sediment type. For example cockles (Cerastroderma edule) reach maximum densities in soils with low lutum content, while the opposite is true for Baltic tellins (Zwarts, 1988b). Through their effect on invertebrate prey densities, sediment characteristics can be used to predict the densities of shorebirds (Yates et al., 1993; van de Kam et al., 1999), and several studies point out that there is in fact a correlation between sediment characteristics and shorebirds densities (e.g. Zwarts, 1988a; Yates et al., 1993; Holloway et al., 1996; Moreira, 1999; Granadeiro et al., 2004; Clark, 2006; Granadeiro et al., 2006). These studies have been conducted at large spatial scale (comparison of mean densities between estuaries) as well as on a much smaller scale (within-estuary comparisons). Clark (2006) showed a positive correlation between the mean proportion of silt and clay in the sediment and the density of Dunlins wintering in 14 estuaries in the UK. Holloway et al. (1996) compared bird densities in 27 estuaries (UK) and showed 4 that in general estuaries on the east coast hold higher densities of birds, except for oystercatcher and bar-tailed godwit. Estuaries along the east coast are generally muddier than the predominantly sandy west coast estuaries (Yates et al., 1996). Geographical position may also explain a great deal of variance in bird community, because this factor might be related to the proximity of breeding grounds or position of flyways. At the same time, Yates et al. (1993) and Granadeiro et al. (2004 & in prep.) found correlations between sediment type and shorebird density in within-estuary researches, respectively in the Wash (UK) en the Tagus estuary (Portugal). Yates et al. (1993) showed that densities of five species of shorebird correlated with at least one particle size category (proportion of clay, silt, fine sand or coarse sand), however, correlation coefficients were generally low. Inclusion of ‘inundation period’ into the model, greatly improved its accuracy. In explaining shorebird distribution, exposure time proved to be the most important factor in the Tagus. Here, sediment type only explained 18.4 % of the variance, whereas exposure period explained 52.3% (Granadeiro et al., 2006). Sediment characteristics do not only influence prey densities, they also have an important effect on prey availability. Sediment type can influence foraging behaviour in a very subtle way. An increased sand content may result in a decrease of the time waders spent foraging there. This was found in an experimental setup in Southern California involving dowitchers, western sandpipers, dunlins and American avocets (Quammen, 1982). In an exclosure experiment, Quammen (1984) found that shallow-feeding shorebirds reduced the density of prey by 26 to 80% at the muddiest mudflats, while there was no effect on similar prey species (occurring in similar densities) in sandier mudflats. The results of these experiments indicate that sand interferes with the detection and/or capture of prey items that are similar in diameter to small sand grains (small oligochaete and polychaete worms), resulting in lower intake rates. Sediment type also correlates with the moisture content of the substrate, which in turn is of great importance to prey availability (see §2.8). 2.3 Emersion time 2.3.1 Low tide counting bias Low tide counts are generally adequate for comparison of total shorebird densities among estuaries (Burton et al., 2004). However, they do not accurately reflect the relative importance of areas with different exposure regimes (Nehls & Tiedemann, 1993; Blomert, 2002; Dias et al., 2006b; Granadeiro et al., 2006). According to Dias et al. (2006b) estimates of bird densities across the intertidal area based on low tide counts alone differ substantially from estimates based on repeated counts over the full tidal cycle. High flats may favour high utilization intensity as they are accessible for a longer time and birds concentrate there at high water levels. By contrast, low flats are accessible for a short period during which the available area for birds is high. An equal exploitation of high and low flats is achieved when birds restrict their feeding activities to the tide line. However if birds simply disperse with flowing tide and concentrate with rising tide, a single survey at low tide would overestimate the utilization of the lower parts of the tidal flats. If the birds stay in a preferred area (e.g. oystercatchers on a mussel bed), a low tide survey may give a reasonable estimate even though the tidal utilization of high flats where birds may have been feeding at higher water levels would be underestimated. Nehls & Tiedemann (1993) thus divide the tidal foraging behaviour in three categories, namely ‘dispersed foragers’, ‘tide line followers’ and ‘preferred area foragers’ (see Figure 2). In a study conducted by Granadeiro et al. (2006), waders are divided into tide followers (black-tailed godwit, dunlin and avocet) and non-followers (grey plover, redshank and bar-tailed godwit). Granadeiro et al. (2006) found that the lower plots offered the highest rate of biomass acquisition. However, this did not compensate for the much longer exposure period of the upper areas. The upper areas were much more important for foraging waders since they provided the largest fraction of the biomass required by birds. There is, however, poor agreement between various studies concerning classification of waders according to their cyclic foraging behaviour as dictated by the tide (compare, for example, Thompson et al., 1986; Nehls & Tiedemann, 1993; Tulp & de Goeij, 1994; van de Kam et al., 1999; Danufsky & Colwell, 2003; Granadeiro et al., 2006). 5 The recognition of the tidal behaviour of waders is thus crucial for the estimation of the exploitation of tidal areas and studies aiming at forecasting shorebird densities across the intertidal habitat have to take this into account (Dias et al., 2006b; Granadeiro et al., 2006). c. preferred area b. tide line Exploitation of intertidal area Density at low tide a. dispersed high low high low high low Figure 2. Hypothetical distribution of birds at low tide representing three different feeding patterns (first row) and results obtained by single low tide counts (second row) and repeated counts over the whole tidal cycle (third row). (after Nehls & Tiedemann, 1993). 2.3.2 Emersion time as a predictor in bird distribution The food present in an estuary is available to shorebirds only when shoals (flats surrounded by water) and intertidal flats (flats alongside the estuary, connected to land) become exposed. An individual wader must digest enough food to cover its daily energy need, if not, it loses fat, and consequently its survival chances decrease (Stillman et al., 2005; Stillman & Goss-Custard, 2006). As the emersion period determines the time intertidal flats are available to foraging shorebirds, it plays an important role in the carrying capacity of an estuary. Furthermore, the quality of an intertidal area is partly determined by its emersion period, since this has a direct influence on prey distribution. In the Wadden Sea, biomass densities are highest at intertidal areas with an emersion period of 3 to 5 hours (Zwarts, 1988b). Some benthic species can only forage when submerged (filter feeders like cockle and mussel (Mytilus edulis)), while others have to wait until the tide retreats (eg. Corophium, Scrobicularia plana). Filter feeders thus exhibit decreased growth rates as the average submersion time decreases. Deposit feeders can continue feeding during low tide, and are therefore expected to occur throughout the intertidal zone. The distribution of benthic organisms, however, is only partly explained by the preference of each species for specific areas within the tidal zone. Competitors and predators play a major role in their distribution. For example, the presence of a mussel bed prevents the settlement of other benthic animals. Small larval animals are likely to be sucked up by the filter feeding mussels. To a lesser extent the same is true for the presence of lugworms (Arenicola marina), cockles or Scrobicularia plana. Apart from feeding on similar foods and making it impossible for others to settle, some benthic animals prey on other benthic species. The most important predators live in the water or on the sediment: fish, shorecrabs (Carcinus maenas) and shrimps. Their predation pressure is highest where the crabs can stay longest: around the low tide line. So although cockles can feed longer and grow more quickly near the low tide line, 6 they are unlikely to survive here. Predation risk explains why cockles are rarely present where growing conditions are optimal (van de Kam et al., 1999). 2.4 Drainage channels Studies on habitat selection by waders in tidal flats have focused mainly on large-scale selection (e.g. Goss-Custard, 1977; Moreira 1993; Piersma et al., 1993, Yates et al., 1993, Rosa et al., 2002). The few studies that have analysed wader distribution at finer scales have documented strong responses to microhabitat features, even at the scale of a few metres (Mouritsen & Jensen, 1992; Colwell & Landrum 1993). Small drainage channels are the most obvious fine scale features in many tidal flats. Some of these channels are formed directly by the drainage of the water that covers the flat during high tide, but many have other origins, such as the drainage of adjacent saltmarshes, saltpans, farmland and built-up areas (Lourenço et al. 2005). Ravenscroft & Beardall (2003) showed significant higher numbers of redshank, dunlin and curlew in the vicinity of freshwater flows. Important prey items of waders, like ragworm (Nereis diversicolor), Corophium volutator and Hydrobia ulvae prefer organically rich areas (Hill et al., 1993; Yates et al., 1993), and freshwater running off land adjacent to estuaries may increase local food abundance through organic enrichment. This is countered by the fact that the flows in the study of Ravenscroft & Beardall (2003) were derived from groundwater and were therefore unlikely to contain many nutrients. A local effect on the salinity may enhance the presence of euryhaline species (Arenicola marina and Corophium volutator). Where channels fan out over the lower and middle shores, the mudflat remains wet at low tide. Waders prefer feeding on wet mudflats because this maintains the activity of invertebrates and the penetrability of the substrate, which both enhance prey availability (see § 2.8). In the Tagus estuary, Lourenço et al. (2005) showed that 44 % of the birds fed on just 12% of the available surface less then 5 meter away from saltwater drainage channels. A study conducted by Granadeiro et al. (2004) in the Tagus estuary shows that ‘distance to drainage channels’ is one of the three most influential variables in modelling wader distribution in the Tagus estuary, followed by the type of sediment and area of oyster beds. The channels in the Tagus estuary drain salt water, originating from the adjacent saltmarshes and saltpans. There were significantly more shorebirds (dunlin, grey plover, redshank, knot, bar-tailed godwit and black-tailed godwit) present close to the channels. Shorebird densities peaked at about 1 to 2 meter from the edge of the channels. Moreover, dunlin, grey plover and redshank had higher turning rates and lower step rates near the channels, which indicates a higher foraging intensity. While characteristics of the sediments did not change significantly with distance to the channels, prey abundance did, with Nereis diversicolor, Hydrobia ulvae and Scrobicularia plana having higher densities and greater biomass closer to the channels. In fact densities of Nereis diversicolor close to the drainage channels are particularly high, locally highest ever found in the Tagus estuary (pers. comm. Pedro Lourenço). The reasons for the higher prey density near the drainage channels are not clear, but may be related to food availability, larval colonization or reduced heat loss in the presence of flowing water during low tide (Thrush, 1991; Ravenscroft & Beardall, 2003). The presence of drainage channels is thus very important for shorebirds since their presence enlarges the carrying capacity and the overall quality of a mudflat. To maintain the quality of a mudflat, it seems necessary to preserve the abundance of drainage channels. Tidal channels created by the action of the tide are more abundant in wide intertidal flats (Leopold et al., 1993) so ideally large areas of flats should be preserved. The drainage of saltmarshes is also responsible for the formation of channels on the adjacent flats: mature saltmarshes result in more developed channel networks (Pye, 1992). 2.5 Salinity Salinity has a major influence on the distribution and the abundance of macrobenthos, and in its turn on prey availability, and thus on distribution of shorebirds (Wolff, 1969; Warwick et al., 1991; Colwell & Landrum, 1993; Ysebaert et al., 2000; Ens et al., 2005). Ysebaert et al. (2000) showed a clear zonation of the water bird communities along the entire salinity gradient of the Schelde estuary. 7 Research in the Westerschelde revealed three abiotic variables that correlate significantly with wader distribution, namely emersion time, salinity and current velocity (Ens et al., 2005). 