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W. HAGEMEIJER & I. TULP, 2004 - Monitoring meadow birds in the Netherlands: monitoring meets policy. In: Anselin, A. (ed.) Bird Numbers 1995, Proceedings of the International Conference and 13th Meeting of the European Bird Census Council, Pärnu, Estonia. Bird Census News 13 (2000):57-65 MONITORING MEADOW BIRDS IN THE NETHERLANDS: MONITORING MEETS POLICY W. Hagemeijer & I. Tulp ABSTRACT. Several species of meadow birds breed in large numbers in wet grasslands in The Netherlands. Wet grasslands make up 28-30% of the total surface area of The Netherlands and are mainly used to graze cattle and for grass production to serve as winter food. The combination of economic and natural values poses high demands to management practices. In Dutch Nature Conservation Policy, wet grassland is an important habitat to be conserved, with meadow birds as one of its major assets. Since these areas harbour a considerable proportion of the European breeding population of many species of meadow birds, conservation of this habitat is of paramount importance to maintain population sizes. Based on historical meadow bird counts trends of six species of meadowbirds were calculated back to the sixties. To unravel the influence of agricultural management schemes on these trends were more closely evaluated for the period 1975-1992. Actual densities were calculated from recent counts. The results indicated that all species had higher densities in reserves and Environmentally Sensitive Areas as compared to intensively used grassland. Trends for Black-tailed Godwit and Redshank show a sharp decline until 1975 and seem to have stabilised since then. Numbers of Ruff and Snipe are rapidly declining. Oystercatcher, and Lapwing to a lesser extend, show an increase throughout the studied period. We discuss problems with regard to the representativeness of the data on which these calculations are based. Results should therefore be interpreted with care. A new monitoring scheme for meadow birds, aiming to result in a randomised and stratified sample is currently under construction by SOVON SOVON Dutch Centre for Field Ornithology, Rijksstraatweg 178, NL-6573 DG, Beek-Ubbergen, The Netherlands INTRODUCTION In the last centuries natural grasslands in Europe (steppe, marshes), breeding ground to several species of waders, disappeared due to human activity. Following deforestation and agricultural developments these species started to inhabit new areas: grassland meadows kept for the grazing of cattle or as hayfields. In the Netherlands these species have adapted very well to this artificial situation and have been classified as 'meadow birds' ever since. Internationally these species might be better known as (wet) grassland birds. Particularly in the North and West large areas of grassland polders are used to keep dairy cattle. The low level, soil type of the polders and the Dutch climate have hampered a good drainage for a long time. In the second half of this century, management of agricultural grasslands has been intensified through increased fertilization and water level control aiming at an increase in the productivity of the land. Meadow birds have been able to adapt to these developments and increased their populations, until the moment that the negative effects of intensification started to overrule the positive effect of increasing biomass availability. Beintema (1986) hypothesised that for each meadow bird species optimal feeding conditions arise in the development from extensively to more and more intensively managed - 57 - grassland. With increasing fertility of the soil, edible soil biomass for full grown birds (worms) increases, resulting in (potentially) higher breeding densities. On the other hand changes occur in the arthropod fauna, that cause a decline in the mean prey size and therefore chicks will have more difficulty to find enough prey per time unit. The optimum intensity of the agricultural management is different for each species. The smaller species are thought to reach their optimum at lower management intensity than the heavier species. The fact that Oystercatchers Haematopus ostralegus and Curlews Numenius arquata, the heaviest among the meadow birds, have only recently (1950-1970, Hulscher 1972; van den Bergh 1986; Beintema 1995) colonized the meadows as breeding habitat is explained by the idea that sufficiently high biomass levels have only recently evolved. The character that distinguishes Dutch polders from similar habitats in other countries is that, being situated in the delta of some of the major rivers of Europe, the soil remains moist throughout the nesting season. The soft wet soil enables adults and chicks to find food in the soil, grass growth is retarded and meadows are only accessible for cattle and machines late in the breeding season because of the limited