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Distribution shifts in wintering Golden Plover
Pluvialis apricaria and Lapwing Vanellus vanellus in Britain
Simon Gillings , Graham E. Austin , Robert J. Fuller & William J. Sutherland
To cite this article: Simon Gillings , Graham E. Austin , Robert J. Fuller & William J. Sutherland (2006) Distribution shifts in wintering Golden Plover Pluvialis�apricaria and Lapwing Vanellus vanellus in Britain, Bird Study, 53:3, 274-284, DOI: 10.1080/00063650609461443
To link to this article: https://doi.org/10.1080/00063650609461443
Published online: 29 Mar 2010.
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Long-term censuses of British breeding birds have revealed declines of up to 80% in farmland bird popu- lations accompanied by marked range contractions since the 1970s (Chamberlain et al.2000, Chamberlain
& Fuller 2000, Fuller 2000) driven by intensification of arable (Robinson & Sutherland 2002) and pastoral (Vickery et al. 2001) farm landscapes. It would seem likely that immigrant wintering populations would be subject to these same changes but few are systemati- cally monitored and population or range changes may be occurring unnoticed.
Lowland farmland in Britain is one of the principal wintering areas in the western Palearctic of Golden Plover Pluvialis apricariaand Lapwing Vanellus vanellus (Cramp & Simmons 1983, Stroud et al. 2004). For most wader species the British Trust for Ornithology/
Wildfowl and Wetlands Trust/Royal Society for the Protection of Birds/Joint Nature Conservation Committee Wetland Bird Survey (WeBS) provides wintering population estimates from counts of the main
estuaries and inland wetlands (Pollitt et al. 2003).
Plovers are amongst the most widespread of European wintering waders, being dispersed across large areas of superficially uniform farmland (Gillings 2003a), and WeBS is thought to inadequately sample populations.
There are few reliable winter population estimates and most are in some way derived from The Atlas of Wintering Birds in Britain and Ireland (the ‘Winter Atlas’, Lack 1986); these are based on peak counts from three winters with very differing weather patterns and are likely to include serious double-counting (Fuller in Lack 1986).
Given the difficulty of producing a baseline popula- tion estimate, it is no surprise that little consideration is given to winter trends despite the fact that some European breeding populations of Golden Plover are thought to be declining (Tucker & Heath 1994, Hancock & Avery 1998, BirdLife International 2004) and Lapwing breeding populations are severely declining (Tucker & Heath 1994, Wilson et al. 2001, BirdLife International 2004). The aim of this paper is to bring together and review recent data on winter status of Golden Plover and Lapwing in Britain. We
Distribution shifts in wintering Golden Plover
Pluvialis apricaria and Lapwing Vanellus vanellus in Britain
SIMON GILLINGS1*, GRAHAM E. AUSTIN1, ROBERT J. FULLER1 and WILLIAM J. SUTHERLAND2
1British Trust for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU, UK and 2School of Biological Sciences, University of East Anglia, Norwich, Norfolk, NR4 7TJ, UK
CapsuleThe winter distribution of Golden Plover and Lapwing has shifted east since the mid-1980s, perhaps in response to climate change.
AimsTo combine analyses of winter trends on wetlands and flocks on farmland to assess the current status of Golden Plover and Lapwing in one of their main wintering regions.
Methods Winter trends were derived from monthly counts on estuaries and wetlands for the period 1974–2002 (the Wetland Bird Survey). Winter distribution on farmland was assessed using casual records of large flocks and surveys of a stratified random sample of 1-km squares (the Winter Farmland Bird Survey).
ResultsRegional trends showed a pronounced increase in numbers of both species since 1974 on the east coast, with a smaller increase on the south coast. Numbers in the west and north tended to decline.
Flocks on farmland were concentrated in eastern Britain unlike the situation in the 1980s.
ConclusionsThe winter distribution of Golden Plover and Lapwing has shifted to the east, resulting in large numbers on the east coast and in the arable east of Britain. The implications of this shift, especially in relation to known habitat associations, are discussed.
* Correspondence author.
Email: simon.gillings@bto.org
specifically ask if there is any evidence for recent changes in distribution or abundance. Such changes in distribution have recently been recorded in estuarine wader populations in Britain and are thought to be a response to changing climate (Austin & Rehfisch 2005).