2.6 Distance to high water roosts In order to predict the ecological impact of human intervention in an estuary, one must keep in mind that apart from the quality and quantity of intertidal habitat, the availability of high tide roosts plays an important role. According to Rehfisch et al. (2003) there are four key criteria that determine whether a roost is suitable or not: • • • • Shelter from exposure Proximity of foraging grounds Minimal disturbance Minimal predation risk The cyclic flights between foraging grounds and high tide roosts involve a high energy expenditure to shorebirds. The tendency to select feeding areas close to their roosts is a logical consequence of the fact that wintering shorebirds may be subject to a critical energy balance. In general, wader densities reach a maximum on the feeding areas close to the high tide roost because, when other environmental factors remain equal, it is profitable to minimize the flight distance between the roost and the feeding areas (Evans, 1976; Zwarts, 1981). Research in the Tagus estuary shows that there is a significant decline in foraging dunlins further away from the roosts (Dias et al., 2006a). Although Dias et al. (2006a) only studied dunlins they suggest a similar response for other waders, namely redshank, turnstone, grey plover and oystercatcher. Based on research in the Wadden Sea, van Gils et al. (2006) conclude that knots trade off energy intake rates against travel costs: sites near a roost are skipped in case they offer low intake rates, while sites offering high intake rates are only used frequently if roosting occurs nearby. In order to maintain the carrying capacity of an estuary, it is important to maintain a network of favourably located high tide roosts (Dias et al., 2006a). Apart from this, shorebird species differ in their fidelity to roost sites. While most species show strong fidelity to high water roosts, knots, bar-tailed godwits and dunlins wintering in Scotland were found to be highly mobile, showing poor fidelity to roost sites. These movements may be induced by reductions in prey availability or by behaviour adapted to the likelihood of such problems occurring (Symonds et al., 1984; Rehfisch et al., 2003). In this respect one should know that knots wintering in tropic areas (North-western Australia and Mauritania) do show high roost fidelity (Leyrer et al., 2003; Rogers et al., 2006). This clear difference in behaviour between tropical and temperate climate zones might result from the fact that availability of prey is much more unpredictable in the temperate zones (Leyrer et al., 2006). In the Westerschelde, densities of shorebirds on the intertidal flats (flats alongside the estuary, connected to land) are often (much) higher than on the shoals, even when sediment type and food availability are equal. It is suggested that intertidal flats are more attractive to shorebirds because of the shorter distance to high water roosts (Graveland, 2005). 2.7 Predators Recently an increasing body of evidence has accumulated indicating that the feeding decisions of shorebirds depend on the risk that they themselves are being eaten by predators (Cresswell, 1994a; Hilton et al., 1999; Lank & Ydenberg, 2003; Whitfield, 2003; Ydenberg, 2004 van Gils et al., 2004; van Gils et al., 2006). In many circumstances there is a trade off between predation risk and the risk of starvation. For example, higher predation risk while feeding (compared to the risk when resting) will make knots feeding more efficient in order to reduce the time they are exposed to this threat (van Gils et al.; 2006). When temperatures are low, metabolic costs increase, and simultaneously, the preparedness of redshanks to accept higher predation risk in turn for greater intake rates increases, as found by Hilton et al. (1999). Cresswell (1994a) in Tyninghame estuary, Scotland, reports that adult and juvenile 8 redshanks segregated into two areas, namely a saltmarsh and a mussel bed. It was showed that a redshank feeding in the saltmarsh was 4 to 8 times more likely to be killed, but had a higher foraging rate and overall energy intake (1 to 7 times). Juveniles were excluded from the mussel beds by the adults and Cresswell (1994a) thus concludes that adult redshanks prefer to minimize predation risk, rather than to maximize intake rate. The adults could afford this because intake rate on the mussel beds was sufficient to meet their daily energy requirements. However, one might expect that they will abandon their mussel bed territories once intake rates fall below a minimum. Indeed, adult redshanks appeared on the saltmarshes in late winter cold weather periods. Whitfield (2003) showed how the distance from cover influences the vulnerability of waders to attacks of Sparrowhawks. He found that the closer to cover they feed, the less waders are able to detect an approaching raptor and escape in time. Hilton et al. (1999) report that when the visibility (e.g. because of precipitation) decreased, redshanks used the more open (safe) areas. When feeding close to cover, redshanks tend to forage in flocks (an effective anti-predation strategy), although this negatively influenced feeding efficiency. On more open flats (far away from cover) birds had a less tendency to flock, demonstrating once more the trade off between maximizing intake rate and minimizing predation risk (Cresswell, 1994b; Whitfield, 2003). In conservational view, it is important to know that predation risk partly determines the carrying capacity. When different patches of an area differ in predation pressure, this may result in a different usage of these patches by shorebirds. Thus, certain areas might be used more often than would be expected based on “common” predictions (van Gils et al. 2004). 2.8 Prey availability As already mentioned, prey availability is a key factor in determining shorebird distribution. In general, prey availability correlates positively with prey density, but several other variables may be equally important in determining how much of the food present on the flats is actually available to shorebirds. Very important are the tidal changes in prey activity and accessibility (Smith & Evans, 1973; Evans, 1976; Evans, 1979; Puttick, 1979 & 1984; Pienkowski, 1983; Colwell & Landrum, 1993; van de Kam et al., 1999; Ribeiro, 2004). Numbers of Nereis (Dugan, 1981) and Arenicola (Smith, 1975) visible at the surface decline steadily after the tide has passed, in line with the drying out of the substrate. Moreover, Nereis buries deeper after the tide has gone out. When the receding tide has just exposed the flats, amphipods are very active, but from then on their activity steadily decreases. Therefore, redshanks feeding on amphipods follow the receding water line (Boates, 1980; van de Kam et al., 1999). The availability of polychaete worms like Scolelepis squamata also largely depends on the tidal state. On Belgian beaches this worm occurs in a narrow zone at the high intertidal area (Degraer et al., 2003). Scolelepis squamata is often presented as the preferred prey of sanderling (Dankers et al., 1983; De Gee, 1984; Glutz von Blotzheim et al., 1984), but on Belgian beaches it seemed available to sanderlings only a short period each tidal cycle, coinciding with the incoming tide crossing the zone with high densities of worms. Most probably this is due to vertical migration of the worms throughout the tidal cycle (De Meulenaer, 2006; Speybroeck et al., 2006). Kalejta & Hockey (1993) state that rapid downward migration of prey following flat exposure is more likely in coarse sediments (beaches) than in muddy environments, due to the lower water retention capacity of coarse sediments. The wetness of the substrate affecting prey activity may partly explain why many waders follow the tide line, though other factors may also be involved. Tactile foragers such as the knot use their sensitive bill tip to locate invisible buried prey. Probing for prey, however, does not work in dry sand or liquid mud, so tactile foraging knots are very much restricted to feed in moderately wet sediments (Piersma et al., 1998). Furthermore, water content is positively correlated with sediment penetrability (Chapman, 1949; Perkins, 1958; Grant, 1984; Mouritsen & Jensen, 1992). Penetrability affects probing depth as well as the cost of foraging in tactile searching birds (Myers et al., 1980; Pienkowski, 1981; Grant, 1984; Mouritsen & Jensen, 1992). Myers et al. (1980) showed a decrease in foraging success in sanderling with decreasing sediment penetrability, owing to reduced probing depth. On a larger spatial scale, Tjallingii (1972) found that the distribution of avocets was determined by the resistance of the sediment to bill movements, rather than by prey density. Kelsey & Hassal (1989) have likewise showed that dunlin preferred the wetter, low lying areas. The firmer ridges, with penetrability being three times lower, attracted fewest birds, while the density of oligochaetes there was greatest. 9 Prey availability may vary seasonally due to variation in prey abundance as well as seasonal differences in the behaviour (activity, burying depth) of benthic prey linked to temperature (Pienkowski 1981 & 1983; Puttick, 1984; Esselink & Zwarts, 1989; Nehls & Tiedemann 1993). For example, when temperature drops beneath 6°C, Corophium volutator, a favoured prey of redshank, becomes less active and the birds take increasing numbers of Baltic tellin or Nereis diversicolor (Goss-Custard, 1969). In other words: changes in temperature can influence prey choice of shorebirds and may thus affect the spatial distribution of birds. Many intertidal organisms move to greater depths within the sediment as temperature drops (Esselink & Zwarts, 1989). At temperatures just above freezing point, annelids and bivalves are buried in the sediment at a depth of more than 15cm and are therefore even out of reach of a curlew’s bill. Apart from vertical migration, there is evidence that marine invertebrates may move to lower tidal levels in response to cold weather (Darby, 1975; Evans, 1979). Similarly, high temperatures and strong winds that dry out the substrate may reduce prey availability for waders as benthic prey moves deeper to avoid desiccation (Evans, 1976). Since good feeding areas attract many foraging birds, this is likely to promote intra- and interspecific interference. In early spring, the total numbers of oystercatcher in the Oosterschelde (NL) are lowest, and only the richest foraging areas are occupied. When numbers rise however, the densities increase to a ceiling level, and from this point on densities in other (less profitable) foraging areas start to increase (Meire, 1993). Interference can be associated with prey depletion and/or disturbance, restriction of the field view and kleptoparasitism (eg. Goss-Custard, 1980 &1985; Zwarts, 1981; Thompson, 1986; van de Kam et al., 1999; Stillman et al, 2003, 2005; Stillman & Goss-Custard, 2006). Studies on the amphipod Corophium volutator showed that these invertebrates retreat into their burrows when a wader passes (Goss-Custard, 1970). Therefore, at high densities of shorebirds amphipod availability decreases quickly. For that reason redshanks feeding on amphipods will occur rather dispersed across the intertidal area. This is in contrast to knots, feeding in large high density groups. Knots can afford this because their presence does not repress the availability of buried Baltic Tellin. Moreover, feeding in large flocks offers advantages in terms of protection against predators, or in finding good foraging spots (van de Kam et al., 1999). 2.9 Shoal morphology The morphology of shoals present in an estuary may largely determine the carrying capacity of the estuary for shorebirds. Obviously, it determines the area of intertidal habitat that is available to staging waders. Moreover, the height of the shoals determines the time they are exposed and the availability of the shoal throughout the tidal cycle. To a lesser extent habitat quality is dictated by shoal morphology. The shape and location determine the prevailing dynamics, which correlate strongly with the sediment composition. For example, in the Westerschelde there is a positive correlation between the height of the shoal and the high tide current velocity. Consequently, there is a positive correlation between the height of the shoal and the coarseness of the soil sediments (Brinkman et al., 2005). At a smaller scale, dynamics vary within the shoal, and shoal-edges exposed to high currents are steeper and show ripples, and these edges are considered to be the most dynamic parts of the shoal. However, highest densities of benthic prey are found at low-dynamic intertidal habitat (Pieters et al., 1991, Vroon et al., 1997). Hence, if widening the shipping channel in the Westerschelde results in higher shoals, the sediment is expected to become muddier. At the same time, emersion periods increase and the shoals become available to foraging waders for a longer period. However, if the shoals grow above the mean high water level, they are no longer attractive as a foraging area to waders, but they may function as a breeding habitat for terns and plovers, or as a high tide roost for waders. In this respect, connection channels play an important role in maintaining enough dynamics to prevent shoals from growing above the mean high water level. Due to the presence of connection channels, there is more diversity in microhabitats, plus, the area of intertidal is more extensive. In the Westerschelde, there is a longterm evolution going on in the disappearance of connection channels. In the middle and the west of the estuary this is probably the result of natural processes, but in the east the disappearance of connection channels is thought to be linked to the channel widening in the early seventies (Vroon et al., 1997). Through widening of the shipping channel, the tide wave experiences less resistance entering and leaving the estuary. Hypothetically spoken this might result in a rise of the high water level and hence stronger dynamics above the shoals. 10 At the end of the eighties, the project OOSTWEST was started up as a result of the concern that human intervention would have a negative effect on the estuary. After the final report ‘Westerschelde: stram of struis?’ in 1995, two parallel projects, namely ZEEKENNIS and MOVE were started up. The monitoring project MOVE aimed at evaluating the effect of the widening of the shipping channel in 1996 (48’/43’), and the project ZEEKENNIS at enlarging the insight in the physical and ecological functioning of the Westerschelde. Concerning morphological changes, it is often very difficult to detach ongoing long term trends from short term trends, or to distinguish natural changes from changes induced by human intervention. Nevertheless, based on the results of the ten-year monitoring programme MOVE, the following changes in the overall morphodynamics of the Westerschelde have taken place. Mean high water levels have not risen since the previous channel widening, but low water levels have decreased. No significant changes in current velocity have been observed, apart from some very local changes near shoal sides and at dump locations. Shoal area in the west and east of the Westerschelde has decreased following the long term trend as observed in the last fifty years. In the middle part of the estuary the shoal area has decreased even more than was expected. Most of the shoal area has been transformed to ‘shallow water’, because of erosion of the shoal sides. This phenomenon is most probably caused by the channel widening. In the Westerschelde as a whole, low dynamic intertidal area has increased, and high dynamic intertidal area has decreased. With respect to the ecological function of the estuary for waders, this is a positive evolution since benthic prey organisms prefer low dynamic habitat (van Eck, 2006). 