mechanical carrying capacity of the wet soil. So the danger of destroying nests or chicks by tramping or mowing stays limited. The Netherlands harbour a substantial proportion of the total breeding population of meadow birds in Europe. Percentages are given in Table 1. For all three species The Netherlands hold the highest population size in Europe (excl. Russia for Lapwing) (Hagemeijer & Blair 1997). Table 1. Percentages of the population sizes of three species of meadow birds in The Netherlands. Values are given as % of total European population and of European population excluding Russia and of the EU. (European values after Hagemeijer & Blair 1997, EU values after Beintema et al. 1995) Species Black-tailed Godwit Limosa limosa Oystercatcher Haematopus ostralegus Lapwing Vanellus vanellus Percentages of population sizes in The Netherlands of European population of EU population Excl Russia (%) Incl Russia (%) (%) 63 36-58 86 38 33-36 56 18 2-5 33 Because drainage-control of the meadows improved and the fertility of the soil increased, multiple grass crops per season became practice and farmers started mowing, on average one month earlier. In addition, cattle density increased and consequently caused a greater risk of trampling. These developments reduced survival chances of nests and chicks considerably. Although meadow birds have partially adapted by starting to breed on average two weeks earlier than in the beginning of the century (Beintema et al. 1985), they were not able to fully compensate for the negative influences. This might be explained by the detrimental effects of cold weather and low food availability early in the season. New laws, enacted to reduce mineral emission, imply injection of manure into the soil. The use of very heavy machines for this purpose further increases the risk of destroying nests and chicks. Besides the changes in grassland management, a lot of grassland area has been turned into arable land. The total amount of grassland surface area has declined remarkably during this century (20-25 % decline). In the Nature Policy Plan of The Netherlands, published in 1990 by the Dutch government, the study project 'Future Perspectives of Meadow birds' was announced. Its goal was to develop a model to estimate the changes in populations of breeding meadow birds in grassland areas in The Netherlands in relation to changes in grassland management. - 58 - The first phase of this project consisted of: -collection of information on trends (1960-'92) in breeding numbers of meadow birds in grassland areas, where possible specified by type of management. Both primary and secondary meadowbirds (Beintema 1995) were subject of the study, with a focus on Black-tailed Godwit Limosa limosa, Lapwing Vanellus vanellus, Redshank Tringa totanus, Oystercatcher, Ruff Philomachus pugnax, Snipe Gallinago gallinago, and Curlew -collection of information on actual densities (breeding pairs/km2; 1988-'92), if possible for different management types and geographical location. In this paper some trends and densities will be presented, as well as some examples of modelling population sizes under different management scenarios. DATA COLLECTION AND ANALYSES The material used to reconstruct population developments consists of data from historical counts. For some areas these counts started in the early sixties. For many species however sufficient amounts of reliable information was only available from 1970 onwards. Population developments are therefore shown starting in 1970. For more recent periods, data were available systematically collected by volunteers within the scope of the SOVON (van Dijk 1996). Furthermore data collected in census work of provincial governments were used. Data had to result from counts using standardized methods and the location and surface area had to be exactly known as well as the years of counting and the bird numbers. Plots had to be counted at least twice. The criteria used to select data for inclusion in the analyses are given in detail in Hagemeijer et al. (1996). Selected data included both very small and very large areas. The census areas were assigned into three different categories: (1) intensively managed agriculture areas with no management restrictions, (2) farmland with voluntary restrictions, the so-called Environmentally Sensitive Areas (ESA's), restriction here relate to late mowing and reduced use of fertilizers and (3) nature reserves. Criteria used for this assignment are given in detail in Hagemeijer et al. (1996). The assignment appeared to be very difficult because management information was available only in a format that was incompatible format to the format of the bird data. This has resulted in a rather large heterogeneity of plots within each category (e.g. an 'intensive farmland' can contain up to 30 % of its surface being ESA or reserve). Table 2 shows