We make use of two extensive surveys: WeBS and the Winter Farmland Bird Survey (WFBS). Although less than a quarter of Britain’s Golden Plover and Lapwing were thought to winter on coastal sites in the 1990s (Cayford & Waters 1996), some coastal sites have seen marked changes in numbers (Taylor et al.
1999) and WeBS data present the only opportunity to assess trends and distribution for the coastal com- ponent of these populations. WFBS was an extensive survey of farmland birds that ran over three winters during 1999–2003 and aimed to provide information on abundance, distribution and habitat selection of a suite of common and declining farmland birds. These two surveys together provide the most recent informa- tion on the winter status of Golden Plover and Lapwing in Britain.
METHODS
The Wetland Bird Survey: 1974/75 to 2002/03 On a specified date each month, volunteer surveyors visit their designated wetland site and count all water- birds present. Since the early 1970s virtually all estuaries have been systematically counted at or near high tide. Counts will often include marshland, salt- marsh and fields adjacent to the coast in which waders often roost. Over the same time period waterfowl were counted at inland sites (e.g. lakes, reservoirs) but waders were routinely counted only since 1991.
Therefore, using data from WeBS it is possible to examine trends in numbers of Golden Plover and Lapwing on coastal sites from winter 1974/75 (since when all British estuaries have been surveyed regularly) to 2002/03 (the latest winter for which data were available), and on inland wetlands from winter 1991/92 to 2002/03. We accept that the latter relate to a relatively short period but we include these for completeness to determine whether these inland counts can tell us anything about plover trends.
Winter Farmland Bird Survey: 1999/2000 to 2002/03
Here we use plover data from two complementary components of the WFBS. The Square Survey
involved detailed surveys of randomly selected 1-km squares. Government agricultural statistics and remotely sensed landcover data sets (Bunce et al.1996, Fuller et al.2002) were employed to ensure that 1-km squares selected for coverage were from lowland areas predominantly used for agricultural production.
Squares were regionally stratified using regions derived from government office regions to ensure representa- tive geographic coverage. Volunteer surveyors were asked to make three visits of up to four hours’ duration to their square in each of the three winters (November to February). On each visit they surveyed as many fields as possible, recording the number of plovers and other farmland birds and recording the habitat present in and around the field.
For some scarce or highly aggregated species, random squares were unlikely to yield sufficient observations to identify patterns of abundance or habitat use. As a sup- plement, Casual Records collated sightings of ‘large’
flocks of birds on farmland habitats throughout Britain.
In the case of plovers, we requested details (location, date, time, abundance and habitat) of any flock totalling 100 or more individuals (of either or both species combined).
The surveys were planned for the three winters 1999/2000 to 2001/02. However, the outbreak of foot- and-mouth disease meant that the final winter of the Square Survey had to be postponed to 2002/03 due to access restrictions on private land.
Analysis
Production of trends from WeBS data
We largely follow the standard methodology employed for deriving national indices for intertidal species (Underhill & Pr^ys-Jones 1994). In brief, counts from sites cannot simply be summed because coverage of wetland sites is not always complete. Instead a model- based approach was first used to impute any missing values as a product of a year factor, a site factor and a month factor (Underhill & Pr^ys-Jones 1994, Pollitt et al.2003). Models were based on counts from December to February and, in order to reduce the degree of impu- tation of missing data, only sites with under 50%
missing values were used (Underhill & Pr^ys-Jones 1994). Predicted values from these models were used to fill missing values and counts were summed across sites for each month in each year to derive annual popula- tion estimates for the suite of sites.
A problem with this approach is that numbers of birds are assumed to respond similarly on all sites (e.g.
the year effects would be applied equally to sites in Orkney and Cornwall). This assumption is probably unrealistic, especially since region-specific trends have been detected for other wader species (Austin et al.
2000). The regional trends themselves may be impor- tant in giving insights into the causes of any national changes. Therefore imputation was performed at the region scale first, then all counts (raw and imputed) were summed to generate the national trends. Regions were delimited by agricultural practices since both Lapwing and Golden Plover principally feed on agricultural fields (Cramp & Simmons 1983). The farmland of Britain is highly polarized, with arable pro- duction in the east and livestock (pastoral) production in the west, with a zone of ‘mixed’ farming in central England. Robinson et al. (2001) used recent agricul- tural statistics to assign each British county into one of these three categories. We use these to derive five regions that are broadly uniform in the type of farmland they contain: East England is predominantly arable;
South-Central England is predominantly mixed in character. The remaining area, mostly pastoral, is split
into the West, North England and Scotland (Fig. 1a).