11 12 3 SPECIES: HABITAT DEMANDS For each species an overview is given of the existing knowledge concerning its habitat demands. The emphasis is on the preferred sediment characteristics and the effect of emersion time on bird densities. Please keep in mind that the effect of emersion time on wader densities very much depends on the way birds were counted. For example, while most studies use low water counts only, Ens et al. (2005) used bird count data obtained by repeated counts (each ½ hour) over the whole of the exposure period. Counts conducted during low tide only tend to underestimate the importance of high intertidal areas (see § 2.3.1). Furthermore, if available more information is given concerning specific habitat demands, the species’ mobility throughout estuaries, nocturnal foraging habits and inland foraging. These factors greatly determine the species flexibility to cope with extreme conditions, and are important towards a correct estimation of the carrying capacity of a certain area. Finally, each species’ diet is discussed, and the effect of the distribution and tidal behaviour of the prey on the foraging behaviour of the birds is considered. 3.1 Oystercatcher (Haematopus ostralegus) There is good agreement in literature concerning the habitat choice of oystercatchers. This species generally prefers sandy soils for foraging. In Guinea-Bissau, the oystercatcher comes second in row after sanderling in a ranking of 15 wader species according to their preference from sandy to muddy sediments (Zwarts, 1988a). In the Wash (UK), oystercatcher densities correlate positively with the area of sand, as well as with the proportion of fine sand (63-125µm) in the sediment, and correlate negatively with the silt content (20-63µm) (Goss-Custard & Yates, 1992; Yates et al., 1993). Comparing densities of oystercatchers in 27 British estuaries showed that highest densities occurred in the less muddy estuaries with a high degree of wave action and exposure to swell (Austin et al., 1996). Hierarchical classification of 109 British estuaries based on their wader communities revealed four groups. The group containing the larger, sandier estuaries, held highest densities of knot, bartailed godwit, sanderling and oystercatcher, as found by Hill et al. (1993). Research in the Netherlands shows that oystercatchers in the Westerschelde occur on a broad range of sediments (Van Kleunen, 1999 (Appendix I); Ens et al., 2005), while in the Wadden Sea they show a preference to the less muddy areas (Brinkman & Ens, 1998). This is confirmed by Figure 3, which is based on the same data as the report of Brinkman & Ens (1998) (Zwarts, unpublished data, in: Blomert, 2002). 0,500 0,500 0,400 0,400 wader density (n/0,1ha) wader density (n/0,1ha) In the Wadden Sea, meadows along the mainland coast and on the islands are sometimes used as feeding grounds, mainly during bad weather when exposure time of the flats is too short, or during cold winter periods when energy requirements are higher (Smit & Wolff, 1981). 0,300 0,200 0,100 0,300 0,200 0,100 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 3. Relation between densities of oystercatcher and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002) In the Westerschelde, oystercatchers occur over a wide range of emersion periods (Van Kleunen, 1999 (Appendix I); Ens et al., 2005), whereas in the Wadden Sea they typically occur in areas with rather short emersion times (Brinkman & Ens, 1998; see Figure 3). Comparatively, Smit & Wolff (1981) state that oystercatchers favour areas that are exposed for 0-6 hours. Meire (1993) describes 13 the tidal behaviour of oystercatchers in the Oosterschelde (NL). Cockles and mussels are the most important prey species, and are not uniformly spread over the tidal flats, but occur on the lower intertidal zone. As the tide ebbs, the birds tend to follow the waterline to reach the lower richer feeding areas as soon as possible. This results in a clear peak of birds moving through the plots in the higher tidal zones, while these plots are nearly abandoned near low tide. When the preferred feeding areas below mid tidal level become exposed, birds occur there in constant high densities until they are flooded again. This tidal behaviour corresponds with the ‘preferred area’ behaviour as described by Nehls & Tiedemann (1993) (see Figure 2). During winter, oystercatchers feed nearly exclusively on shellfish, especially cockles and mussels, but also other larger shellfish species like Baltic tellin, Mya arenaria and Scrobicularia plana. In summer Nereis is also eaten, mainly by females (eg. Hulscher, 1996; Zwarts et al., 1996b). In the Westerschelde, cockles are the most important prey, since mussel beds are rare (Van Kleunen, 1999; Ens et al., 2005). Cockles typically occur in soils with rather low lutum content, and intermediate exposure periods (Zwarts, 1988b, grafiek in: van de Kam et al., 1999). According to Smit & Wolff (1981), the majority of oystercatchers in the Wadden Sea concentrates on the mussel beds and cockle fields, or on places with concentrations of Mya arenaria and Scrobicularia plana. Studies in the UK and the Netherlands indeed show a strong association of oystercatcher to mussel beds (Ens et al., 1993; Yates et al., 1993). There are, however, always small numbers feeding on tidal flats characterised by Arenicola marina, where Arenicola, Nereis and Baltic tellin are eaten (Smit & Wolff, 1981). Meire (1993) states that without a detailed knowledge of the foraging behaviour of the predator, it is not possible to give an adequate estimate of the harvestable prey density, which in turn is essential to calculate the carrying capacity of a given area. In this respect, Hulscher (1996) found that oystercatchers reject mussels smaller than 20-25 mm in length because of low profitability, and take all larger size classes in proportions conform to the likelihood of encounter. The profitability of hammered mussels decreases when they are larger than 50-55 mm because some of these cannot be opened at all, and time is spent in failed attacks. As a result, the profitability curve shows a peak for mussel lengths around 50-55mm (Meire, 1993). When foraging on cockles, oystercatchers refuse first year cockles smaller than 10mm when older cockles are present (Sutherland, 1982). The average mussel length taken by oystercatchers in the Oosterschelde (NL) is 30-45mm (Meire, 1993). The results obtained by Meire (1993) can well be understood within the framework of the optimal foraging theory (birds maximizing their intake rate at all times) when availability of the different mussel length categories is taken into account. Moreover, oystercatchers select mussels with thin shells, since the mean shell thickness of mussels opened by oystercatchers is less than that of the mussels present. Goss-Custard et al. (2004) modelled five major estuaries in the United Kingdom and France (Exe, Wash, Bangor flats, Burry Inlet, Baie de Somme). They concluded that between 2,5 and 7,7 times the shellfish biomass that is actually consumed, must be available in autumn if most birds are to survive until spring. Main reason here for is that by reducing the biomass of shellfish available per bird, interference competition for food is intensified. When oystercatchers are unable to take in enough food during the day, they will feed at night as well. Zwarts et al. (1996b) found that oystercatchers feeding on Baltic tellin and Scrobicularia plana began feeding at night after these bivalves migrated deeper into the soil in the course of autumn. Several studies appear to contradict each other when it comes to nightly foraging success (Hulscher, 1996). Zwarts & Drent (in: Smit & Wolff, 1981) suggest that the intake rate of mussels during night is only half compared to day time intake rate. Sutherland (1982) states that oystercatchers switched from visual detection of prey during day time to tactile feeding at night, and in doing so, found cockles of smaller size, at a lower rate than they did during the day. By contrast, under controlled conditions Hulscher (1976) found that oystercatchers reached a higher intake rate at night using tactile foraging techniques. Probing for food, however, is energetically more expensive, and in this study the net energy intake could not be evaluated. 14 3.2 Avocet (Recurvirostra avosetta) The avocet strongly prefers the most muddy sediments for foraging. Research in the Netherlands showed a preference of avocet to muddy areas in the Delta area (Wolff, 1969; Van Kleunen, 1999; see Appendix I) and a strong positive correlation with the area of muddy (high) intertidal zones in the Wadden Sea (Ens et al., 1993). Looking at Figure 4, one sees that in the Wadden Sea, avocets occur nearly exclusively on sediments with a lutum content of 5% or more. In the Tagus estuary, the avocet exclusively forages on mud and sandy mud, and of all wader species present, avocets (and blacktailed godwits) feed on the finest sediments (Moreira, 1999). A study conducted in Humboldt Bay, California, showed a negative correlation of American avocet with sediment particle size, with birds occurring nearly exclusively in areas of fine sediment (Evans & Harris, 1994; Danufsky & Colwell, 2003). At a larger spatial scale, a DCA-analysis of 109 estuaries (UK) according to their wintering shorebird communities, revealed that avocets reach highest densities in the very muddy estuaries and coastal lagoons (Hill et al., 1993). Increasing the sand content of the substrate from 2 to 14% showed that sand interferes with the detection and/or capture of prey (Quammen, 1982). This could be deducted from the fact that the time spent in the treated plots decreased, and avocets started pecking at the surface in addition to ‘scything’ (see below), the more common way of feeding. Tjallingii (1972) reported that the resistance of the soil to the bill movements determines the spatial distribution of foraging birds more than density of their prey did. 0,005 0,010 0,004 0,008 wader density (n/0,1ha) wader density (n/0,1ha) In the Wadden Sea, the density of avocets in relation to emersion time shows two peaks: one at short emersion periods (4 hours) and one at higher emersion periods (6,5 hours) (Figure 4). Overall densities in the Wadden Sea being very low, this may enhance unreliable results. Other research in the Wadden Sea showed a strong positive correlation between avocet densities and the area of high intertidal zones (Ens et al., 1993). Studies conducted at the Tagus estuary also report a positive correlation between densities of avocet and the exposure period, with avocets being more numerous on the higher reaches of the intertidal area, mainly consisting of muddy sediments retaining a thin layer of water (Moreira, 1995; Granadeiro et al., in press). On the other hand, Granadeiro et al. (2006) classifies avocet as being a follower, suggesting a preference to the lower intertidal. Evans & Harris (1994) describe the intertidal behaviour of wintering American avocets in Humboldt Bay, California. There, most birds seem to follow the retreating tide edge, while a few birds always remained on the higher exposed mudflats containing visible surface water. Avocets foraging on the intertidal mudflats mostly waded in water covering their feet (43%), in water up to belly (34%), or walked on mudflats with visible surface water (22%). In coastal lagoons in Ghana, avocets favour water depths of 2-11 cm (Ntiamoa-baidu et al., 1998). 0,003 0,002 0,001 0,006 0,004 0,002 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 4. Relation between densities of avocet and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). In the Wadden Sea, Nereis diversicolor was found to be the most important food item for avocets. The same was found for the Ventjagersplaten in the Delta area. Other small prey like Corophium and insects complete the diet. Locally, avocets feed in brackish channels on small crustaceans (Paleomonetus and Neomysis) (see literature review in Leopold et al., 2004). Avocets use their 15 upward curved bill for a characteristic way of (tactile) feeding: while walking through shallow water or soft sediment, they quickly sweep their bill through the top layer of the silt or the water (scything) (Tjallingii, 1972). Avocets also peck for prey visible in clear water, on the sediment or amongst the vegetation on the edge of the water. They sometimes forage in dense flocks hunting for crustaceans (shrimps, mysidacea), often accompanied by other species like spotted redshank (Ntiamoa-baidu et al., 1998; van de Kam et al., 1999). Boettcher & Haig (1995) showed that in South island, South Carolina, impoundment use by American avocet is far greater than intertidal mudflat use. There, avocets invariably used areas covered by shallow water (5-17 cm) with little or no exposed substrate. Evans & Harris (1994) found that avocets, who normally forage at intertidal mudflats during low tide, occasionally fed exclusively at oxidation ponds taking advantage of large but variable availability of Daphne magna. Several authors point out that night feeding can be very important. In (non-tidal) coastal lagoons in the gulf of Guinea (Ghana), avocets probably forage more during night time (Ntiamoa-baidu et al., 1998). American avocets in Humboldt Bay, California, are also presumed to forage effectively during night time, since the birds use primarily tactile feeding methods and were observed feeding in the same locations as during daytime (Evans & Harris, 1994). Hötker (1999) studied time activity budgets of avocets and reports that activity pattern in estuarine areas was mainly influenced by the tide. Activity patterns by day and at night were essentially the same, except during very dark nights, when foraging activity was reduced. By contrast, Zwarts et al. (1990) observed daytime feeding only, in a non-tidal lagoon in Senegal. Evans & Harris (1994) report that nearly all American avocets foraged within 3 km of the roosts. 