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Which features of UK farmland are important in retaining territories
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of the rapidly declining Turtle Dove Streptopelia turtur?
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JENNY C DUNN1*
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ANTONY J MORRIS1
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Bedfordshire, SG19 2DL. UK.
Centre for Conservation Science, RSPB, The Lodge, Potton Road, Sandy,
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* Author and address for correspondence:
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Short title: Turtle Dove breeding habitat selection
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Keywords: nesting habitat, foraging habitat, competition, habitat selection, migratory
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bird
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Capsule: Turtle Doves continue to show a strong population decline; territories were
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more likely to be retained in areas with more nesting habitat, and more suitable
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foraging habitat.
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Aim: To determine which features of farmland in England are important for retaining
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Turtle Dove territories
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Methods: Fifty-eight grid squares with recent records of territorial Turtle Doves were
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re-surveyed, and squares retaining Turtle Dove territories compared with those from
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which Turtle Doves had been lost.
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Results: Turtle Dove territories were detected in 48% of squares resurveyed. When
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correcting for the 70% detection rate of the survey methodology, territories were
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present in 66% of squares surveyed suggesting a 34% decline over a two-year period.
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Established scrub and hedgerows >4 m tall positively influenced Turtle Dove
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presence and abundance, as did standing water. Bare ground and fallow had positive
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effects on turtle dove abundance whereas grazed land negatively impacted abundance.
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Conclusion: The positive effects of area of established scrub and volume of large
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hedgerow are likely to represent a declining density of birds selecting the best quality
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nest sites. We suggest instead that foraging habitat may be limiting distribution.
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Introduction
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Numbers of both farmland and migratory birds have been declining since the 1970s
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(Eaton, et al. 2010), and understanding the cause of these declines is crucial in
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determining strategies to aid population recovery. The European Turtle Dove
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Streptopelia turtur is the UK’s only migratory dove, over-wintering in sub-Saharan
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Africa and relying on a source of seed food on European farmland during the summer
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months in order to raise its young (Murton, et al. 1964; Browne & Aebischer 2003).
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The UK population declined by 91% between 1970 and 2009, and the decline is
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ongoing (Eaton, et al. 2011), a trend that is paralleled by a 69% decline across Europe
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since 1980, leading to this species being one of the most strongly declining birds in
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Europe (PECBMS 2010).
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Population declines can usually be attributed to a combination of reductions in habitat
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or food availability, which have implications for survival and breeding productivity.
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In the case of the Turtle Dove, an analysis of CBC data between 1965 and 1995
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concluded that nesting habitat availability on farmland was restricting Turtle Dove
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distribution, as changes in density were positively related to the amount of hedgerow,
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scrub and woodland edge on farmland plots (Browne, et al. 2004). Territories also
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contained a higher proportion of woodland, grassland and other non-cropped habitats
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that would be expected from availability, and a lower proportion of cropped habitats
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(Browne & Aebischer 2004), suggesting that the availability of suitable foraging
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habitat was also important, although to a lesser extent. Productivity per nesting
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attempt was unchanged between 1941 and 2000, suggesting that nest predation was
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not important (Browne, et al. 2005), although birds ceased breeding earlier and
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showed markedly reduced August re-nesting when compared to the 1960s, resulting
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in half the overall number of fledged young (Browne & Aebischer 2004).
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Over-winter survival of Turtle Doves is strongly related to cereal production in
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Africa, suggesting that conditions on wintering grounds are also important for this
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species (Eraud, et al. 2009) and it has been suggested that a combination of breeding
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and over-wintering factors are responsible for population trends (Thaxter, et al. 2010);
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however, the reduction in the number of fledged young is by itself sufficient to
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explain the population decline (Browne & Aebischer 2004). Thus, understanding the
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drivers of the decline in reproductive output is essential if we are to understand and
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reverse the mechanisms responsible for the negative population trend.
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This study aimed to identify sites retaining territorial Turtle Doves and to compare
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these to sites from which Turtle Dove territories have been lost within the past two
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years. Whilst individual Turtle Doves tend not to be site faithful (Browne &
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Aebischer 2001), the same sites are used by birds year after year (BBS data). We did
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not aim to determine changes in occupancy as a response to changing habitat; indeed,
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we ensured that habitat change at our survey sites was minimal through examination
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of maps and talking to landowners. Instead, we aimed to detect more subtle
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differences between preferred (still occupied) sites and less preferred sites where
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territorial Turtle Doves can no longer be detected. This comparison allows us to test
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four hypotheses:
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1) An overall decline in numbers means birds arriving in the south east of England
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(BTO/RSPB/BWI/SOC 2011) are not travelling as far to find suitable breeding
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territories and thus fewer birds are reaching the northern and western edge of their UK
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range.