For each of the five regions, two runs of the imputing algorithm were performed, one on coastal counts and one on inland counts. This was necessary because, if run together, the 50% rule for missing values automat- ically excluded all inland sites because they were only surveyed for a maximum of 12 winters (1991/92 to 2002/03) which is less than 50% of the maximum run for coastal sites (29 winters, 1974/75 to 2002/03). For visual representation, trends in each region were smoothed using a General Additive Model (GAM; Hastie & Tibshirani 1990) fitted using the GAMproce- dure in SAS (SAS Institute 2001). Following the recommendations of Fewster et al.(2000), models used a smoothing spline with 0.3T degrees of freedom rounded to the nearest integer, where Tis the number of years for which count data were available.
Distribution mapping
The winter distribution of Golden Plover and Lapwing has not been described since the ‘Winter Atlas’ of the early 1980s (Lack 1986). Here we combine the most
Figure 1.Coverage for the respective surveys. (a) Distribution of WeBS count sites with counties shaded according to the type of agriculture practised (black = arable, grey = pastoral, white = mixed). These are used to define five geographic regions within which agriculture is rela- tively uniform. (b) Distribution of 10-km squares containing 1-km squares surveyed for Square Survey. (c) The 10-km squares from which Casual Records (any farmland species) were received (small dots, 1–5 observers; large dots, >5 observers). In (b) and (c), grey shading indicates upland areas from which squares were not selected.
recent winter distribution information to describe the current range and geographical abundance of plovers.
Given that the distribution of both species can be strongly affected by cold weather (Kirby & Lack 1993), we produce a snapshot mid-winter distribution map based only on December records. We use the most recently available data from WeBS (December 2002 counts) and Casual Records from Decembers 1999–
2002 to derive the best estimate of winter distribution from the available information. We map all records and follow Fuller & Lloyd (1981) in highlighting flocks of over 1000 individuals. Maps from the ‘Winter Atlas’
are reproduced for comparison. Ideally, statistical comparisons would have been made between the
‘Winter Atlas’ maps and those presented here, but the differing resolution of the data sets (10-km square maximum counts versus 1-km squares, flocks and WeBS count sectors) prevented quantitative com- parisons.
RESULTS
Survey coverage
Since the winter of 1974/75, WeBS has collected wader counts from 3048 sites. Of these, 679 and 1494 sites recorded Golden Plover and Lapwing, respectively, at least once during December to February between 1974 and 2003. Not all of these were visited with sufficient frequency (at least 50% of possible occasions:
Underhill & Pr^ys-Jones 1994) to allow data from them to contribute to the generation of trends. This reduced the sample of sites to 322 for Golden Plover and 581 for Lapwing (Fig. 1a).
At least 700 WFBS squares were surveyed in each of the three winters (Table 1) and coverage was attained throughout the British lowlands (Fig. 1b). The propor-
tion of squares visited differed slightly but significantly between regions (Table 1, P< 0.05 in each winter) so weightings had to be used to correct for unrepresenta- tive regional coverage. Survey visits to these squares yielded at least one survey visit to over 18 000 fields across the three winters. A total of 1022 Casual Records forms were received from throughout the British lowlands (Fig. 1c). Since these were unstruc- tured and observers could choose where to visit, there was potential for geographic biases. However, the number of forms received from each 10-km square was relatively uniform across Britain (Fig. 1c).
Trends in winter numbers on wetland sites
In each region, trends were based on counts from at least 16 sites and the percentage of imputed values was generally low, though tending to be higher for inland sites than coastal sites (Table 2).
Between 1974/75 and 2002/03 numbers of both species changed markedly on coastal sites, with varying trends in each region (Fig. 2). In Scotland, North England and the West region, coastal trends were characterized by a decline during the 1970s, followed by a partial recovery, followed by another decline.
South-Central trends were mostly stable with a slight increase during the 1990s and indication of a slight recent decline. The greatest change was evident in the East region where a five-fold to seven-fold increase in coastal numbers was apparent since 1974/75 (Fig. 2).