16 3.3 Ringed plover (Charadrius hiaticula) In general, ringed plovers are encountered on intermediate to muddy substrates. In the Westerschelde, highest densities occur on intermediate substrate (Ens et al., 2005). In the Wadden Sea, their habitat preference appears to be segregated according emersion period: ringed plovers favour intermediate sediments at the higher intertidal zones, while they prefer muddy sediments at the lower tidal flats (Brinkman & Ens, 1998). Looking at Figure 5, densities of ringed plovers in the Wadden Sea in relation to lutum content of the sediment show two distinctive peaks, one on sandy and one on muddy substrates. When the data are further analysed, it appears that in areas with a long emersion time, the preferred sediment is muddy, and in areas with shorter emersion times, favoured sediment is sandy (Figure 6). This is the opposite of what is stated in Brinkman & Ens (1998)! Out of 15 species of shorebirds wintering in Guinea-Bissau, ringed plover showed the strongest association to muddy sediments (Zwarts, 1988a). In the Tagus estuary, Moreira (1999) found a preference for sandy mud, while muddy as well as coarser sediments were clearly avoided. However, through a cluster analysis, Granadeiro et al. (in press) distinguished four wader communities, in which ringed plover was associated with Kentish plover, Greenshank, knot and turnstone due to their preference to the less muddy sediments. Danufsky & Colwell (2003) state that plovers are non-followers, probably to avoid inter- and intraspecific concurrence. Smit & Wolff (1981) report that ringed plovers primarily feed on the high eulittoral, and even on the supralittoral zones. Brinkman & Ens (1998) and Ens et al. (2005) also found a preference for higher parts of the intertidal zone, respectively in the Wadden Sea and the Westerschelde. Looking at Figure 5, ringed plovers in the Wadden Sea reach highest densities on mudflats with long emersion periods. However a secondary peak density occurs at the lower intertidal zone. 0,016 0,008 wader density (n/0,1ha) wader density (n/0,1ha) 0,010 0,006 0,004 0,002 0,012 0,008 0,004 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) lutum content (%) Figure 5. Relation between densities of ringed plover and lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). wader density (n/0,1 ha) 0,06 0,05 0,04 0,03 0,02 0,01 0 <0,5 .5-1.5 1.5-3.0 3-5 lutum content (%) 5-8 8-12 12-18 emersion time 5u emersion time 7u Figure 6. Relation between densities of ringed plover and the lutum content of the substrate, after segregation according to emersion time (after Zwarts, unpublished data, in: Blomert, 2002). 17 Nereis diversicolor is often mentioned as the most favoured prey item of ringed plovers. Along the east coast of the UK, the species mainly feeds on thin worms (Scoloplos and Notomastus). Other prey mentioned in literature involves insects, amphipods, Hydrobia and Carcinus maenas. Bivalves have never been found as prey (see literature review by Leopold et al., 2004). Ringed plovers forage almost entirely by visual means, so they greatly depend on the activity pattern of their prey. For example, capture rates are positively correlated to sand temperature (increased prey activity), and inversely related to wind force (decreased activity through desiccation of the substrate) (Pienkowski, 1981 &1983). This only applies when air temperature is fairly low. When air temperature is higher, capture rates decline again despite continuing increase in prey activity. Mean prey size increased however, so it is likely that the plovers become more selective for larger prey when prey availability increases. In situations of low availability, ringed plovers are reported to induce activity through ‘foot vibration’, which leads to an increase in activity of small crustaceans (Pienkowski, 1981 & 1983). As discussed above, ringed plovers in the Wadden sea favour the muddiest sediments when foraging on the high intertidal (see Figure 6). This seems logical, since prey activity correlates positively with the moisture content of the sediment (see §2.8), and muddy soils have a higher water retention capacity compared to coarser soils. It could however also be the other way around, since sandy flats are often characterised by surface pooling, while muddy flats are often convex and water runs off more quickly (pers. comm. Bruno Ens). Despite being a visual forager, a high degree in night time foraging is reported by Ntiamoa-baidu et al. (1998) in coastal lagoons in Guinea-Bissau. In the Berg River estuary, South Africa, Kalejta (1992) found a ratio of night to day foraging numbers of Charadrius spp. of 2,85. On the other hand, Zwarts et al. (1990) reports a clearly lower feeding activity at night, since densities at night are but a fourth of feeding densities during daytime. According to Pienkowski (1982), the peck rate of ringed plovers during night time is about half of that during the day. However, feeding success and prey size could not be measured, so they might achieve higher intake rates when larger prey would come available at night (see §3.5). 18 3.4 Kentish plover (Charadrius alexandrinus) Little was found on the habitat preference of foraging Kentish plovers. In Guinea-Bissau the Kentish plover reaches highest densities on sandy mud and soft mud, and takes 10th position in a ranking of 15 wader species according to their sediment preference from sandy to muddy (Zwarts, 1988a). In the Tagus estuary, Kentish plovers occur nearly exclusively on sandy muds (Moreira, 1999), but show a negative correlation to the mud content according to Granadeiro et al. (2004). Also at the Tagus estuary, Granadeiro et al. (in press) distinguished four wader communities, in which Kentish plover was associated with ringed plover, Greenshank, knot and turnstone, a community that is said to favour the less muddy sediments. According to Smit & Wolff (1981), Kentish plovers in the Wadden Sea use shallows in the higher eulittoral or supralittoral. Compared to ringed plovers, they prefer moister substrates, and are more often found on the intertidal flats. In general, plovers are non-followers, preferring the higher reaches of the intertidal (Danufsky & Colwell, 2003). Perez-Hurtado (1997) found that in Cadiz Bay, Spain, Kentish plovers were mainly foraging in the marshes and saltpans with low water levels adjacent to the intertidal mudflats. Cramp & Simmons (1983) state that polychaete worms, molluscs and crustaceans are important prey in coastal and inland saltwater areas. In Schleswig-Holstein stomachs of six birds contained insects and their larvae, crustaceans (Carcinus maenas), Nereis, gastropods (Hydrobia, Littorina) and a bivalve (Lange, 1968; Höfmann & Hoerschelmann, 1969). Dropping analysis in southwest Spain showed larvae of diptera and coleoptera (Perez-Hurtado, 1997). Kentish plovers forage in the typical plover’s “stop-run-peck”-mode. They have not been observed feeding by night by Zwarts et al. (1990) in Mauritania, Banc d’ Arguin. 19 20 3.5 Grey plover (Pluvialis squatarola) 0,020 0,020 0,016 0,016 wader density (n/0,1ha) wader density (n/0,1ha) In general, grey plovers occur on a broad range of sediment types, but appear to favour muddy sediments. In the Wadden Sea, grey plovers exploit sandy as well as muddy tidal flats, and also occur on the mussel beds and cockle fields (Smit & Wolff, 1981). Brinkman & Ens (1998) state that highest densities in the Wadden Sea are found on intermediate sediments. Figure 7 and Figure 8 show more detailed information of the Wadden Sea, in which appears that grey plovers occur on a broad range of sediments with two peak densities: one on muddy, and one on rather sandy sediments. When the data are segregated according to emersion time, the same two folded habitat preference as found in ringed plover appears: in areas with high emersion times, muddy sediments are preferred, but in low intertidal areas densities are highest on the more sandy substrates. In the Westerschelde, Ens et al. (2005) scored the preferred habitat as muddy. According to the graphs presented by van Kleunen (1999), grey plovers in the Westerschelde occur in maximum densities in the muddiest areas, but there is also a second lower peak at sandy sediments (see Appendix I). In the Wash (UK), Goss-Custard & Yates (1992) found a positive correlation between grey plover densities and the area of mud. By contrast, Yates et al. (1993) showed a negative correlation between grey plover density and the silt content (2063µm) of the sediment. In Guinea-Bissau, grey plovers occur on the somewhat muddier areas (Zwarts, 1988a). In the Tagus estuary, several studies point out that grey plovers prefer the muddier sediments. According to Moreira (1999) grey plovers occur nearly exclusively on sandy mud and muds, with a clear optimum on sandy muds. Granadeiro et al. (2004) found a positive correlation between bird densities and the mud content of the sediment. Furthermore, Granadeiro et al. (in press) report that high densities of grey plovers coincide with high densities of dunlin, redshank and bar-tailed godwit, a wader community with strong preference to muddy substrates. 0,012 0,008 0,004 0,012 0,008 0,004 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) lutum content (%) Figure 7. Relation between densities of grey plover and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). wader density (n/0,1 ha) 0,02 0,016 0,012 0,008 0,004 0 <0,5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 emersion time 4u emersion time 7u Figure 8. Relation between densities of grey plover and the lutum content of the substrate, after segregation according to emersion time (after Zwarts, unpublished data, in: Blomert, 2002). Grey plovers are regarded as non-followers (Townshend et al., 1984; van de Kam et al., 1999; Granadeiro et al., 2006). Being a typical visual forager (Pienkowski, 1983), this is an expected result, since followers are generally tactile foragers (Granadeiro et al., 2006). In the Wadden Sea, grey 21 plovers show peak densities in areas with intermediate emersion periods of 5,5 hours (see Figure 7), while Brinkman & Ens (1998) report a preference to the high intertidal. In the Westerschelde, van Kleunen (1999) showed that grey plovers occur in high densities at high intertidal areas (emersion period of 60-100%) as well as on low intertidal areas (emersion time 0-25%) (Appendix I). Ens et al. (2005) report a preference to high intertidal areas in the Westerschelde. Others studies, in the Wash (UK) and the Tagus estuary (Portugal), show a negative correlation between grey plover densities and the emersion period (Yates et al., 1993; Granadeiro et al., 2004). The greater part of prey items caught by grey plovers consists of large worms (Arenicola, Nereis), but smaller worms (principally Notomastus) can also be a very important food source. Small cockles, mussels and Baltic tellins have also been noticed, mainly in the Wash estuary. Crustaceans are largely absent in the species’ diet (Pienkowski, 1982; see also literature review by Leopold et al., 2004). Grey plovers strongly depend on the activity of their prey, and as in ringed plovers, capture rates are related to sand temperature, wind force and rainfall (Pienkowski, 1981 & 1983). Furthermore, Pienkowski (1983) found an inverse relationship between the capture rates of small and of large worms, so the rate of capturing thin worms was depressed when the rate of taking Arenicola increased. The observations confirmed that grey plovers actively select the larger Arenicola (greater calorific value), obeying the optimal foraging theory. Thus, when falling temperatures or increasing time after high water cause a decrease in Arenicola activity, more thin worms were taken. This causes a shift in food source during each tidal cycle. Desiccation of the substrate also influences prey activity. In the Berg River estuary, South Africa, the species is most abundant on the intertidal areas with most vegetation cover. Kalejta & Hockey (1993) suggest that this is caused by the fact that the vegetation cover reduces the rate of mudflat desiccation during exposure. Reduced desiccation in muddy areas may also explain why grey plovers foraging on the high intertidal favour the muddiest areas as shown in Figure 8. Pienkowski (1981 & 1983) states that the tidal behaviour of grey plover is partly influenced by the presence of bar-tailed godwits. Being a visual forager, grey plovers acquire a large area available to each bird, and are excluded by the tactile foraging godwits that are able to forage efficiently in high density groups. After departure of the godwits in spring, grey plovers spread nearer to the tide edge where prey availability is highest. Night foraging is reported by several authors (Pienkowski, 1980 & 1982; Dugan, 1981; Zwarts et al., 1990; Kalejta, 1992; Ntiamoa-baidu et al., 1998). Kalejta (1992) found a mean ratio of night to day foraging numbers of 0,72 from December until March in the Berg River estuary, South Africa. The ratio night/day density during premigration period amounts to 0,69 as found by Zwarts et al. (1990) in Banc d’ Arguin, Mauritania. According to Pienkowski (1982), the peck rate of grey plovers during night time is about half of that recorded during the day, and Kalejta (1992) also found grey plovers to forage significantly slower at night. However, this does not necessarily mean a decrease in intake, since feeding success could not be measured in these studies. In this respect, Pienkowski (1980) showed a marked increase in Nereis-availability during night time, which may compensate for the decrease in foraging rate. Dugan (1981) found that grey plovers change their feeding site at night, and probably also changed diet from Nereis diversicolor to the much larger Nereis virens. He suggests that the major part of the energy requirements of some individual grey plovers are met at night rather then by day. According to Lourenço et al. (2005), grey plovers show strong association to intertidal drainage channels. 