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2) Territory loss is associated with a reduced availability of nesting habitat. As the
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population is declining, then birds may just be at lower overall densities and thus
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selecting areas with better quality nesting habitat.
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3) Territory loss is associated with a reduction in quality or quantity of suitable
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foraging habitat. Again, as the population is declining, then birds may be selecting
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areas with better quality foraging habitat.
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4) Territory loss is associated with higher abundance of resident columbid species,
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suggesting either direct competition within suitable habitat for nesting or foraging
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sites, or indirect competition through increased disease or increased nest predation
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risk in areas of high nest density.
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Methods
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Site selection
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Records of Turtle Dove presence in East Anglia (Bedfordshire, Buckinghamshire,
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Cambridgeshire, Essex, Hertfordshire, Norfolk and Suffolk) during 2008 and 2009
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were obtained from sources detailed in the legend to Figure 1. Any records that were
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recorded at a coarser scale than 1km2 (i.e. tetrads) were excluded unless that record
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could be isolated to a 1km grid square resolution or higher.
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All observations of transitory (flying) or foraging birds were removed. Records from
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earlier than 1st June and later than 31st July were also excluded unless breeding
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activity (calling and/or displaying males, nests or feeding young) was specified, as
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during these periods it is likely that Turtle Doves will be moving to or from breeding
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sites (Browne & Aebischer 2004). From these 460 records of breeding Turtle Doves
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in East Anglia, those with <50% agricultural land (as determined from 1:25,000
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Ordnance Survey maps) were excluded to leave 379 records from which survey sites
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could be selected (Fig 1a).
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The remaining squares were prioritised and survey sites were selected according to
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Turtle Dove density within each grid square, proximity to other records (i.e. focussing
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on high density areas at the greater than 1km scale) and then according to logistics
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such as access to landowner details and location to reduce travel distances.
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58 squares (Fig 1b) were surveyed on two occasions by one of three surveyors: the
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first survey took place between 11th May and 19th June 2010, and the second between
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21st June and 3rd August 2010; there were at least 4 weeks between the first and
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second survey at each site (range 28 – 74 days; mean ± SE 46.3 ± 1.7 days). No
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surveyor bias was evident, with Turtle Doves detected in between 44 and 58% of
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squares covered by each surveyor.
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Additional single bird and habitat surveys were carried out at three sites where Turtle
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Doves had been reported breeding during 2010 in order to supplement the sample size
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of squares with Turtle Doves present for habitat data; breeding activity was confirmed
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at all three sites.
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Survey methodology
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Surveys aimed to detect the presence and abundance of singing male Turtle Doves
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within each survey area. Each survey began at sunrise and lasted between 1 and 2
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hours (mean ± SE: 91.7 ± 6.8 minutes): 70% of singing Turtle Doves should be
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detected within the first two hours after sunrise, after which vocal activity decreases
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markedly, reducing detection rates (Calladine, et al. 1999). The time taken for the
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survey depended on the structure of habitat (density of field boundaries). Surveys
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were loosely based on Breeding Bird Survey (BBS) methodology (Marchant, et al.
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1990): routes followed field boundaries and were designed so that the surveyor
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crossed each 1km square at least twice (with transects a maximum of 500m apart
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depending in habitat, and approximately parallel) in order to maximise the surveyor’s
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chance of hearing singing Turtle Doves should they be present. Where grid squares
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were consecutive and habitat allowed adequate coverage during the two-hour survey,
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up to two grid squares were covered on the same morning. Surveyors also assessed
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the abundance of Woodpigeons Columba palumbus (WP) within each survey area on
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a categorical scale of 1 – 5 (where 1: 0 – 10 WP; 2: 10 – 25 WP; 3: 25 – 50 WP; 4: 50
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– 100 WP; and 5: 100+ WP per 1 sq km) to determine whether competition for nest
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sites or food may influence Turtle Dove distribution. We aimed to include only
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breeding Woodpigeons in this number, including calling birds, and birds flushed off
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nests and out of hedgerows, and excluding flocks foraging in fields. Ideally, we
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would have investigated associations with the abundance of Collared Doves
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Streptopelia decaocto, as their numbers have increased through natural range
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expansion as Turtle Dove numbers have declined in England and elsewhere in Europe
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(Rocha & Hidalgo De Trucios 2000); however Collared Dove numbers at our sites
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were very low and thus, as the most abundant columbid, Woodpigeon numbers were
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used as a surrogate for overall columbid abundance as they also show overlap in terms
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of both nesting and foraging ecology with the Turtle Dove, although they are more
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generalist (Murton, et al. 1964).