Unlike all the other regions, there is no evidence of a very recent decline in the East. In each region, coastal trends of the two species were highly correlated (East rs
= 0.97; South-Central rs= 0.82; West rs= 0.85; North
Table 1.Numbers of WFBS squares surveyed in each region (per- centage in parentheses) in each winter (W1 = 1999/2000, W2 = 2000/01, W3 = 2002/03).
Region All (%) W1 W2 W3
East 22 150 (17) 125 (16) 123 (17)
South-Central 30 313 (36) 242 (31) 240 (32)
North 13 88 (10) 97 (13) 109 (15)
West 22 219 (25) 199 (26) 174 (23)
Scotland 13 101 (12) 107 (14) 99 (13)
Total 871 770 745
The ‘All’ column gives the percentage of all lowland farmland squares falling in each region. See Methods section for the defini- tion of ‘lowland farmland’ squares.
Table 2.Number of sites used in the indexing process for each region. Also shown is the mean annual percentage of counts that were imputed.
Golden Plover Lapwing No. Imputed No. Imputed
Region Type sites (%) sites (%)
East Coastal 16 11 16 11
Inland 39 28 57 30
South-Central Coastal 22 17 27 20
Inland 75 24 152 27
North Coastal 20 21 20 20
Inland 32 32 78 35
West Coastal 26 16 38 17
Inland 37 26 83 29
Scotland Coastal 28 22 32 23
Inland 27 25 78 33
Figure 2.Regional trends in numbers of Golden Plovers (GP) and Lapwings (L) on wetland sites. The solid symbols show the trend for tidal sites (1974/75–2002/03) and the open symbols show the trend for non-tidal, primarily inland sites (1991/92–2002/03). Dashed lines are the smoothed trends.
rs = 0.81; Scotland rs = 0.84. All significant at P <
0.001 and n= 29). At the regional level, little could be inferred from the inland wetland trends (Fig. 2) and indices were poorly correlated between species (East rs
= 0.10 ns; South-Central rs= −0.29 ns; West rs= −0.49 ns; North rs= −0.69, P< 0.05; Scotland rs= 0.40 ns.
All n= 12).
At the national level, Golden Plover and Lapwing coastal trends were highly correlated (n= 29, rs= 0.89, P < 0.0001). The population trajectory was almost identical from 1974/75 to 1991/92 (Fig. 3), but there- after indices diverged and were no longer significantly correlated (n = 12, rs= 0.02, P> 0.9), although the annual highs and lows were still synchronized (Fig. 3).
From 1991/92 onwards, when both inland and coastal counts were undertaken, there were weak significant correlations between coastal and inland trends (Golden Plover n= 12, rs= 0.59, P< 0.05; Lapwing n
= 12, rs = 0.64, P < 0.05). Inland trends were not correlated across species (n= 12, rs= –0.16, P> 0.6).
Abundance and distribution
For both species, and in each of the three years, occu- pancy of squares differed significantly between regions (Table 3). A higher percentage of squares was occupied by Golden Plover in the East than elsewhere (Table 3).
Occupancy of squares by Lapwings was highest in the North and East regions throughout, plus in Scotland in winter 1 (Table 3). For Golden Plover and Lapwing, respectively, 42% and 35% of Casual Records and 66%
and 50% of the WeBS December 2002 count came from the East region (Table 4). This region accounted for only 19% of the land area. The South-Central region accounted for 32% of the land area but only 26–28% of Casual Records and 9–16% of the wetland total (Table 4). The remaining regions accounted for small propor- tions of records and wetland totals (Table 4).
Distribution maps based on WeBS and WFBS are shown in Fig. 4 along with those from the ‘Winter Atlas’
from the early 1980s for ease of comparison. Most Golden Plover records, and a large proportion of flocks numbering over 1000 individuals, were in eastern Britain (Fig. 4aii). A similar though less well-defined pattern is evident for Lapwing (Fig. 4bii).
DISCUSSION
Trends in numbers on coastal and inland wetlands Numbers of Golden Plover and Lapwing wintering on coasts have changed in synchrony, with a decrease evident from 1974/75, followed by a short period of stability from 1978/79 to 1986/87, followed by an increase. A similar trend is apparent from capture rates of Golden Plover in The Netherlands (Piersma et al.