the number of plots and area sizes in the dataset per management category, used for calculating the densities. Sample sizes for calculating year-indices are given in Fig. 1. Table 2. Number of census areas and their area per management category as used for calculations of the current densities Category Intensive agriculture ESA Reserve area (ha) grassland (ha) 155 443 6 429 16 374 Number of areas 643 58 94 Total area 1 024 759 24 838 14 012 To calculate the current densities the most recent census per area was used, and densities were calculated as the number of breeding pairs km2. National density per species was weighted for the total area of the three different management categories in The Netherlands. - 59 - Figure 1 : Year-indices for the six meadowbird species in the period 1970-1992. The dots represent an average value for an average census area. Numbers are indexed and presented relative to 1992. 95 % confidence limits are indicated by lines Note the different scales along the Y axis. Total number of areas and number of birds counted are indicated. Likelihood ratio tests are performed to examine how well the model fits the data. X2 values and degrees of freedom for each species are: Oystercatcher: X2 = 3713, df= 2203, p<0.001, Lapwing: X2 =10796, df= 2261, p<0.001, Ruff: X2 = 1131, df= 457, p<0.001, Snipe: X2 = 967, df= 616, p<0.001, Black-tailed Godwit: X2 = 8628, df= 2239, p<0.001, Redshank: X2 = 3283, df= 2196, p<0.001. Note the different scales along the Y axis. Population indices of meadow birds were reconstructed by performing a loglineair poisson regression on the data matrix (Ter Braak et al. 1994; Pannekoek & van Strien 1994). Calculations were performed by the program TRIM 0.95 (Statistics Netherlands). This program can calculates linear trends and year-effects, with the possibility to include covariates in the linear as well as the year-effect model. The last model (year effects with covariates) requires a very complete dataset however and was not applicable on this data matrix. The influence of different management types was evaluated by adding the categories as covariates to the linear model. - 60 - TREND AND YEAR-INDICES Year-indices for the six meadowbird species in the period 1970-1992 are given in Fig. 1. Oystercatcher and Lapwing show a similar development: a constant level until the early eighties followed by an increase of 30-50 %. Breeding numbers of both Ruff and Snipe decreased dramatically especially in the seventies, but after that continued to decrease. The steep decrease in numbers of Black-tailed Godwits in the early seventies stabilised later and even showed a moderate increase. A similar pattern is found for Redshanks, although their numbers seem to remain on a constant level in the eighties, rather than increase. Figure 2 : Trends and year-indices for Black-tailed Godwit for two consecutive periods: 1960-'75 and '75-1992. The index for 1975 was set to 100. The amount of information available for the Black-tailed Godwit allows to give year-toyear indices starting in 1960. Fig. 2 shows the trends and year-to-year indices for this species when the data are split in two periods: 1960-'75 and 1975-'92. The reason for the split at 1975 is that the management under evaluation (ESA, 'relatienotabeleid' in Dutch) was implemented in 1975. The early period is characterized by a marked decline (100 %, index change from 200 to 100). The latter period shows a slight increase (max. 15%, index change 100-115). Note the difference in the width of the confidence interval. This is due to the fact that the latter period contains a much smaller proportion of missing values as compared to the earlier period. Indices differ somewhat between Fig. 1 and Fig. 2. This is the result of the fact that a larger dataset is used for Fig. 2 and to the fact that is Fig. 2 the index for 1975 was set to 100, whereas in Fig. 1 this was done for 1970. The overall trend is similar however. The effects of different management categories could only be analysed using the linear model. Insufficient data were available per category to calculate year effects. Fitting a linear model with management as covariate to the data for the 6 species of Fig. 1 results in trends shown in Fig. 3. For the Godwit the 'national' trend (NL linear) shows a moderate increase (the same as in Fig. 2, after '75). In the intensively farmed areas the trend is nearly horizontal, whereas the categories ESA and Reserve show an increase. Lapwing shows a very similar picture to the Black-tailed Godwit (ESA and Reserve better than intensive). The development in Oystercatcher numbers shows a large increase in reserves and an increase of 20 % in other - 61 - Figure 3 : Linear trends for Black-tailed Godwit, Lapwing, Oystercatcher, Ruff, Snipe and Redshank in the period 1975-'92, discriminated by management category. The output of TRIM (model 4) is