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Habitat data collection
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Following the first Turtle Dove survey at each site, habitat data were collected from
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the area surveyed (the entire grid square). Data collected were the crop type in each
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field, along with details of any non-cropped habitat such as game cover, pollen and
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nectar or wildflower margins, tracks and paths, field corners, bare ground, trees
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>10m, scrub, wood, hedgerows, and standing water. The height and width (± 1m) of
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each length of hedgerow was noted to allow subsequent calculation of hedge volume
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(height x width x length) in two height categories (<4m and >4 m; Browne &
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Aebischer 2004). Areas of each crop type or non-cropped habitat were calculated
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subsequently from 1:10,000 GB Ordnance Survey maps using MapInfo software.
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Statistical analysis
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Two models were run to determine factors influencing a) the presence and absence of
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territorial Turtle Doves, and b) the abundance of territorial Turtle Doves. Firstly, we
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modelled the likelihood of detecting one or more territorial Turtle Doves on a survey:
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the presence and absence of territorial Turtle Doves within each grid square (over
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both surveys) was used as the response variable in a general linear model with
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binomial error structure. In the second model, the maximum abundance of territorial
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males during either survey was used as the response variable in a generalised linear
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model with poisson error structure.
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Variables tested against a minimal model were terms related to geographical location,
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nesting habitat, potential foraging habitat, and Woodpigeon abundance to examine the
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potential for competition (direct or indirect) for nest or foraging sites. A full list can
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be found in Appendix 1. All terms pertaining to an area or a volume were log
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transformed prior to inclusion in any model.
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Variables were screened using the ‘dredge’ function in the ‘MuMIn’ (Burtoń 2012)
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package in R (R Development Core Team 2012). We fitted models to all possible
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combinations of explanatory variables (detailed in Appendix 1), then ranked models
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using second-order Akaike’s Information Criteria (AICc) (Burnham & Anderson
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2002). AICc measures the relative goodness of fit of a model whilst taking into
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account the number of variables within each model, and penalising models for the
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addition of variables. Thus, AICc selects a model with the maximum goodness of fit
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whilst retaining the minimum number of explanatory variables (Burnham & Anderson
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2002).
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No single model fit the data better than others, so we averaged the top models using a
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cut-off of ΔAIC < 2 to provide parameter estimates adjusted for shrinkage according
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to the number of top models within which each term was found (Burnham &
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Anderson 2002).
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Results
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Fifty-eight grid squares with records of territorial Turtle Doves were resurveyed
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during 2010. Of these, 36 records were from 2008 and 22 from 2009. Turtle Doves
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were recorded in 28 squares (48 %) during 2010 (Fig 2); of these, 15 (42 %) were
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from 2008 records and 13 (59 %) from 2009 records. Controlling for the 70 %
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detection rate of the survey methodology (Calladine, et al. 1999) and correcting for
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the number of squares within which birds were detected during both surveys, 66 % of
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squares re-surveyed retained Turtle Dove territories, suggesting a loss of territories
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from 34 % of squares.
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Habitat influences on Turtle Dove presence or absence
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Three terms were retained within all eleven top models, and all three had confidence
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intervals for parameter estimates that didn’t span zero, indicating a high confidence in
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the effects of these terms. Turtle Doves were more likely to be retained in sites with
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larger areas of established scrub and greater volumes of hedges, and less likely to be
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retained in sites with larger areas of grazed land (Tables 1 & 2; Figures 3a & b). The
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area of fallow and area of standing water were both retained in the top-ranked model,
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and in six and four of the eleven top models respectively (Table 1), and parameters
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estimates suggest a small positive influence of these variables on the presence of
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Turtle Doves, although confidence intervals overlapped zero slightly for both terms
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(Table 2). Woodpigeon abundance, and the area of young scrub were retained in five
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and four of the top models respectively (Table 1), and both showed negative
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associations with Turtle Dove presence; however, confidence in the importance of
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these terms is lower given that confidence intervals for both terms spanned zero
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(Table 2).