22 3.6 Knot (Calidris canutus) In general, knots seem to prefer intermediate sediments, with slightly differing results obtained by several studies. In the Westerschelde, knots were found to prefer intermediate substrates (van Kleunen, 1999 (Appendix I); Ens et al., 2005). According to Smit & Wolff (1981), preferred feeding areas in the Wadden Sea vary from place to place, but slightly muddy tidal flats and mussel beds are favoured most. Figure 9, based on low tide counts in the Wadden Sea, shows avoidance of the most sandy sediments, and a preference for intermediate substrates with lutum content between 1,5 to 5%. At the shores of the Wash, Goss-Custard & Yates (1992) found that densities of knots correlate positively with the area of sandy substrate. However, sediment characteristics in this study were assessed using satellite imagery. In a more detailed study including soil sampling, Yates et al. (1993) found a positive correlation between density of knots and the proportion of fine sand (63-125µm), but equally so with the proportion of silt (20-63µm) in the sediment. In defining four groups of estuaries based on their wader communities, Hill et al. (1993) found that high densities of knot coincide with high densities of sanderling, bar-tailed godwit and oystercatcher in the larger, sandier estuaries. In Guinea-Bissau, knots reach highest densities on muddy sand, and end up 4th in a ranking of 15 species of shorebird according to their habitat preference from sandy to muddy sediments (Zwarts, 1988a). In a similar ranking of 13 species of wader in the Tagus estuary, the knot takes in an intermediate position, favouring sandy muds (Moreira, 1999). 0,100 0,100 0,080 0,080 wader density (n/0,1ha) wader density (n/0,1ha) Little was found concerning the relation between densities of knot and the emersion period. According to Goss-Custard (1977), knots do not follow the tide line once their preferred feeding areas are exposed. Such tidal behaviour (‘preferred area’, see Figure 2) is described by Nehls & Tiedemann (1993). The modelling of wader distribution in the Westerschelde and the Wadden Sea showed a preference for intermediate emersion periods (Brinkman & Ens, 1998; Ens et al., 2005). 0,060 0,040 0,020 0,060 0,040 0,020 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 9. Relation between densities of knot and lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). Knots use their sensitive bill tip to locate invisible buried prey. The efficiency of this tactile foraging depends on the water content of the sediment, and this mechanism does not work in dry sand, nor in liquid mud, so knots are only found foraging on moderately wet sediments (Piersma et al., 1998). Knots show a strong preference for bivalves (especially Baltic tellin and small cockle and mussel). Worms lack nearly completely in their diet, which makes the knot unique in the range of waders studied here (see literature review by Leopold et al., 2004). When birds arrive from the arctic they have small gizzards and rely heavily on soft bodied prey for some time. In July and August, knots arriving at the Wadden Sea thus take advantage of great abundance of crustaceans (Crangon crangon and young Carcinus maenas). Crustacean densities rapidly decline, and knots switch to their normal bivalve prey, while their gizzards grow larger to cope (van Gils et al., 2005b). Most shorebirds behave according to the contingency model, which implies that the birds spend all their time foraging (handling and searching), and that their maximum energy intake rates are constrained by the rate at which prey can be externally handled (cfr. oystercatcher, Meire, 1993). Knots however ingest their shelled prey as a whole, which is subsequently crushed in the gizzard. A knot obeying the contingency rules, maximizing its energy intake per unit foraging time, does not 23 necessarily maximizes its intake per total unit of time (foraging plus digestive pauses). Therefore, it is expected that knots are selective towards prey that is rapidly digested, so time that would be lost to digestive pauses can be used to search for easy-to-digest prey. Experimental tests conducted by van Gils et al. (2005a & 2006) indeed shows that knots obey the ‘digestive rate model’ much more than the ‘contingency model’. Knots show a tremendous flexibility (inter- and intra-individual variation) in digestive organ size (Piersma et al., 1993b; van Gils et al., 2003), and it has also been verified that gizzard size constrains digestive processing rate in knots (van Gils et al., 2003). This influences the patch choice as shown by van Gils et al. (2005b). Comparing two contrasting patches, the patch with high densities of hardshelled prey attracted birds with large gizzards, while the patch with low densities of soft-shelled prey attracted birds with smaller gizzards. Birds with small gizzards are even forced to extend their foraging time beyond the 12 hour tidal system. This is possible because in the Wadden Sea, there are large spatial differences from west to east in the timing of the tidal cycle, western flats being exposed two hours earlier than the eastern ones. By gradually moving eastwards during low tide, birds can extend their low tide feeding time to 16-17 hours. Based on extensive research, van Gils et al. (2006) concludes that the presence of foraging knots on the mudflats in the Wadden Sea matches most closely with an ideal, non-free model. ‘Non-free’ since flights between foraging areas incur time and energy costs. ‘Ideal’, since knots appear to have good information about the expected intake rate at each site. This may be necessary since prey densities and hence, intake rates vary tremendously between sites, seasons and years (Zwarts, 1992). van Gils et al. (2006) conclude that knots make state-dependent decisions by trading off starvation against foraging-associated risks, including predation. Knots seem to share public information about resource quality which enables them to behave in a more or less ideal manner. The home range of individual knots in the Wadden Sea is large and amounts up to 800 km². With respect to area use, van Gils et al. (2006) conclude that knots trade off energy intake rates against travel cost. Thus, sites near a roost are skipped in case they offer low intake rates, while sites offering high intake rates are only used if roosting occurs nearby. For example, Ballastplaat is more often used by birds that roost at Griend (8-9 km away) than by Richel-roosters (16-18 km away). Knots wintering in Scottish estuaries also show no or little site fidelity between years or within wintering seasons (Symonds et al., 1984; Rehfisch et al., 2003), and roam over areas of several tens of square kilometres in course of winter. By contrast, knots in Banc d’Arguin, Mauritania, were found to be very faithful to their roosts, and used an area of only 2-16 km². This is in conformity with the behaviour of birds wintering in tropic North-western Australia, where birds roam within an area of 20km² (Rogers et al., 2006). The reason for this clear difference between tropical and temperate climate zones might be that availability of prey is much more unpredictable in the temperate zones (Leyrer et al., 2006). Prater (1972) found that stomachs of knots feeding by day contained four times the quantity of food compared to the stomachs of night feeding birds. On the other hand, during premigration period, (February-March) knots in Banc d’ Arguin seem to reach higher densities at night than during the day, and Zwarts et al. (1990) found a night/day density ratio of 1,39. Sitters et al. (2000) reports night foraging too, and also found a change in distribution during night time compared to daytime distribution. According to Lourenço et al. (2005), knots show strong association to intertidal drainage channels. 24 3.7 Sanderling (Calidris alba) In literature, sanderling is often presented as typically occurring on sandy intertidal areas. In the Wadden area they occur almost exclusively on sandy beaches and sand flats (Zwarts, 1981; van de Kam et al., 1999); in the Delta area the majority of observations originate from sandy beaches (Wolff, 1969). In the Westerschelde, there exists a negative correlation between sanderling densities and the silt content of the substrate (van Kleunen, 1999; Appendix I), and a comparable negative correlation was found in the Tagus estuary by Granadeiro et al. (2004). In defining shorebird communities, Granadeiro et al. (in press) found high densities of sanderling occurring on the coarser substrates, coinciding with high densities of gulls. At the Orkneys, sanderlings, together with bar-tailed godwits favour sandy sediments (Summers et al. 2002). Danufsky & Colwell (2003) found that substrate particle size correlated positively with sanderling density in Humboldt Bay, California. Some authors (van Turnhout & van Roomen, 2005) report a (small) difference in sediment preference between Nearctic and Siberian populations. Sanderlings of the Nearctic population should have a preference for muddier substrate compared to the Siberian birds. This is one of the reasons why some authors (e.g. Engelmoer & Roselaar, 1998) plead for a division into two subspecies. The sanderling shows a strong association with the tide line (De Meulenaer, 2006; Speybroeck et al., 2006). Therefore one might expect sanderlings to be followers. However they behave in a very opportunistic manner while feeding, and when food is abundant at emerged parts of the intertidal (eg. concentration of wreck), sanderlings will readily gather there to form so-called feeding frenzies. This way, sanderlings either behave as followers, or they behave in direct response to food abundance as ‘preferred area foragers’ (see Figure 2). According to van Kleunen (1999), sanderlings in the Westerschelde occur throughout the intertidal area with a slight preference to areas with emersion periods of 25-40% (Appendix I). The species’ diet is very diverse and ranges from worms, over insects, to small amphipods and wreck washed ashore. Scolelepis squamata is often mentioned as the most important food item for sanderling (Dankers et al., 1983; De Gee, 1984; Glutz von Blotzheim et al., 1984). Recent research on the Belgian coast however shows a rather marginal importance of Scolelepis squamata, and sanderlings seem to depend largely on opportunistic feeding on wreck (all kinds of prey washed ashore) (De Meulenaer, 2006; Speybroeck et al., 2006). Apparently, foraging on Scolelepis is profitable only when the tide line crosses the narrow zone at the high intertidal where Scolelepis is abundant. Whether this is due to deep burying of the worms in quickly drying sandy soils, or due to high energy costs of probing in dry substrates is unclear. 25 26 3.8 Dunlin (Calidris alpina) Dunlins show a clear preference for muddy substrates. Research in the Wadden Sea and the Westerschelde estuary showed a positive correlation between dunlin densities and muddy sediments (Ens et al., 1993; Brinkman & Ens, 1998; van Kleunen, 1999 (Appendix I); Ens et al., 2005). The data of the Wadden Sea are visualised in Figure 10, and one sees that dunlins are encountered on a broad range of sediment types, but reach highest densities on sediments with lutum content of 5-8%. In the Tagus estuary in Portugal, Granadeiro et al. (2004 & in press) showed a positive correlation of dunlin densities with the mud fraction of the soil. The same accounts for dunlins in the Wash (UK), where densities are positively correlated with either the area of mud, or with the clay fraction (<20µm) of the sediment, while a negative correlation was found with the sand fraction (>125µm) (Goss-Custard & Yates, 1992; Yates et al., 1993). While investigating 494 coastal trajects on the Orkney islands (UK), dunlins were found mostly on the muddy ones, together with Snipe, avocet, redshank and curlew (Summers et al., 2002). On the other hand, Moreira (1999) found that the species’ habitat preference in the Tagus estuary was not as explicit as reported by most, as dunlin took in an intermediate position in a ranking of 13 species of waders according to their preference for sandy to muddy sediments. Nehls & Tiedemann (1993) showed a seasonal pattern in habitat utilization and foraging-behaviour. Generally, densities of dunlin were highest on muddy substrates, but in late summer, when there was high availability of shrimps (Crangon crangon), dunlins preferred foraging on higher sandy sediments. 0,050 0,100 0,040 0,080 wader density (n/0,1ha) wader density (n/0,1ha) Some studies are conducted at a large special scale, and compare total numbers of wintering dunlin between estuaries (McCulloch & Clark, 1992; Hill et al., 1993; Holloway et al., 1996). McCulloch & Clark (1992) present a graph that shows the relationship between mean densities of dunlin and the mean percentage of silt and clay in the sediments of 14 British estuaries (see Figure 11). Hill et al. (1993) compared 109 British estuaries and identified four groups of estuaries with comparable wader communities. The group containing the smaller and muddier estuaries was characterised by high densities of dunlin, redshank and curlew. Finally, Holloway et al. (1996) and Austin et al. (1996) related bird densities to estuary characteristics, and also found that dunlins are associated with the more muddy estuaries. 0,030 0,020 0,010 0,060 0,040 0,020 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) lutum content (%) Figure 10. Relation between densities of dunlin and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). mean Dunlin density birds/ha 20 15 10 5 0 0 20 40 60 80 100 m ean percentage silt+clay Figure 11. The relationship between the mean proportion of silt and mud in the sediments of estuaries and the densities of overwintering dunlin (birds/ha) (after McCulloch & Clark, 1992). 