2005; Jukema & Hulscher 1997), though they subsequently indicate a decline since 1990 that is not apparent in Britain. At the regional level most striking is the large increase in numbers for both species on the East coast, a fact already noted for other wader species by Austin et al. (2000). Numbers were generally stable on the South-Central region’s coastline, with an increase only evident from the early/mid-1990s.
Elsewhere coastal trends were characterized by a period
Figure 3. Smoothed trends in numbers of Golden Plover (filled symbols) and Lapwing (open symbols) on coastal wetlands (squares) and inland wetlands (circles). For ease of comparison of trends, all are standardized to an index value of 1 in winter 1991/92. Numbers in parentheses in the legend are the estimated number of birds present in the index base year.
Table 3. Percentage occupancy of WFBS squares in different regions in each of the three winters (W1 = 1999/2000, W2 = 2000/01, W3 = 2002/03).
Golden Plover Lapwing
Region W1 W2 W3 W1 W2 W3
East 14 18 24 30 32 46
South-
Central 6 6 8 19 19 30
North 6 8 13 40 35 43
West 2 5 6 13 19 26
Scotland 8 2 6 34 20 21
χ24 22.7***29.3*** 37.7*** 39.5*** 19.2*** 27.3***
Overall 7 8 12 25 24 33
χ2 tests for significant difference in square occupancy patterns between regions. Occupancy is weighted to account for regional coverage patterns. ***P < 0.001
of decline from 1974/75 to the mid-1980s followed by a partial recovery and then another recent decline (i.e.
similar to the trend in The Netherlands). Inland trends are presented for the first time, but at present little can be inferred due to the short timescale and high inter- annual variation. That these counts are not correlated with coastal trends is noteworthy.
In interpreting these trends it is worth considering the reliability of ‘wetland’ counts of Golden Plover and Lapwing. Whilst count accuracy is likely to be relatively good (compared to the huge estuarine flocks of some species, e.g. Knot Calidris canutus), there are perhaps issues regarding the area to which these counts relate. In some regions large numbers of plovers are associated with inland wetlands, mostly for roosting.
The area of farmland from which these are drawn and the degree of fidelity to roosts are unknown and probably vary with weather and lunar conditions (Spencer 1953, Gillings 2003b). Similarly on the coast, fewer Golden Plover are typically seen during high tide count periods than at low tide, when large flocks may form on some exposed mudflats (Mason & Macdonald 1999a). For these reasons WeBS counts on and adjacent to wetlands should not be used in isolation to estimate population sizes for wintering plovers. They do, however, provide a useful means of monitoring plover trends, and constitute the only long-term infor- mation available for these species.
Winter distribution
Currently most Golden Plover winter in eastern Britain, with most Casual Records, especially of large flocks, being in the East region. Since these records were of a casual nature they could be biased by observer density and effort. However, the distribution of Golden Plover is strikingly different from that of all Casual Records (cf. Figs 1c and 4aii). Data from the systemat-
ically visited sample of 1-km squares show a similar pat- tern, with higher rates of square occupancy in the East than any other region. Furthermore, of all Golden Plover counted on wetlands, 66% were at sites in the East region. Both the 1977/78 Golden Plover survey (Fuller & Lloyd 1981) and the ‘Winter Atlas’ of the early 1980s (Lack 1986) recorded a more uniform distribution, with large flocks present in all regions.
The current distribution of Lapwing was similarly east- erly biased, though not as marked as for Golden Plover.
Population trends or shifting distribution?
Before considering the causes of these changes it is worth reviewing what we know about numbers and origins of plovers wintering in Britain. From data from the early 1980s, Fuller (in Lack 1986) estimated minima of 500 000 Golden Plover and 1 000 000 Lapwing in Britain and Ireland. Cayford & Waters (1996) combined ‘Winter Atlas’ data (from 1980–84) and WeBS data (from 1987–92) and estimated 250 000 Golden Plover and 1 500 000 Lapwing in Britain alone. Rehfisch et al.(2003) noted increases in coastal numbers of both species over the period 1994/95 to 1998/99 but did not produce updated popu- lation estimates due to the large numbers outside wetlands. Recently there has been a formal attempt to overcome this problem for these and other dispersed waterbirds using a stratified random sampling approach (Jackson et al. in press). Specifically, the Dispersed Waterbird Survey (DWS) derived updated British winter estimates (with 95% confidence limits) of about 580 000 (0.3–1.1 million) Golden Plover and about 1 400 000 Lapwing (0.6–2.9 million), although geo- graphic coverage was patchy.