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Habitat influences on the abundance of territorial Turtle Doves
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Three terms were retained in all twelve top models investigating associations with
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Turtle Dove abundance: the area of established scrub, the volume of large hedges, and
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the area of standing water (Table 3). All three variables had positive impacts on
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Turtle Dove abundance and none of their confidence intervals spanned zero,
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indicating a high degree of confidence in the importance of these terms (Table 4).
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Several other terms were found within the top set of models (Table 3); however, these
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terms each appeared in between one and three models and parameter estimates
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indicated low effect sizes (ranging from -0.065 for year of previous record, to 0.015
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for longitude) with low confidence in these effect sizes as all confidence intervals
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spanned zero (Table 4). In addition, as we screened a relatively large number of
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explanatory variables, there is also a possibility of Type 1 error. Thus, whilst the
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importance of these terms cannot be ignored, there is only weak support for their
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influence on Turtle Dove abundance.
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Discussion
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Grid squares with recent records of territorial Turtle Doves were resurveyed, and
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available habitat in squares retaining doves was compared to those from which
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territories had been lost. Our results indicate a 34% decline in occupancy over a
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maximum of two years, suggesting a continuing sharp decline in the UK breeding
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Turtle Dove population. Our sites were selected from previous records based on the
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abundance of territorial males, so should perhaps have had a higher likelihood of
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retaining territories than squares with records of only one territory; however, our sites
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also encompassed areas on the edge of the geographical range detected by our record
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search, which may have been more likely to lose territories as the Turtle Dove
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population declines and its range contracts (Fuller, et al. 1995), although we only
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found weak support for this within our data. Whilst our results may not necessarily be
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representative of the scale of the population decline, due both to the scale of our
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study, and as we only examined losses over a two-year period, evidently site
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occupancy by this species is still decreasing rapidly, supported by BBS data
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indicating a 21% population decline between 2009 and 2010 (Eaton, et al. 2011). At
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the current rate of decline, Turtle Doves may be lost as a UK breeding bird by 2021
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and thus it is crucial that further work attempts to identify the mechanisms
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underpinning this trend.
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The habitat composition of grid squares retaining Turtle Dove territories was
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compared to those from which territories have recently been lost and these data were
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used to test four hypotheses. We found strong support for two of these hypotheses,
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but only weak support for the other two. First, longitude was retained in only one of
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the top models investigating Turtle Dove abundance, and none of the top models
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investigating presence. It is likely our data are not sufficient to detect any effect
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however, as a range contraction has been occurring in this species for a number of
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years (e.g. Browne, et al. 2005). No evidence was found to support any effect of
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latitude, although Turtle Doves are still found further north than our study sites and
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thus any southwards range shift is unlikely to have been detected.
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Second, Turtle Dove territories were both more likely to be retained, and more
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abundant, in squares with a greater area of established scrub, and a greater volume of
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hedgerow >4m high. This concurs with results of previous work that suggested Turtle
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Doves were limited by the availability of suitable nesting habitat (Browne, et al.
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2004). Browne, et al. (2004) found Turtle Doves to be associated with hedgerows
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and woodland edge in farmland, and found territory densities to be much higher in
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woodland plots than on farmland plots. However, Browne, et al. incorporated
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woodland and scrub into one category, whereas our results suggest that it is likely to
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be the scrub component of this that is most important to Turtle Doves as we found a
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very small negative effect of the area of woodland on the abundance of territories.
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One interpretation of these results is that Turtle Doves are limited by the availability
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of suitable nesting habitat. However, this seems unlikely as rates of scrub removal, or
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deterioration in quality, are unlikely to match the rate of Turtle Dove decline in recent
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years and territories are being lost from habitat where there is no evidence of
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destruction or deterioration in quality of scrub in recent years, as determined through
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examination of maps and talking to landowners. An alternative interpretation,
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supported by the continuing decline and range contraction shown by the species, is
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that a lower density of birds results in the fewer birds selecting areas containing the
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best quality nesting habitat as territories.