27 While in the Westerschelde van Kleunen (1999)(Appendix I) and Ens et al. (2005) found preference for areas with long emersion periods, other studies show the opposite (Yates et al., 1993; Ens et al., 1993; Brinkman & Ens, 1998; Granadeiro et al., 2006). In the Westerschelde, foraging dunlins possibly make use of surface pools at the higher shoals (pers. comment Peter Meininger). Low tide water counts in the Wadden Sea showed a clear preference for the lower parts of the intertidal (short emersion times, see Figure 10). According to Granadeiro et al. (2006), dunlins behave like real tidefollowers, probably as a result of better prey availability (Esselink & Zwarts, 1989; Kelsey & Hassall 1989; Nehls & Tiedemann, 1993). Several authors stress on the importance of substrate penetrability for tactile feeders like dunlin (Quammen, 1982 & 1984; Gerritsen & Van Heezik, 1985; Kelsey & Hassal, 1989). For example, Kelsey & Hassal (1989) found that dunlins prefer low-lying wet areas with high penetrability, even though density of the favoured prey was higher on the firmer ridges. Gerritsen & Van Heezik (1985) showed that dunlins preferred to feed upon soft sediment rather than on firmer substrate types, even if this choice reduced their foraging success. Dunlin is said to be a typical worm-feeder, and the diet is dominated by oligochaetes and polychaetes (mainly Nereis diversicolor). In case of high availability, the diet is added with Hydrobia ulvae, small crustaceans (Crangon crangon, Corophium volutator) and small bivalves (Baltic tellin) (Wolff, 1969; Cramp,1983; Kelsey & Hassall, 1989; Mouritsen & Jensen; 1992; Nehls & Tiedemann, 1993; Mouritsen, 1994; Leopold et al., 2004). According to Symonds et al. (1984) wintering dunlins are quite mobile within the estuary Firth of Forth (Scotland), and they don’t seem to have great fidelity to high tide roosts. Dunlins even fly up to 18 km between roosts and foraging areas. Rehfisch et al. (2003) also found poor fidelity to roosts, dunlin being the third most mobile species out of eight in the Moray Basin, Scotland. These findings are in contrast to what Dias et al. (2006a) report from the Tagus estuary. They found that dunlins only feed close to their roost, and suggest that due to a lack of roosts, suitable foraging areas could remain unexploited. According to studies conducted in the Wash estuary, adult dunlins also seem to be less mobile (Minton, 1975; Cooper, 1988; Rehfisch et al., 1996) than reported from other estuaries in the UK. However this is not the case in juvenile dunlins, as these are among the most mobile shorebirds in the Wash. Rehfisch et al. (2003) offers an overview of several explanations for this age-related difference. Juveniles may be more mobile in order to sample possible alternative future wintering sites. Furthermore, juveniles may be forced to range more widely because of their lesser experience, socially subordinate status, higher risk of being predated and possibly different diet. Night foraging by dunlin is reported by several authors (e.g. Zwarts et al., 1990; Mouritsen, 1994; Dodd & Colwell, 1996). Mouritsen (1994) reports that pecking is the dominant foraging technique by day (interpreted as visual foraging), whereas probing was the preferred technique at night (tactile foraging). They also tend to utilize different habitats during day and night time, with relatively more birds aggregating on soft sediment containing high densities of Corophium volutator at night. When tactile feeding, selected sites should indeed contain high densities of infaunal prey to maximize prey encounter during probing. Soft sediment furthermore offers reduced costs and better prey accessibility when probing. The results obtained by Mouritsen (1994) support the ‘supplementary hypothesis’ (shorebirds feeding at night only when daytime feeding has proved insufficient), however it does not refute the ‘preference hypothesis’ (shorebirds feed at night because it is more profitable). 28 3.9 Bar-tailed godwit (Limosa lapponica) Literature shows that bar-tailed godwits prefer intermediate to sandy environments. In the Wadden Sea, Ens et al. (1993) found a positive correlation with the area of sandy sediments, and Brinkman & Ens (1998) state that bar-tailed godwits avoid muddy areas, and prefer intermediate substrates. This is visualised in Figure 12, showing that bar-tailed godwits occur on a broad range of sediments, but clearly avoid sediments with lutum content of more than 8%. In the Westerschelde, the species was found to avoid coarser substrates (van Kleunen, 1999)(Appendix I) requiring a silt content of at least 25%. According to Ens et al. (2005), bar-tailed godwits in the Westerschelde occur in a narrow range of intermediate sediments. In the Wash (UK) the species shows positive correlation with the area of sand (Goss-Custard & Yates, 1992), as well as with the content of fine sand (63-125µm) (Yates et al., 1993). In the Orkneys too, the species prefers sandy areas (Summers et al., 2002). Hill et al. (1993) defined four types of estuaries based on shorebird communities, and found that high densities of bartailed godwits generally occur in the broad and sandy estuaries (UK), and coincide with high densities of knot, sanderling and oystercatcher. In western Africa, Guinea-Bissau, the bar-tailed godwit is placed 3rd in a ranking of 15 shorebird species according to their preference from sandy to muddy sediments (Zwarts, 1988a). Moreira (1999) reports an intermediate sediment preference in the Tagus estuary. By contrast, in defining four wader communities in the same estuary, Granadeiro et al. (in press) found that highest densities of bar-tailed godwit coincide with high densities of dunlin, redshank and grey plover, a community occurring on muddy sediments. Foraging bar-tailed godwits are encountered on mussel beds too, and inland meadows can be important as additional feeding grounds during winter and spring (Smit & Wolff, 1981; van de Kam et al., 1999). Bar-tailed godwits generally favour the low intertidal areas with short (Smith, 1975; Ens et al., 1993; Tulp & de Goeij, 1994; Granadeiro et al., 2004) to intermediate emersion periods (Brinkman & Ens, 1998; van Kleunen, 1999 (Appendix I); Ens et al., 2005). Figure 12 shows clear avoidance of the higher reaches of the intertidal area. The bar-tailed godwit is regarded as a follower by Thompson et al. (1986), whereas it is classified as a non-follower by Granadeiro et al. (2006). This seems odd, since the latter found a preference to areas with short emersion times during another study at the same site (Granadeiro et al., 2004). 0,160 wader density (n/0,1ha) wader density (n/0,1ha) 0,160 0,120 0,080 0,040 0,120 0,080 0,040 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 12. Relation between densities of bar-tailed godwits and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). In the Westerschelde, Ruiters (1992) studied faeces of bar-tailed godwits. He found that worms (Nereis and others) form the staple food, added with few bivalves (mussel, cockle, Baltic tellin) and Hydrobia. When temperature drops beneath 3°C intake rate of Arenicola decreases, and bar-tailed godwits switch to Scoloplos and Notomastus, which were found by the sewing technique, and thus most probably by tactile means (Smith, 1975). The species’ diet is diverse however, and recent research in the German Wadden Sea showed a total of 17 prey species, the most important items still being worms (Nereis, Nephtys and Scoloplos). In autumn Arenicola marina seems very important, and comprises 24% in the male’s and 65% in the female’s prey mass (Scheiffarth, 2001). According to Smith (1975), bar-tailed godwits follow the tide edge closely, on both ebb and flood, for the numbers of 29 their prey (Arenicola marina) visible at the surface declines steadily after the tide has passed. Smith (1975) also found that the godwits found more Arenicola in wet sediments, and therefore avoid sandy areas that dry out during low tide. Lourenço et al. (2005) reports a strong association with drainage channels (where sediments also remain wet), probably due to exceptionally high densities of Nereis diversicolor near these channels (pers. comm. Pedro Lourenço). Night foraging has been reported by Zwarts et al. (1990) and Ntiamoa-baidu et al. (1998). During premigration period in Banc d’ Arguin, Mauritania, bar-tailed godwits reach highest densities during night time, with a night/day density ratio of 1,22 (Zwarts et al., 1990). Both et al. (2003) report a different use of the tidal flats by males compared to females, as a result of sexual dimorphism (males having shorter bills). Males were concentrated on exposed mud flats, while females occurred more along the waterline. According to Both et al. (2003) also, the waterline seems to be a better foraging habitat for bar-tailed godwits, since intake rates are higher there. Females thus seem to monopolise the better quality foraging areas, since males were more susceptible to intraspecific kleptoparsitism. Bar-tailed godwits are mobile birds, and show poor fidelity to high tide roosts, as shown by research in the Firth of Forth and the Moray basin, Scotland (Symonds et al., 1984; Rehfisch, 2003). 30 3.10 Curlew (Numenius arquata) In general, densities of curlew show a positive correlation with muddy substrates. This is found by research conducted in Guinea-Bissau (Zwarts, 1988a), in the Wash (UK) (Goss-Custard & Yates, 1992; Yates et al., 1993), on the Orkney islands (UK) (Summers et al., 2002), in the Westerschelde (van Kleunen, 1999 (Appendix I); Ens et al., 2005) and in the Wadden Sea (Ens et al., 1993). Only Brinkman & Ens (1998) report that the favoured habitat in the Wadden Sea consists of intermediate to muddy sediment. The results of low tide counts in the Wadden Sea indeed show that foraging curlews can be found at a broad range of sediment types with a preference for intermediate substrates (lutum content 3-5%). When distinguishing shorebird communities, Hill et al. (1993) found that high densities of curlew occur in the more muddy estuaries, and coincide with high densities of dunlin, redshank and avocet. Furthermore, curlews may show a strong association with mussel beds (Smit & Wolff, 1981; Ens et al., 1993). There is poor agreement in literature about the tidal behaviour of curlews. Several authors report curlews are followers (Thompson et al., 1986), or favour the low intertidal (Yates et al., 1993; Brinkman & Ens, 1998; van Kleunen, 1999). Looking at Figure 13 it seems that curlews in the Wadden Sea avoid the highest intertidal areas, and show a preference for relatively short to intermediate emersion times of 4 to 5,5 hours. In the Westerschelde estuary, van Kleunen (1999) found curlews occurring throughout the intertidal range with a preference for emersion periods less than 40%. This is in contrast to what Ens et al. (2005) found in the Westerschelde, namely curlews preferring areas with rather long emersion periods. Tulp & de Goeij (1994) state that in contrast to the real followers, Eastern curlews in Roebuck Bay, North-western Australia, also use parts of the intertidal that are already exposed for longer periods. Some authors state that curlews are non-followers (Recher, 1966; Townshend et al., 1984 in: Danufsky & Colwell, 2003). 0,400 wader density (n/0,1ha) wader density (n/0,1ha) 0,400 0,300 0,200 0,100 0,300 0,200 0,100 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 13. Relation between densities of curlew and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). Based on an extensive literature review, Leopold et al. (2004) concludes that the diet of curlews is very diverse, consisting mainly out of bivalves and large worms (Nereis and Arenicola), added with 20% other prey such as Carcinus maenas and shrimps. Small prey like little worms or Hydrobia ulvae are unimportant to the large curlew. In the Westerschelde, most important prey items are Nereis, followed by Carcinus maenas and bivalves (Ens et al., 2005). Curlews aren’t strictly bound to intertidal flats and are often seen on inland meadows where they forage on earthworms (Lumbricus terrestris) and other typical inland prey (Diptera-larvae). Some curlews spend the whole winter inland, others only during severe weather or high tides (van de Kam et al. 1999). Thomspon et al. (1986) showed a negative correlation between curlew and densities of Nereis. This is in fact not a contradiction since the largest (and most profitable) Nereis were rare where overall Nereis density was high. According to Zwarts (1996b), curlews select prey of the most profitable sizes. For example, amongst the prey caught by curlew, Nereis of 12 cm occurred 5 times more often than Nereis of 6 cm. Taking prey densities and accessibility (burying depth varies with worm length) into account this means that Nereis of 12 cm are selected 23 times more often. Similarly, curlews preferred Mya arenaria of 4-5 cm. In the case of Carcinus maenas, individuals smaller than 10mm are ignored. 31 Zwarts & Esselink (1989) report two different methods that curlew use to capture Nereis diversicolor. They search for worms at the surface which are taken with a single peck, or they search for visual cues such as burrow entrances or traces (water current) due to filter feeding activity, and probe deep to extract the worm from the burrow. Nereis taking through probing is common in summer, but this behaviour disappears in autumn due to a decreased accessibility (increase of burrowing depth and a reduction of the filter feeding). Foraging behaviour of curlews is strongly influenced by the behaviour of its prey. Worms cannot filter-feed on dry surfaces and thus filter-feeding is generally restricted to the first 2 hours after exposure of the intertidal area. Afterwards worms must surface to graze. In other words: during the low water period filter activity decreases, while grazing activity increases, resulting in a shift between foraging modes of curlew. Nereis activity is also influenced by the sediment type. Generally, in muddy soils, Nereis will tend to graze more often, since silt particles interfere with filtering activities. Also, filtering is less profitable when the water contains few food particles as at the end of winter, so grazing will then be more efficient (Esselink & Zwarts, 1989). The sexual dimorphism in curlews may result in a segregation in habitat choice (Townshend, 1981), as well as in diet choice (Zwarts & Wanink, 1984). In the Tees estuary (North-east England), relatively more males compared to females feed on adjacent fields during winter. Townshend (1981) suggest this is due to the deep burying of Nereis as temperatures drop and Nereis becomes more and more unavailable to the short-billed male curlews. Zwarts & Wanink (1984) state that in the Wadden Sea, male curlews take few Mya arenaria only, while it is a main prey item of females. These authors suggest that the accessible proportion of Mya arenaria above the lower size limit is too small for the shorter-billed males. Curlews prefer feeding during the day, but may also feed at night when daytime food intake becomes insufficient (Cremer & Hupkes, 1984, in: van de Kam et al., 1999). The foraging activity of curlews that were followed from July until November increased from 65% to 80%, however, this did not compensate for the decreasing foraging time available. In the meanwhile, night time foraging increased, from ½ hour to nearly 5,5 hours. Thus, birds in November foraged 1,5 times as long as birds in July, but overall intake rate stayed nearly the same throughout the period, as night time foraging success was 30% lower compared to day time foraging success. During premigration period in Guinea-Bissau (February-March), Zwarts et al. (1990) found a ratio of night to day foraging numbers of 0,58. Ntiamoa-baidu et al. (1998) also report curlews feeding by night. In Scotland, curlews show high fidelity to high tide roosts as reported by Rehfisch et al. (2003). Through colour ring marking of several hundreds of curlew in the Wadden Sea, high (foraging) site fidelity could be shown. Marked birds seemed to forage at exactly the same sites for years in a row, some defending an area as small as 0,2 to 1 ha. If in particular years, there was not enough food within their own territory, the birds often kept foraging within 1 km of that site (Ens & Zwarts, 1980; Cremer & Hupkes, 1984; in: van de Kam et al., 1999). 