indexed. The index for 1975 was set to 100. (The lines merely show the extent of the change over the period, they do not indicate the relative abundance between the categories). Wald tests were performed to indicate the significance of the covariate. Oystercatcher X2=166, df=2, p<0.0001; Lapwing X2=286, df=2, p<0.0001; Ruff X2=12, df=2, p<0.005; Snipe X2=45, df=2, p<0.0001; Black-tailed Godwit X2=54, df=2, p<0,0001; Redshank X2=3.1, df=2, n.s. categories. Redshank shows no significant differences between the categories. Ruff and Snipe show sharp declines, Snipe less steep in reserves. DENSITIES Densities of meadowbirds for The Netherlands are given in Table 2. Lapwings and Blacktailed Godwits are the most common meadowbirds in Dutch grassland areas. Oystercatcher and Redshank show moderate densities, while Ruff and Snipe are very rare breeders. Curlews have only recently started to colonize grassland areas, their common breeding areas are found in the dunes, and occur in low densities. The results for the density calculations carried out per management category are given in Table 3. All species show highest densities in reserves or in ESA's. Results are presented in detail in Hagemeijer et al. (1996). - 62 - Average densities per km2 in the Netherlands, weighted for area of each management category as present in 1992. Species Oystercatcher Lapwing Ruff Snipe Black-tailed Godwit Curlew Redshank area(ha) 176 528 177 046 99 848 103 235 177 714 92 128 176 472 n area (ha) density Average interval 95%-conf. interval n (ha) Surface area Reserve 95%-conf. interval ESA Average density 11.4 23.9 0.1 0.5 17.3 10.5-12.4 22.2-25.5 0.1-0.2 0.3-0.7 15.9-18.7 640 641 389 414 646 154 543 154 996 81 340 84 670 155 663 18.4 36.3 0.1 0.9 33.1 15.3-21.6 30.7-41.9 0.1-0.3 0.4-1.5 26.1-40.2 58 59 54 58 59 6,362 6,429 5,820 6,362 6,429 26.2 12.8-39.5 94 37.4 31.6-43.2 94 0.7 0.3-1.2 74 2.2 1.2-3.3 75 33.0 28.1-37.9 94 15 621 15 621 12 688 12 202 15 622 0.6 6.5 0.4-0.8 5.9-7.2 393 638 78 665 154 571 0.6 12.3 0.2-1.0 9.6-15.0 46 59 5,086 6,429 1.9 9.9 8 376 15 472 0.6-3.2 7.6-12.2 (ha) Species n areas 792 794 517 547 799 499 791 Average densities and 95 % confidence intervals per km2 for the three management categories. Intensive agriculture Oystercatcher Lapwing Ruff Snipe Black-tailed Godwit Curlew Redshank 95%-conf. 10.6-13.0 22.5-26.2 0.1-0.2 0.3-0.7 16.3-19.5 0.4-0.8 6.0-7.5 interval Table 4. Average 11.8 24.3 0.2 0.5 17.9 0.6 6.7 density Table 3. 60 93 DISCUSSION ESA and reserves seem to be effective measures for the conservation of some species of meadowbirds. The highest densities are found in areas with most restrictions on agricultural use. This finding cannot be solely contributed to the difference in management. The selection of areas appointed to become a reserve is by density of meadowbirds. So, even before the adjusted management becomes into practice differences in densities already exist. The fact that the trends in population size are more positive in reserves is a better indication for the effectiveness of the management. Although the question remains whether this is not also a result of better conditions (source-sink differences). - 63 - For Black-tailed Godwit and Lapwing trends are more positive in ESA and reserve areas than in intensively farmed land (Fig. 3). For Snipe there is a smaller decline in reserves than in other management types. ESA seems to be less effective for this species. Ruff shows a decline in all categories and the analysis indicates a significantly better situation in intensively farmed land as compared to ESA and Reserves. Numbers are extremely low however and we consider this result an artefact. For its conservation in The Netherlands, the Ruff is almost totally depending on the population in reserves. The representativeness of the data used for calculating year-indices and trends is a serious problem. The data do not result from a stratified, random sampling effort. They turn out to be heavily skewed towards the better areas for meadow birds, especially so for the category 'intensive'. This is the result of volunteers choosing their own census plot. The consequence is that areas with low numbers of birds are underrepresented. For ESA and Reserve this presumably does not seriously hamper the assessment of national trends but for 'intensively farmed land' results present a too positive picture. The same problem arises in calculating the densities. The intensive agricultural areas are not represented in the dataset. Most areas that were censused represent the better meadowbird areas of The Netherlands. Therefore the presented densities show a too optimistic picture. These figures are therefore not suitable for extrapolation on a national scale. In order to be able to develop policies for the conservation of grasslands in general and meadowbirds in particular, a prediction of future population