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Third, Woodpigeon abundance had a negative impact on the presence of Turtle Dove
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territories, and a much smaller negative impact on territory abundance. These effect
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sizes are not large, especially when taking into account the categorical scale of
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measurement and thus it seems unlikely that this is a result of direct competition for
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nest sites or food resources, although this cannot be entirely discounted as
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Woodpigeons begin nesting before the arrival of migrant Turtle Doves. Although
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Woodpigeons are more cosmopolitan in their nesting habitats, many do nest in areas
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of large hedgerows and scrub, and it is possible that density-dependent effects such as
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increased nest predation or disease risk (Chalfoun & Martin 2009, Hochachka &
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Dhondt 2000) may deter Turtle Doves from nesting in areas containing high nesting
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densities of the resident species. Historically, the diet of Turtle Doves and
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Woodpigeons had minimal overlap in early summer as Woodpigeons took mostly
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unripe cereal grain and Turtle Doves weed seeds (Murton, et al. 1964). However,
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more recently Browne, et al. (2003) found that 61% of the Turtle Dove diet is now
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cereal seeds and thus it remains possible that these two species do now compete for
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food resources. An alternative, and possibly more likely, explanation is that
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Woodpigeon abundance correlates with an unmeasured habitat variable avoided by
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Turtle Doves. Whilst the following suggestions are purely speculative, it is possible
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that microhabitats at either nesting or foraging sites may mean that the negative
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relationship between Turtle Dove presence and Woodpigeon abundance is habitat-
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driven. For example, the greater weight and size of Woodpigeons may mean that they
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nest in slightly more open and structurally supportive scrub than Turtle Doves, whose
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smaller size allows them to exploit denser scrub with less substantial supporting
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foliage. Additionally, Woodpigeon often forage at sites with enclosed canopies such
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as mature oil seed rape crops, whereas Turtle Doves favour open foraging sites
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(Browne & Aebischer 2003) and thus may be less able to exploit landscapes with a
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high area of enclosed canopy, e.g. autumn-sown crops. This association requires
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further investigation before any firm conclusions can be drawn.
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Finally, we found some indications that foraging habitat might be limiting Turtle
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Dove distribution: the area of grazed land had a negative association with territory
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presence, and an increased area of fallow had a positive association, albeit small, with
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territory presence. Additionally, the areas of bare ground and fallow had a weak
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positive association with territory abundance, which also had a negative association
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with the area of spring cereals. Bare ground and fallow areas can both potentially
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provide suitable foraging habitat for Turtle Doves, although there will be considerable
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variation within these habitats in terms of the suitability of plant species, production
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of seeds and the accessibility of seeds to foraging birds. The negative effect of grazed
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land on territory presence suggests that this land type may now be too intensive to
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provide the weed seeds historically found in Turtle Dove diet (Murton, et al. 1964):
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vegetation may be too intensively grazed to set seed, herbicide and fertiliser use may
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reduce the weed species present, and many grazed fields are initially established as
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monocultures and now lack the diversity historically important in Turtle Dove diet
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(Murton, et al. 1964). Additionally, Turtle Doves will fly up to 10km to find food
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(Browne & Aebischer 2003) and thus it is likely that if foraging habitat is limiting the
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species, we would not detect large effects at the 1km square scale. Of note is the
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importance of standing water in both our models. As well as farmland, Turtle Doves
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also nest in reedbed and marsh habitats (Robinson, 2005); whilst this association may
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be driven by an association between scrub and standing water, the r2 for this
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correlation within our dataset was only 0.32, suggesting an independent, positive,
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influence of standing water per se on Turtle Dove presence and abundance. On the
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breeding grounds, Turtle Doves are known to regularly visit standing water with
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sparse surrounding vegetation to drink (Naumann, 1833).
