Population estimates for waders Charadrii wintering in Great Britain, 1987/88–1991/92

Biological Conserration 77 (1996) 7-17 Copyright O 1996 Ehlevier Science Limited

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POPULATION ESTIMATES FOR WADERS CHARADRII WINTERING IN GREAT BRITAIN, 1987/88-1991/92

J o h n T. C a y f o r d & R. J. W a t e r s

British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK

(Received 2 April 1994; accepted 15 August 1995)

Abstract Britain 'S wintering wader populations were estimated for the period 1987/88-1991/92 from data provided from three sources: the Wetland Bird Survey (WEBS), the Winter Shorebird Count and the Winter Atlas. WeBS is the annual monitoring scheme for non-breeding waterfowl populations in the United Kingdom. It achieves excellent coverage of the important concentrations of waders living within estuaries, but the coverage of populations using non-estuarine coasts and inland habitats is incomplete. The Winter Shorebird Count, conducted in 1984/85, was a comprehensive survey of waders wintering on non-estuarine coasts while the Winter Atlas provides distributional data for all bird species in Britain and Ireland for three winters in the early 1980s and is a source of information on inland wader populations. The wader populations on non-estuarine coasts were estimated from sites counted in both 1984/85 and in 1987/88-1991/92 as part of WEBS, enabling calibration of the Winter Shorebird Count data. The results suggest that Britain's coastline supports approximately 1.65 million waders. Values are presented from which site evaluations based on 1% of the national population can be derived. Comparison of the new coastal population estimates with the previous estimates (for the period 1981-85) suggests that numbers of most species of wintering waders have increased on estuaries and non-estuarine coasts during the last decade. These population changes are compared with trends in British wintering wader populations as shown by application of a model- based method of indexing populations. The possible causes of between-year fluctuations in wader populations are discussed and the importance of long-term monitoring schemes emphasised Copyright © 1996 Elsevier Science Ltd

Keywords: population, waders, wintering, Great Britain.

INTRODUCTION

Britain's estuaries, coasts and inland wetlands are important habitats for large numbers Of non-breeding

Correspondence to: R. J. Waters, Tel. +44 (0)1842 750050; fax +44 (0)1842 750030.

waders Charadrii and wildfowl which breed in Canada, Greenland, the European arctic and Siberia (Prater, 1981; Waters & Cranswick, 1993). Waders are attracted to Britain's extensive intertidal estuarine mudflats because they support high densities of invertebrate prey and remain relatively free from ice during the winter because of the influence of the Gulf Stream (Evans, 1984; Moser & Summers, 1987).

The Atlantic coasts of Europe and Africa are wintering grounds for almost seven million waders (NOME, 1982; Smit & Piersma, 1989) which form part of the biogeographic unit termed the East Atlantic Flyway (Smit & Piersma, 1989). In addition to their role as wintering areas, estuaries can also act as refuges for waders during cold weather, as moulting sites for large numbers dispersing from the Wadden Sea in the autumn, and as staging posts for waders on migration to the Atlantic coast of Morocco and the Banc d'Arguin (Dick, 1975; Engelmoer et al., 1984; Smit & Piersma, 1989). Waders therefore require a network of different sites at different times of the year which means that their conservation depends, inter alia, on internationally co-ordinated monitoring and conservation programmes.

The distribution and numbers of non-breeding waterfowl on Britain's estuaries are well studied. Twenty-six species of wildfowl and 18 species of waders regularly occupy the 124 estuaries, which together hold more than 80% of Britain's overwintering coastal waterfowl population (Moser, 1987). More than half of Britain's wintering waders occur on just 10 estuaries, of which The Wash "and Morecambe Bay support the largest populations. Oystercatcher Haematopus ostralegus, dunlin Calidris alpina and knot C. canutus are the most abundant species which together form c.70% of the total wintering wader population. Waders differ in the extent to which they occur on Britain's estuaries (Davidson et al., 1991), non-estuarine coastline (Moser, 1987; Moser & Summers, 1987) and inland habitats. It has been reported that more than 90% of the black- tailed godwit Limosa limosa, bar-tailed godwit L. lapponica, knot, grey plover Pluvialis squatarola and dunlin occur on estuaries; whilst purple sandpiper C. maritima, sanderling C. alba, turnstone Arenaria interpres

8 J .T . Cayford, R. J. Waters

and ringed plover Charadrius hiaticula are most numerous on non-estuarine beaches and rocky coastlines (Moser, 1987). Lapwing Vanellus vanellus, golden plover Pluvialis apricaria, and snipe Gallinago gallinago occur mainly in inland habitats.