32 3.11 Spotted redshank (Tringa erythropus) According to Blomert (2002), the spotted redshank prefers muddy sediments, and Smit & Wolff (1981) also describe the preferred habitat in the Wadden Sea as very muddy shallows. By contrast, Brinkman & Ens (1998) mention spotted redshank as a bird of intermediate substrates. Low tide counts in the Wadden Sea revealed a strong preference to soils with a lutum content of 5-8%, and an exposure period of 7 hours (Figure 14). Feeding generally takes place in shallow water, and foraging birds are often encountered in the creeks of saltmarshes or in ditches in inland polders. Foraging birds usually walk in the water and sometimes even swim (van de Kam et al., 1999). In tidal areas they search for Nereis, Carcinus maenas, shrimps and smaller prey like Corophium volutator. In summer spotted redshanks also hunt for young fish and shrimps trapped in pools (van de Kam et al., 1999; Leopold et al., 2004). The bird sometimes uses side-to-side scything movements like avocet, with bill inclined at shallow angle to surface (Cramp, 1983). 0,020 0,020 0,016 0,016 wader density (n/0,1ha) wader density (n/0,1ha) Just like avocet, Ntiamoa-baidu et al. (1998) classify spotted redshank as a pelagic forager. This type of wader hunts for fish in the water layer, and uses visual as well as tactile senses to detect prey. When foraging on fish or shrimps, social feeding is often seen, not seldom in association with avocets (Ntiamoa-baidu et al., 1998; van de Kam et al., 1999). Ntiamoa-baidu et al. (1998) reports night foraging. 0,012 0,008 0,004 0,012 0,008 0,004 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 14. Relation between densities of spotted redshank and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). 33 34 3.12 Redshank (Tringa totanus) Redshanks generally prefer muddy sediments, although some authors found a preference to intermediate substrates. Brinkman & Ens (1998), for example, report a preference to intermediate sediments in the Wadden Sea. Ens et al. (1993), however, found positive correlations with the low muddy areas (most significant), as well as with the low sandy areas, as with the area of mussel bed. Looking at Figure 15, redshanks in the Wadden Sea show a clear avoidance of the coarse sediments and reach peak densities at areas with a fairly high lutum content of 5-8%. In the Westerschelde, redshanks clearly prefer the muddy areas (van Kleunen, 1999 (Appendix I); Ens et al., 2005). In Guinea-Bissau, Zwarts (1988) found well defined peak densities at muddy sand, and redshank took in an intermediate position in a ranking according to the species’ preference from sandy to muddy substrate. In the UK, results of research show good agreement. In the Wash, densities of redshank show a positive correlation with the area of mud, as well as with the clay content (<20µm) of the substrate, and moreover, densities show a negative correlation to the sand content (>125µm) (Gosscustard & Yates, 1992; Yates et al., 1993). Hill et al. (1993) compared wader communities in 109 British estuaries, and distinguished four types of estuaries. The smaller, more muddy estuaries held highest densities of redshank, together with dunlin, curlew and avocet. At the Orkney islands, redshanks occur on muddy substrates, and are associated with snipe, curlew, dunlin and avocet (Summers et al., 2002). 0,500 0,500 0,400 0,400 wader density (n/0,1ha) wader density (n/0,1ha) The redshank’s diet is diverse, comprising of several species of worms (Nereis, Nephtys, Lanice, Scoloplos, Harmothoe), Crustacea (small crabs as well as shrimps and Corophium) and Hydrobia (see literature review by Leopold et al., 2004). Inland meadows can provide an alternative and additional food source (van de Kam et al., 1999). 0,300 0,200 0,100 0,300 0,200 0,100 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 15. Relation between densities of redshank and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). According to van Kleunen (1999) there is a negative correlation between redshank densities and the emersion period (Appendix I). Yates et al. (1993) and Ens et al. (1993) both found a positive correlation between densities of redshank and area of low lying mud, respectively in the Wash (UK) and the Wadden Sea. Goss-custard & Yates (1992) report that redshanks are mainly associated with creeks and the tide edge. Likewise, Granadeiro et al. (2004) report a negative correlation between redshank densities and exposure period in the Tagus estuary, while Lourenço et al. (2005) reports a strong association with drainage channels. On the other hand, Granadeiro et al. (2006) states redshank being a non-follower. Furthermore, research in the Netherlands showed that the species occurs on habitats with a wide range of emersion periods (Brinkman & Ens, 1998), in the Westerschelde even favouring areas with long emersion times according to Ens et al. (2005), in contrast to van Kleunen (1999). According to the low tide count in the Wadden Sea as shown in Figure 15, redshank occur on a broad range of exposure periods, but clearly avoid areas with long emersion times. The amphipod Corophium is often mentioned as a favoured prey species (Goss-Custard, 1969 & 1977b). When preying on Corophium, the habitat preferences and tidal and seasonal variation in activity and availability of this benthic prey species greatly determines the foraging behaviour of redshanks. For example, when temperature drops beneath 6°C, Corophium becomes less active and 35 the birds take increasing numbers of Baltic tellin or Nereis diversicolor (Goss-Custard, 1969). On a tidal scale, when receding tide has just exposed the flats, Corophium is very active. Their activity then steadily decreases, thus redshanks follow the retreating tide line to maximize intake rate. This suggests that redshank are so-called followers, favouring the low tidal flats (Zwarts, 1974; Thompson et al., 1986; van de Kam et al., 1999). Boates (1980) found that on the receding tide line large sized adults of Corophium are active. This is a short-lived phenomenon however, and within half an hour of the tide passing, only 5% of the initial number of Corophium is active on the surface. Redshanks foraging on Nereis obey the optimal foraging theory and select larger prey in respect to their profitability. The preferred size classes of worms had the highest ratio of energy content to handling time, as predicted by the optimal foraging theory. On the other hand, redshanks feeding on Corophium did not seem to actively select larger prey. When larger Corophium were taken, this appeared to reflect the greater chance of large individuals being detected as large Corophium provide larger visual clues (Goss-Custard, 1977). Several authors report nocturnal foraging by redshank (Goss-Custard, 1969; Zwarts et al., 1990; Ntiamoa-baidu et al., 1998; van de Kam et al., 1999; Burton & Armitage, 2005). Normally redshanks in the Ythan (UK) pecked at the mud surface for Corophium, but swished their bills from side to side in darkness and ate gastropods (Goss-Custard, 1969). Burton & Armitage (2005) found that at night, redshanks in the Severn estuary use different foraging areas than during the day. Notwithstanding the fact that the site visited at night held very high densities of Corophium, this is probably explained by the presence of a heliport causing disturbance during the day. Zwarts et al. (1990) found a ratio of night/day densities of 0,39 in Banc d’ Arguin, Mauritania. In Scotland, redshanks are not very mobile and show high fidelity to their roosts (Symonds et al., 1984; Rehfisch et al., 2003). 36 3.13 Turnstone (Arenaria interpres) Turnstones show a strong association with hard substrates (rocky shores but also all kinds of human constructions like groynes, dykes etc.). However, as a highly opportunistic species it is also found at various types of tidal flats (sand as well as mud) and along the tide line, where it feeds on all kinds of prey washed ashore (Smit & Wolff, 1981; Cramp, 1983; van de Kam et al., 1999; Engledow et al., 2001; Summers et al., 2002; Becuwe et al., 2006; Speybroeck et al., 2006). The small numbers of turnstones present in the Wadden Sea seem to prefer the more sandy substrates (see Figure 16). A preference for sandy substrate in the Wadden Sea was also found by Ens et al. (1993). In GuineaBissau however, turnstones reach peak densities on muddy sands and take in 11th position in a ranking of 15 shorebird species according to their preference from sandy to muddy substrates (Zwarts, 1988a). In the Westerschelde, van Kleunen (1999) encountered highest density of turnstones on muddy sediments with silt content of more than 50% (Appendix I). Along the Belgian coast, turnstones may also show a preference for muddy areas, especially during summertime, for example in the IJzer estuary and the harbour of Zeebrugge. During high tides in winter, foraging turnstones can also be found in meadows and fields in the polder land (Becuwe et al., 2006). The name of the species refers to their unique behaviour of turning stones, shells or other items; once the item is turned over, all kind of prey may come available (Cramp, 1983; van de Kam et al., 1999). Their strong, stout bill can also be used to break open shells and crabs, and to peck barnacles from rocks. According to Becuwe et al. (2006) their diet along the Belgian coast diet is very diverse and prey taken ranges from Crustaceans (crabs, Gammarus) and Barnacles, over worms (Nereis, Eulalia) and insects to mussels and even bread thrown by humans. Leopold et al. (2004) state that turnstones probably take anything that is eatable, and the number of invertebrate species taken as food varies enormously, depending on place and time of the year. In the Westerschelde, Ruiters (1992) found shellfish (Baltic tellin, Hydrobia; most probably eaten as wreck), Nereis, Corophium and insects as main prey items. Turnstones foraging on beaches often occur along the tide line, searching for food washed ashore. However, being highly opportunistic, they respond quickly to local food abundance, and readily exploit emerged parts of the intertidal when food in the form of wreck is abundant (Speybroeck et al., 2006). Turnstones in North-western Australia were also observed foraging along the tide line as well as on parts already exposed for a longer period (Tulp & de Goeij, 1994). In the Wadden Sea, Ens et al. (1993) found a positive correlation with the area of low intertidal sand flats. Based on the low tide counts in the Wadden Sea (Figure 16), one sees two very distinct peak densities, one at intermediate emersion time (5,5h), and one at very short emersion time (3h). Because of very low densities encountered, these results may be unreliable. In the Westerschelde, turnstones are most often encountered on areas with rather short emersion periods of less than 40% (van Kleunen, 1999; Appendix I). 0,016 wader density (n/0,1ha) wader density (n/0,1ha) 0,004 0,003 0,002 0,001 0,012 0,008 0,004 0,000 0,000 <0.5 .5-1.5 1.5-3.0 3-5 5-8 lutum content (%) 8-12 12-18 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 emersion period (hours) Figure 16. Relation between densities of turnstone and the lutum content of the substrate (left) and emersion period (right), based on low tide counts in the Wadden Sea (after Zwarts, unpublished data, in: Blomert, 2002). Since turnstones largely depend on visual information while foraging they are exclusively day-active in a day-night rhythm. During the day, turnstones show a tidal rhythm; they forage around low tide and 37 aggregate in roosts at high tide (Becuwe et al., 2006). Other studies confirm their diurnal foraging pattern, like for example in the Banc d’ Arguin, Mauritania (Zwarts et al., 1990) and in Ghana (Ntiamoa-baidu et al., 1998). In the UK, turnstones show a high degree of site fidelity to foraging grounds as well as to high water roosts (Symonds et al., 1984; Rehfisch et al., 2003). 