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Whilst birds are likely to prioritise territory locations according to suitable nest sites,
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this does not necessarily mean that food availability is not problematic. That Turtle
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Doves have halved the number of young they produce since the 1960s (Browne &
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Aebischer 2004), along with their dietary switch from weed seeds to cereal grain
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(Browne & Aebischer 2003), suggests that the species may be energetically
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challenged following migration and during breeding, possibly due to a reduction in
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diet quality and reduction in food availability early in the breeding season. This
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reduction in reproductive output is enough, by itself, to explain their population
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decline (Browne & Aebischer 2004. Currently, agri-environment schemes such as the
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Entry- and Higher-Level Stewardship Schemes in England offer limited provision for
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seed-eating birds during the breeding season, as the majority of passerines and
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columbiformes will consume invertebrates during this time whilst Turtle Dove diet is
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almost entirely granivorous (Browne & Aebisher 2003). Thus, we suggest that the
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provision of seed-rich foraging habitat in close proximity to patches of established
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scrub currently retaining Turtle Dove territories is likely to be beneficial for this
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species, a hypothesis that will be further tested by a more detailed study of foraging
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habitat supplementation and use.
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Acknowledgements
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We would like to thank the many farmers and landowners who allowed access to their
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land for surveys, and to the BTO and many county recorders and birdwatchers who
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provided their records of Turtle Doves. Thanks are due to Jenny Bright and Chris De
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Ruyck for carrying out much of the survey work alongside JCD. This work was
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funded through the Action for Birds in England (AfBiE) partnership by Natural
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England and the RSPB.
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Fuller, R.J., Gregory, R.D., Gibbons, D.W., Marchant, J.H., Wilson, J.D., Baillie,
S.R. & Carter, N. 1995. Population Declines and Range Contractions among
Lowland Farmland Birds in Britain. Conserv. Biol. 9: 1423 - 1441.
Hochachka, W.M. & Dhondt, A.A. 2000. Density-dependent decline of host
abundance resulting from a new infectious disease. PNAS 97: 5303-5306.
Marchant, J.H., Hudson, R., Carter, S.P. & Whittingham, P.A. 1990. Population
Trends in British Breeding Birds, BTO, Tring.
Murton, R.K., Westwood, N.J. & Isaacson, A.J. 1964. The feeding habits of the
Woodpigeon Columba palumbus, Stock Dove C. oenas and Turtle Dove Streptopelia
turtur. Ibis 106: 174-188.
Naumann, J. A. 1833. Naturgeschichte der Vogel Deutschlands. Ed. J. F. Naumann
& E. Fleischer, Leipzig. 6: 391.
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433
434
435
436
437
438
439
440
441
442
443
444
445
446
PECBMS 2010. Trends of common birds in Europe, 2010 update. European Bird
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23/02/2011
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(Streptopelia decaocto). Servicio de publicaciones de la Universidad de Extremadura.
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Hypotheses to explain patterns of population change among breeding bird species in
England. Biol. Conserv. 143: 2006-2019.
447
Table 1. Summary of the top models from a binomial GLMM examining which
448
habitat features influence the retention of Turtle Dove territories.
450
451
452
453
454
Y
2
Y
Y
Y
3
Y
Y
Y
4
Y
Y
Y
5
Y
Y
Y
6
Y
Y
Y
Y
7
Y
Y
Y
Y
8
Y
Y
Y
9
Y
Y
Y
10
Y
Y
Y
11
Y
Y
Y
Weight
Area of standing water
Y
Delta
Area of fallow
Y
AICc
Volume of large hedgerows
Y
df
Area of grazed land
Y
Woodpigeon abundance
Area of established scrub
1
Area of young scrub
Model number
449
6
52.11
0.00
0.14
4
52.71
0.59
0.11
Y
5
52.73
0.62
0.11
Y
6
52.76
0.65
0.10
Y
6
52.88
0.77
0.10
Y
7
53.09
0.98
0.09
5
53.10
0.99
0.09
5
53.53
1.42
0.07
7
53.66
1.55
0.07
Y
5
53.76
1.65
0.06
Y
7
53.78
1.67
0.06
Y
Y
Y
Y
Y
Y
Y
Y
Y
455
Table 2. Weighted averaged parameter estimates (back-transformed where necessary)
456
from the top models ± weighted 90% CI for terms upheld in the top GLMs (Table 1).
457
Terms in bold are those retained in all top models.
458
459
460
Term
Estimate
Lower CI
Upper CI
Intercept
-10.848
-18.449
-3.246
Area of Established scrub
0.557
0.241
0.954
Area of grazed land
-0.165
-0.285
-0.025
Volume of large (>4m) hedgerow
1.558
0.225
4.341
Area of fallow
0.099
-0.006
0.217
Area of standing water
0.065
-0.005
0.140
Woodpigeon abundance
-0.278
-0.575
0.019
Area of young scrub
-0.036
-0.081
0.012
461
Table 3a. Summary of the top models from a poisson GLM examining which habitat features influence the number of Turtle Dove territories.