In terms of origins, Golden Plover wintering in Britain may come from three populations: most are from two populations of altifrons breeding in Iceland and
Table 4.Regional breakdown of Golden Plover and Lapwing abundance.
Golden Plover Lapwing
Region Casual WeBS Casual WeBS Area (%)
East 165 (42) 86 830 (66) 358 (35) 144 652 (50) 19
South-Central 101 (26) 12 189 (9) 283 (28) 47 418 (16) 32
North 38 (10) 13 166 (10) 125 (12) 44 525 (15) 14
West 41 (10) 8594 (7) 131 (13) 36 227 (12) 21
Scotland 51 (13) 11 200 (8) 123 (12) 18 469 (6) 15
Casual, the number of Casual Records, approximating to number of flocks reported; WeBS, the total number of individuals recorded on inland and coastal wetlands by WeBS in December 2002; area, approximate percentage of lowland area falling in each region. Figures in paren- theses are column percentages.
Figure 4.Maps showing the distribution of (a) Golden Plover and (b) Lapwing during (i) the early 1980s (‘Winter Atlas’ data; Lack 1986) and (ii) 1999-2002 (December 2002 WeBS data and December 1999 to December 2002 WFBS data). In (i), increasing dot sizes indicate maximum counts for 10-km squares: (a) small, 1–120; medium, 121–495; large, >496; (b) small, 1–435, medium, 436–1500, large, >1501.
In (ii), flocks numbering over 1000 birds are marked by a larger open symbol. Upland areas are shaded grey
a Golden Plover
i 1980–84 (Winter Atlas)
b Lapwing
i 1980–84 (Winter Atlas)
ii 1999–2002 (WFBS/WeBS)
ii 1999–2002 (WFBS/WeBS)
Scandinavia/western Russia, and fewer are from apricaria breeding in Britain, Denmark and Germany (Wernham et al. 2002, Stroud et al. 2004). The Lapwing is mono- typic and UK wintering birds include local breeders and birds from Scandinavia and continental Europe (Wernham et al. 2002, Stroud et al. 2004).
The common trends in winter abundance and distri- bution suggest a common process. The Scandinavian populations are the only shared breeding origins but the species breed in different habitats (Cramp &
Simmons 1983). Therefore it seems unlikely that processes acting on the breeding grounds would give rise to such synchronous winter trends. Furthermore, most countries in which British wintering plovers are likely to have their breeding origins report either stable or declining breeding populations (Tucker & Heath 1994, Hagemeijier & Blair 1997, Byrkjedal &
Thompson 1998, Wilson et al. 2001, BirdLife Inter- national 2004). Golden Plover mortality due to hunting pressure has probably declined (Piersma et al.
2005) but how this balances with declining breeding populations and apparently increasing wintering numbers in some regions is unclear.
The wetland bird trends, distribution maps and square-occupancy rates point to a change in winter distribution towards eastern Britain. This is especially true of Golden Plover and less clear in Lapwing. Such changes in other wader species have been convincingly tied to changing weather patterns. Austin & Rehfisch (2005) demonstrated this for estuarine waders (exclud- ing these two species) whilst Rehfisch et al. (2004) showed a shift to the east and/or north by several species, including Lapwing, on non-estuarine coasts between 1984/85 and 1997/98. These changes are probably part of a larger-scale redistribution. Reductions in Golden Plover populations in The Netherlands in autumn/early winter seem to be balanced by greater numbers and later staging in Denmark and south Sweden (van Eerden & Keij 1979, Rasmussen 1994, van der Winden et al. 1997, Jukema et al. 2001, Rasmussen 2004). In Britain, interpreting these coastal trends and potential changes in distribution requires knowledge of inland flocks which are not systemati- cally monitored. However, some formerly occupied areas in western and central England are now vacant (Lock 1994, Byrkjedal & Thompson 1998, R.J. Fuller unpubl. data).
Causes of changing distribution
Here we consider several alternative causes for the
observed patterns, some of which cannot be rejected with current data.