38 4 EXPERT INQUIRY 4.1 Introduction Five out of twenty contacted shorebird experts were prepared to fill in our inquiry form. These were Bruno Ens (SOVON, the Netherlands), Pedro Lourenço (Universidade de Lisboa, Portugal), Peter Meininger (RIKZ, the Netherlands), Geert Spanoghe (INBO, Belgium) and Michael Yates (Centre for Ecology and Hydrology, United Kingdom). Please note that the order of experts as listed in the previous sentence is alphabetically and does not correspond to the numerical order as given in the tables below. The first question in this inquiry dealt with sediment preference of waders. The experts were asked: • • • to classify the waders according to their habitat preference from muddy to sandy substrates (ranking), to generally describe the preferred habitat using the terms sandy, intermediate and muddy and to score the dependence of the wader on the preferred sediment type. In the second question the experts were asked to classify the wader species according to their tidal preferences (from low to high intertidal areas), and to point out which species are followers and which are non-followers. The third question was established to learn more about the dependency of the birds to one or more prey species, the most important prey species and the habitat demands of the prey species. Finally, the experts were asked to sum up any other factors that might be of significance for the distribution of waders, and to ventilate remarks concerning our investigation. In Appendix III one can find the original question form. 4.2 Sediment There is good agreement among the experts about the habitat preferences of avocet, sanderling, dunlin and redshank Table 1. By contrast, for knot, bar-tailed godwit and grey plover there is poor agreement concerning their habitat preference. Table 1. Results of the inquiry for the sediment preference, including a ranking of the birds according to their sediment preference (1 = occurring on most sandy sediments … 13 = occurring on most muddy sediments), and a classification of the favoured sediment (M = muddy, I = intermediate, S = sandy). EXPERT 1 EXPERT 2 EXPERT 3 EXPERT 4 EXPERT 5 oystercatcher 2 S 6 6 I 4 S 4 I avocet 13 M 13 12 M 13 M 13 M ringed plover 6 I 5 3 S 3 S 7 I Kentish plover 5 I 1 1 S 2 S grey plover 9 I 11 4 S 4 I 6 M knot 4 I 4 8 I 5 I 11 M sanderling 1 S 2 2 S 1 S 1 S dunlin 10 I 10 7 I 7 I 10 M bar-tailed godwit 8 I 3 10 M 2 S 12 M curlew 7 I 8 9 I 7 I 5 I spotted redshank 11 M 12 13 M 10 M 8 M redshank 12 M 9 11 M 10 M 9 M turnstone 3 I 7 5 I 8 I 3 S 39 For each species we calculated an average ranking based on the expert rankings (Table 2). Next, a mean number was calculated based on the experts’ classifications of the preferred habitat, which was consequently converted to one of four following categories: sandy = <1,5; muddy sand = 1,5-2; sandy mud = 2-2,5; muddy = >3). The same was done for the dependency of the waders to their favoured habitat (high specifity= 1-1,7; intermediate specifity= 1,7-2,3; poor specifity= 2,3-3). Table 2. Mean ranking according to habitat preference of waders based on expert judgement; classification of preferred habitat in 4 categories (sandy, muddy sand, sandy mud, mud). The degree of dependency on the sediment is expressed by the colour (yellow=high specifity; green=intermediate specifity; orange= low specifity). Avg. Class Specifity Species Preferred sediment Ranking sanderling Sandy 1,4 1 1 Kentish plover Sandy 2,3 1,333 1,5 oystercatcher Intermediate (muddy sand) 4,4 1,5 2,2 ringed plover Intermediate (muddy sand) 4,8 1,5 2 turnstone Intermediate (muddy sand) 5,2 1,75 2,6 knot Intermediate (sandy mud) 6,4 2,25 1,8 grey plover Intermediate (sandy mud) 6,8 2 2 bar-tailed godwit Intermediate (sandy mud) 7,0 2,25 1,8 curlew Intermediate (sandy mud) 7,2 2 2,6 dunlin Intermediate (sandy mud) 8,8 2,25 2,2 redshank Muddy 10,2 3 2,2 spotted redshank Muddy 10,8 3 1,6 avocet Muddy 12,8 3 1,2 Based on these results we can conclude that sanderling and Kentish plover strongly depend on sandy sediments. Moreover, expert judgement shows that they are strongly bound to this sediment type (Table 2). A similar strong preference, but for muddy substrate, was suggested for spotted redshank and avocet. According to the experts, all other wader species exhibit a less strong habitat preference, with turnstone and curlew exhibiting the lowest habitat preference. Oystercatcher is third in row, and this matches well the general findings in literature that this species is found in highest densities on rather sandy soils. Ringed plover, turnstone, knot occur on intermediate substrates with a slight preference for the sandy ones. Grey plover, bar-tailed godwit, curlew and dunlin are also mainly found on intermediate substrates, but show a preference for the more muddy substrates. Redshank, spotted redshank and avocet end up at the other extreme of the ranking. In general, these results are quite consistent with what could be deducted from literature. The result for Kentish plover however is in contrast to what was found in the very few literature reporting on habitat preference by this species (preference to muddy substrates). Again it appears that knowledge on habitat preference by Kentish plover is largely unavailable. 40 4.3 Emersion time There is very poor agreement among experts in the results of the inquiry question dealing with emersion time preference (Table 3). Possibly, the disagreement reflects large variability of foraging behaviour of shorebirds, both in time and in space. Therefore, in situ results obtained by van Kleunen (1999) and Ens et al. (2005) are probably of much more value. Table 3. Results of the inquiry for emersion time preference, including a ranking of the birds according to their habitat preference (1 = occurring on lowest parts of the intertidal … 13 = occurring on the highest parts), and a classification of the birds according to their tidal behaviour (follower/non-follower). EXPERT 1 EXPERT 2 EXPERT 3 EXPERT 4 EXPERT 5 oystercatcher 2 FO 11 NF 7 NF 4 NF 10 NF avocet 12 NF 1 FO 6 FO 10 NF 1 FO ringed plover 5 NF 7 NF 11 NF 12 NF 8 ? Kentish plover 13 NF 8 NF 13 NF ? ? 11 ? grey plover 10 FO 6 NF 12 NF 6 NF 6 NF knot 4 FO 9 NF 3 NF 8 NF 3 FO sanderling 8 FO 12 NF 10 FO 1 FO 12 FO dunlin 1 FO 10 NF 2 NF 10 FO 2 FO bar-tailed godwit 11 FO 5 FO 1 FO 6 NF 7 NF curlew 3 FO 4 FO 8 NF 4 NF 9 ? spotted redshank 9 FO 3 FO 4 FO 2 ? 4 ? redshank 7 FO 2 FO 5 NF 12 FO 5 NF turnstone 6 NF 13 NF 9 FO 6 FO 13 ? Table 4 shows the average ranking according to the preference of the waders going from low to high intertidal flats. Since there is poor consistence between the results obtained, one may doubt the value of this ranking. In the column ‘agreement among experts’ the difference between minimum and maximum ranking values given by the experts is shown (as presented in Table 4). In the case of tidal behaviour (follower/non-follower), the result is shown in bold when there is good agreement among experts. Table 4. Mean ranking according preference from lowest to highest intertidal areas, based on expert judgement, and classification of tidal behaviour. Agreement Mean Species Tidal behaviour among ranking experts spotted redshank follower 4,4 7 dunlin ? 5,0 9 knot ? 5,4 6 curlew ? 5,6 6 avocet ? 6,0 11 bar-tailed godwit ? 6,0 10 redshank ? 6,2 10 oystercatcher non-follower 6,8 9 grey plover non-follower 8,0 6 ringed plover non-follower 8,6 7 sanderling follower 8,6 11 turnstone ? 9,4 7 Kentish plover non-follower 11,3 5 Spotted redshank and sanderling are regarded as typical followers, whereas oystercatcher, grey, ringed and Kentish plover should behave like non-followers. In literature too, plovers are generally regarded as non-followers. A contradictory result was obtained for sanderling. While this species is 41 regarded as a follower by four out of five experts, it ends third last in the ranking, suggesting sanderling mainly occurs on the higher intertidal areas. Personal comment obtained by two of the experts did not result in a satisfactory explanation for this contradiction. 4.4 Food choice When looking at Table 5, one sees that avocet, bar-tailed godwit, and especially knot and oystercatcher are highly specialised feeders, depending on a limited range of prey species. It seems obvious that these specialised species will suffer more of changes in their environment than opportunistic feeders like turnstone or sanderling would do. These results are consistent with what was found in literature. Table 5. Results of the inquiry: food specialisation and diet of shorebird species (yellow = highly specialised; green = moderately specialised; orange = non-specialised, opportunistic). The order of the bird species is according to their food specialisation; in the second column, the order of the prey species is according to their importance. Species Prey items turnstone diverse, bivalves, crustaceans, polychaetes sanderling crustaceans (!), worms, polychaetes, insects Kentish plover worms, insects, crustaceans dunlin worms (!), polychaetes, crustaceans curlew Carcinus maenas, worms, Nereis, Arenicola, Mya arenaria, insects, crustaceans spotted redshank crustaceans, polychaetes, fish redshank crustaceans, Corophium, Nereis, Hydrobia, polychaetes ringed plover worms, polychaetes, insects, crustaceans grey plover worms, Arenicola, polychaetes, insects, crustaceans avocet (small) Nereis, Hydrobia, worms, insects, crustaceans bar-tailed godwit worms (!), Arenicola, polychaetes knot cockle, Baltic tellin, other (small) bivalves oystercatcher cockle (!!), mussel, Baltic tellin 42 5 RECOMMENDATIONS This literature study shows that there is quite good agreement concerning sediment preference by waders. Moreover, the results obtained by the inquiry are consistent to what is found in literature. The opposite is true for tidal foraging behaviour and emersion period preference. Literature as well as the results of the inquiry show very poor agreement, which makes it difficult to make definite statements. As far as we can judge, tidal behaviour and emersion period preference seems to vary seasonally (in accordance to variation in food availability) and varies among estuaries. However, good knowledge on this subject is essential to correctly predict shorebird distribution. Lourenço et al. (2005) showed the importance of the small-scale effect of drainage channels on shorebird densities. However, little was found on the importance of other small-scale features that characterise intertidal areas, such as mussel beds and surface pooling. Mussel beds, however, are encountered at only two locations in the Westerschelde. 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Oystercatcher: Avocet: Grey plover: 55 Knot: Sanderling: Dunlin: 56 Bar-tailed godwit: Curlew: Redshank: 57 Turnstone: 58 APPENDIX II: Shorebird rankings according habitat preferences Table 6. Bird species ranked according to their preference from sandy to muddy substrates in four different estuaries. Waddenzee Guinea-Bissua Tagus estuary Westerschelde Moreira (1999) (Van Kleunen, 1999, (Zwarts, unpublished (Zwarts et al., 1988a) data, in: Blomert, in: Blomert, 2002) 2002) oystercatcher 3 3 2 avocet 9 11 10 ringed plover 4 10 2 Kentish plover 7 4 grey plover 5 8 8 8 knot 2 7 4 7 sanderling 1 1 1 dunlin 7 6 5 bar-tailed godwit 6 2 3 6 curlew 4 5 6 3 spotted redshank 10 redshank 8 9 5 9 turnstone 10 1 9 Table 7. Bird species ranked according to their preference from low to high intertidal areas in two different estuaries. Westerschelde Waddenzee (Van Kleunen, 1999, (Zwarts, unpublished in: Blomert, 2002) data, in: Blomert, 2002) oystercatcher avocet ringed plover Kentish plover grey plover knot sanderling dunlin bar-tailed godwit curlew spotted redshank redshank turnstone 5 9 3 6 11 6 8 7 10 4 3 9 8 1 2 7 2 4 10 5 1 59 60 APPENDIX III: Question Form When answering this survey, please keep in mind that only intertidal flats are dealt with. Please answer the questions according to your own experience and to the situation in your own study area, irrespective of the specific (and possibly different) situation in the Westerschelde. However, even if you are not familiar with some of the mentioned shorebird species, please do include them in the rankings as this is important for further analysis. 1) Sediment Sediment influences shorebird distribution in several manners. Most importantly, sediment type has a direct impact on the macrobenthos community, both on diversity and densities of benthos, as on the availability of this food source for shorebirds. • • • Classify the birds according to their sediment preference: do this by assigning number 1 to the species with the strongest preference for sandy sediments, and number 13 to the species with the strongest preference for muddy sediments. Describe the preferred sediment for each species, using only the following three categories: sandy, intermediate, muddy. Range: this column is added to get an idea of the dependency of the shorebirds on the preferred substrate: 1 = the birds forage nearly exclusively on their favoured sediment type 2 = the birds forage mostly on their favoured sediment type 3 = the birds forage on a broad range of sediment types, with a slight preference for the favoured sediment type Table 1. Sediment preference of 13 species of waders . Sort by sediment Describe sediment preference type oystercatcher avocet ringed plover Kentish plover grey plover knot sanderling dunlin bar-tailed godwit curlew spotted redshank redshank turnstone Range 61 2) Emersion time Emersion time has an important influence on the distribution of birds on mudflats. First of all, the emersion time stipulates the availability of the flat to waders and consequently, determines the total carrying capacity of a certain area. Secondly, emersion time has an influence on the prey density as well as on the availability of this prey (Yates et al., 1993; Blomert, 2002; Granadeiro et al., 2003; Graveland, 2005). • • Please sort the following species according to their preferred part of the intertidal. The number 1 is given to the species that favours most the lowest part (short emersion time), and number 13 is given to the species that favours most the highest part of the intertidal (long emersion time). Counts performed during low tide underestimate the use of the high-tidal areas. This is because some species are strongly associated with the waterline so it seems that during low tide counts they prefer the low intertidal zone while in fact they use the whole intertidal equally. Thompson et al., (1986); Nehls & Tiedemann, (1993); van de Kam et al., (1999); Danufsky & Colwell, (2003) and Granadeiro et al. (2006) separate shorebirds in so-called “followers” and “non-followers” to distinguish between waders that follow the tide line and those that do not. Can you tell us, according to your experience, if these species are: FO = NF = ?= Followers Non-followers No idea Table 2. Emersion time preference of 13 species of wader. Sort according to Follower / exposure time Non-follower oystercatcher avocet ringed plover Kentish plover grey plover knot sanderling dunlin bar-tailed godwit curlew spotted redshank redshank turnstone 62 3) Prey-choice and specificity Highly specialised species are likely to be less flexible in their foraging behaviour. • • • We first would like you to assign a code indicating the flexibility in prey choice: 0 = great flexibility, can easily switch to other food sources, 1 = quite flexible, preys on a rather broad range of prey, 2 = very strong preference for certain prey; (almost) no alternative prey. In the second column please enter the most targeted prey (species or higher taxon) for each bird species. In the third column you may indicate how abiotic factors determine the presence of their favoured prey: sediment preference (sandy, intermediate, muddy), tidal zone preference (high, mid, low tidal) Tabel 3. Prey specifity and prey choice of 13 species of wader. Prey specifity Favoured prey Habitat preference of favoured prey oystercatcher avocet ringed plover Kentish plover grey plover knot sanderling dunlin bar-tailed godwit curlew spotted redshank redshank turnstone 4) Other abiotic factors Do you believe there are other important (abiotic) factors (apart from salinity, emersion time and sediment) affecting shorebird distribution in estuaries? 5) Remarks and recommendations Do you have any remarks or recommendations that could be important for the continuation of this survey? 63