Y
Y
4
Y
Y
Y
5
Y
Y
Y
6
Y
Y
Y
7
Y
Y
Y
Weight
Y
Delta AICc
3
AICc
Y
0.00
0.17
117.45
1.14
0.10
117.50
1.19
0.09
117.58
1.27
0.09
Y
117.71
1.39
0.08
Y
117.97
1.66
0.07
118.04
1.72
0.07
Y
Y
Y
Y
Area of trees
Y
Area of young scrub
Y
Woodpigeon abundance
2
116.31
Longitude
Y
Area of spring cereals
Area of standing water
Y
Area of fallow
Volume of large hedgerows
Y
Area of bare ground
Area of established scrub
1
Year of previous record
Model number
462
Y
463
464
465
466
467
8
Y
Y
Y
9
Y
Y
Y
10
Y
Y
Y
11
Y
Y
Y
12
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
118.16
1.84
0.07
118.19
1.88
0.07
118.21
1.90
0.07
118.28
1.96
0.06
118.28
1.97
0.06
468
Table 4. Weighted averaged parameter estimates (back-transformed where necessary)
469
from the top models ± weighted SE for terms upheld in the top GLMs (Table 1).
470
Terms in bold are those retained in all top models.
471
472
473
Term
Estimate
Lower CI
Upper CI
Intercept
-5.831
-8.588
-3.074
Area of Established scrub
0.236
0.118
0.434
Volume of large (>4m) hedgerow
0.294
0.053
0.710
Area of standing water
0.103
0.026
0.197
Year of previous record
-0.065
-0.152
0.022
Area of bare ground
0.010
-0.003
0.023
Area of fallow
0.006
-0.002
0.014
Area of spring cereals
-0.008
-0.019
0.004
Longitude
0.015
-0.013
0.044
Woodpigeon abundance
-0.007
-0.023
0.009
Area of young scrub
-0.002
-0.006
0.002
Area of trees
-0.002
-0.005
0.002
474
Figure Legends
475
476
Figure 1. Distribution of a) Turtle Dove abundance in squares with >50% farmland
477
across East Anglia during 2008 and 2009, b) sites re-surveyed during 2010 coded
478
according to the year of original record. Abundance refers to the number of singing
479
males, and numbers in brackets in the legends represent the number of records. Site
480
records were obtained from the BTO Breeding Bird Survey, Bedfordshire Bird Atlas,
481
Cambridgeshire Bird Report, Norfolk County Bird Records, previous RSPB farmland
482
bird surveys, the 2008 Suffolk Wildlife Trust Turtle Dove Survey and personal bird
483
records obtained following e-mails to regional bird forums.
484
485
Figure 2. Results of 2010 Turtle Dove surveys according to presence/absence of
486
territorial Turtle Doves
487
488
Figure 3. Turtle Dove territories were more likely to be retained in areas a) with a
489
greater area of established scrub, and b) with greater volumes of hedgerows greater
490
than 4m in height. A: Turtle Doves absent; P: Turtle Doves present.
491
492
493
Figure 1.
494
495
a)
496
497
498
499
500
501
502
503
504
505
506
507
508
b)
509
510
511
512
513
514
Figure 2
515
a)
516
517
518
519
520
Figure 3
522
a) Established scrub
3
2
1
0
Area of established scrub (ha)
4
521
A
523
524
b) Hedge >4m
P
A
525
526
527
P
0
10000
20000
30000
40000
Volume of large (>4m high) hedges (cubic m)
50000
528
Appendix 1. Terms screened to determine their influence on Turtle Dove presence, or
529
on the abundance of territorial birds.
530
531
Geographical:
532
Longitude
533
Latitude
534
535
Nesting habitat:
536
Area of established scrub
537
Area of young or newly planted scrub
538
Area of trees with no scrub understorey
539
Volume of large hedge >4m high
540
Volume of medium and small hedges <2-4m high
541
542
Foraging habitat:
543
Area of bare earth
544
Area of spring cereals
545
Area of fallow
546
Area of broad-leaved crops
547
Area of grazed land
548
Area of standing water
549
550
Year of previous record (2008 or 2009)