sizes is calculated using adjusted densities. Professional judgement values were used to substitute the figures for densities in intensively managed agricultural land (without 'meadow bird friendly' measures). The results are given in Table 5. Estimates of population size on basis of the densities for 'intensively farmed land' gave much higher numbers for most species as compared to other sources (Hötker 1991, Hustings 1992). Table 5. Modelling population sizes of Black-tailed Godwit. Using densities from the above study for ESA and reserves and professional judgement values for intensively farmed land, population sizes are calculated for 1995 and for the future, after full realization of goals for surfaces of ESA and reserve. For goals see Den Boer 1995 (adapted for division into high and low). Intens. high = Intensively farmed land in the higher parts of The Netherlands (dry, sandy soils in E of country); Intens. low = same for low parts ('wet', clay and peat, W of country). Prot. nest = nest protection by volunteers against impact of agricultural practices. Intens high Densities (n/100 ha) Model 1995 area (ha, x1000) pop size Model 'goal' area (ha) pop size Intens low Prot. nest ESA Reserve 3 7 18 36 57 535 500 125 12 13 16 100 35 000 22 500 4 300 7 700 228 200 425 80 60 6 840 14 000 76 500 28 800 34 200 - 64 - Total 85 600 161 140 Policies are based on, and evaluated by the use of simple calculations like this. It is of paramount importance therefore to be able to retrieve reliable density and trend information.For the interpretation of these figures, e.g. in policy documents one must keep in mind some potential drawbacks. For instance, densities in reserves are generally high, but new reserves will be obtained in sub-optimal areas since the best are reserve already and will therefore not hold the same densities. In order to be better able to assess representative trends and densities in the future, a national monitoring scheme for meadowbirds is under construction by SOVON. Important parameters to be measured in this scheme are (1) changes in numbers, (2) reproductive success and (3) survival and population structure. In order to be able to answer questions regarding the effectiveness of management and conservation measures, it is crucial to follow a stratified approach, stratifying the sample according to the parameters to be measured and the factors to be analysed. The Dutch government partially finances the scheme under construction. Hopefully, in the near future, we will be able to calculate reliable trends and figures for most meadow bird species. More importantly these figures then ought to be used in studies supporting policy documents in order to develop cost-effective plans for meadow birds conservation. REFERENCES Beintema, A.J., R.J. Beintema-Hietbrink & G.J.D.M. Müskens. 1985. A shift in the timing of breeding in meadow birds. Ardea 73: 83-89. Beintema, A.J. 1986. Man-made polders in the Netherlands: a traditional habitat for shorebirds. Colonial Waterbirds 9: 196-202. Beintema, A., O. Moedt & D. Ellinger 1995. Ecologische Atlas van de Nederlandse Weidevogels. Schuyt&Co, Haarlem. Boer, den, T. 1995. Meadowbirds: facts for conservation. Technisch rapport Vogelbescherming, no. 16. Zeist Dijk van, A.J. 1996. Broedvogels inventariseren in proefvlakken. (Handleiding Broedvogel Monitoring Project). SOVON, Beek-Ubbergen. Hagemeijer, E.J.M., Tulp, I., Groot, H., van der Jeugd, H. & Sierdsema, H. 1996. Weidevogels in graslanden in Nederland: trends en dichtheden. SOVON onderzoeksrapport 96/07, SOVON, Beek-Ubbergen. Hagemeijer, E.J.M. & Blair, M.J. 1997. The EBCC Atlas of European Breeding Birds: their distribution and numbers. Poyser, London. Hötker, H. 1991. Waders breeding on wet grasslands in the countries of the European community- a brief summary of current knowledge on population size and population trends. Wader Study Group Bulletin 61, Suppl.: 50-55. Hötker, H. (ed.) 1991. Waders breeding on wet grasslands. Wader Study Group Bulletin 61, Suppl. Hustings, F. 1992. Aantallen en trends van Nederlandse broedvogels in 1960-91. Documentatie ten behoeve van de herziening Rode Lijst. Intern rapport SOVON, Beek-Ubbergen. Pannekoek, J. & van Strien, A.J. 1994. Developments in wildlife statistics: new methods for meeting new demands. Netherlands Official Statistics 9: 42-45. Ter Braak, C.J.F., van Strien, A.J., Meijer, R. & Verstrael, T.J. 1994. Analysis of monitoring data with many missing values: which method? In: Hagemeijer, E.J.M. & Verstrael, T.J. (eds.) 1994. Bird Numbers 1992. Monitoring, Distribution and Ecological Aspects. Proceeding of the 12th International Conference of IBCC and EOAC, Noordwijkerhout, The Netherlands. Statistics Netherlands, Voorburg-Heerlen; SOVON, Beek-Ubbergen. 663-673. TRIM, Trend analysis and Indices for Monitoring data, CBS. (unpublished manual)5): 197-209. - 65 -