Terrestrial habitat change
There have been marked changes in the extent and management of pastures (Fuller & Lloyd 1981, Barnard
& Thompson 1985, Tucker 1992, Milsom et al.1998, Chamberlain & Fuller 2000, Vickery et al. 2001) and arable fields (Chamberlain et al. 2000, Robinson &
Sutherland 2002). It is unclear whether these have meant that the arable east is now relatively more suitable to wintering plovers, although in arable farm- land plovers are unable to meet their daily energy requirements by feeding during daylight alone and must feed at night as well or instead (Gillings 2003b, Gillings et al. 2005).
An increase in intertidal feeding
The coastal increases appears to coincide with an increasing tendency to feed on mudflats at low tide.
Neither species regularly fed on mudflats (Fuller &
Lloyd 1981), and only three of 68 WeBS counters of major sites indicated an increasing tendency for plovers to feed on mudflats (S.G. unpubl. data).
However, in two mild winters large numbers of both species fed on mudflats in Essex estuaries (Mason &
Macdonald 1999a, 1999b) and at a site in Germany over half of Golden Plover used mudflats, where they achieved intake rates four times higher than on nearby farmland (C. Ketzenberg & K.-M. Exo unpubl. data).
It is clear that more work is needed to understand how plovers use intertidal and coastal farmland habitats.
Milder winters
Changes in weather patterns may be important since the distribution and migrations of both species are strongly affected by winter weather conditions (Fuller
& Youngman 1979, Jukema & Hulscher 1988, Kirby &
Lack 1993, Byrkjedal & Thompson 1998). Mean winter temperatures have increased during this period (Hulme & Jenkins 1998) and there has been a reduc- tion in the frequency of cold spells (Watkinson et al.
2004), potentially allowing waders generally to winter closer to their breeding grounds (Rehfisch et al. 2004, Austin & Rehfisch 2005). In the case of Golden Plover and Lapwing this would mean wintering further north/east and east, respectively. Evidence from this study, from Rehfisch et al. (2004) and from studies on the continent (Jukema et al. 2001) point to exactly this phenomenon.
Implications
Wintering Golden Plover and a large proportion of Britain’s Lapwing are now largely concentrated in the arable zone of Britain. There have been few autecolog- ical studies of plovers using arable farmland (Gregory 1987, Mason & Macdonald 1999b, Gillings 2003b) and more work is needed to understand how plovers use the complexes of habitats available in the coastal zone, with special reference to interactions between tide, moon phase and weather.
These results have implications for the production of population estimates. Counts from farmland and wet- lands can only be combined if from a similar period.
DWS (Jackson et al.in press) is a constructive step in acknowledging the large numbers of Golden Plover and Lapwing that occur on farmland outside systematically counted wetland sites. In countries where plovers winter on large expanses of farmland, complete censuses are often ineffective and sampling methods are the only practical means of counting (Gillings 2003a). However, for both DWS and WFBS, Golden Plover occupied less than 10% of the 1-km sample squares making population extrapolations problematic for this highly aggregated species. The large difference between existing published population estimates for Golden Plover and the DWS estimate, plus the appar- ently high percentage of the winter population that now exists on, or in proximity to, wetlands indicate a clear need for a dedicated winter plover survey, ideally coordinated across several countries (Gillings 2003a).
To provide precise population estimates will require better sampling of plover populations, perhaps using larger sampling units (at least 2 km ×2 km squares) in combination with extensive atlas-style surveys. Any survey must acknowledge that wetland and farmland habitats frequently serve different functions for these species.
ACKNOWLEDGEMENTS
We wish to thank the thousands of volunteers who have participated over many years to make WeBS and WFBS such a success. WeBS is a partnership between the BTO, the Wildfowl and Wetlands Trust, the Royal Society for the Protection of Birds and the Joint Nature Conservation Committee (JNCC, on behalf of English Nature, Scottish Natural Heritage, the Countryside Council for Wales and the Environment and Heritage Service in Northern Ireland).
WFBS was funded under a partnership between BTO and JNCC. The analysis was done as part of a NERC Case indus- trial studentship at the University of East Anglia. We wish to
thank Andy Wilson for help running WFBS and Phil Atkinson for help generating the GAMs. We would like to thank David Stroud, Rowena Langston and two referees for comments that improved this paper.
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(MS received 21 February 2005; revised MS accepted 15 August 2005)