Estuaries attract a variety of human activities which can result in the loss and/or degradation of intertidal feeding areas and therefore threaten waterfowl populations. Recreational and tidal power barrages, marinas and housing schemes make up more than half the current estuarine land claim proposals in Britain (David- son et al., 1991). Additional threats to intertidal feeding habitats come from the development of ports and reser- voirs, pollution from organic effluents, oil and heavy metals, shellfish fisheries, agricultural land claim and the disturbing effects of recreation (Davidson & Roth- well, 1993).

Protection of waterfowl and wetlands in the UK is achieved through both international and national legis- lation and designations. The UK is a signatory of the Convention on Wetlands of International Importance especially as Waterfowl Habitat (the Ramsar Conven- tion) (Matthews, 1993) and is bound by EC Directive 79/409 on the Conservation of Wild Birds (the 'Birds Directive'). These international statutes contain provi- sions for protection of sites of international importance (Ramsar sites and Special Protection Areas respec- tively). Some of the criteria for selecting Ramsar sites and SPAs are quantitative, including the 1% criterion whereby a wetland is considered internationally important if it regularly holds >1% of the individuals in a biogeographical population of a species or subspecies of waterfowl (Spagnesi, 1982). This criterion has been widely accepted and adopted as a method of identify- ing sites requiring statutory protection (Stroud et al., 1990; Matthews, 1993). For most wader populations in western Europe, the appropriate international comparison population is the East Atlantic Flyway. The quali- fying levels for international importance (i.e. the number of birds used as the nominal 1% of the population) are termed '1% thresholds' (Rose & Stroud, 1994).

In Great Britain, sites of national importance, as well as ones of international importance, are notified as Sites of Special Scientific Interest (SSSIs) under the Wildlife and Countryside Act 1981. Guidelines for selection of SSSIs include national 1% thresholds, adopted by analogy to the international 1% thresholds (Nature Conservancy Council, 1989). Thus, periodic revision of population sizes is important to aid site selection as well as having a general monitoring function.

Two previous population estimates have been made for Britain's wintering coastal waders. The first was based on estuarine data collected during 1969-1975 (Prater, 1981) and the second on estuarine and non- estuarine coastal data collected during 1981-1985 (Moser, 1987). These suggested that c. 1.3 million waders wintered on British coasts. The aim of this

paper is to present new population estimates for 22 species of waders (listed in Table 4) which regularly overwintered in Britain (excluding the Channel Islands and the Isle of Man) during the period 1987/88-1991/92 and to define new national 1% thresholds. The population trends of individual species are presented and the revised estimates broadly compared with the international (East Atlantic Flyway) populations derived from the Western Palearctic International Waterfowl Census (Smit & Piersma, 1989; Rose & Scott, 1994). A similar exercise has recently been conducted for British wintering populations of divers Gaviidae, grebes Podicipitidae, swans, geese and ducks Anatidae (Kirby, 1995).

METHODS

Sources of bird data Population estimates were calculated separately for estuarine, non-estuarine coastal, and inland habitats excluding species for which Britain's wintering population is <10 individuals. Population estimates in estuaries have been derived from the Wetland Bird Survey (WEBS), which incorporates data from the former Birds of Estuaries Enquiry (BoEE) (Prater, 1981) and the former National Waterfowl Counts (NWC) (Cran- swick et al., 1992). The Wetland Bird Survey is administered and funded by the British Trust for Ornithology (BTO), The Wildfowl & Wetlands Trust (WWT), the Royal Society for the Protection of Birds (RSPB) and the Joint Nature Conservation Committee (JNCC) and represents the most comprehensive database available on Britain's non-breeding waterfowl populations. Monthly counts are made by experienced volunteers on estuaries, sections of open coast and inland wetlands throughout the UK on predetermined dates. Figure 1 shows the locations of estuaries regularly counted by WEBS. Counts are synchronised to minimise errors resulting from bird movements and at estuarine sites are generally made at spring tide roosts where birds can be quickly and accurately counted.

Population estimates for waders in non-estuarine coastal habitats are based on two datasets. First, the Winter Shorebird Count, conducted in December 1984 and January 1985. This survey remains the only comprehensive count of wintering populations of waders on all non-estuarine coasts around the UK and the Channel Isles (Moser, 1987; Moser & Summers, 1987). All open rocky and soft sediment shores were surveyed in the period 3h either side of low water to reduce the chances of birds being re-counted or missed as a result of small-scale movements. Further details of the methods are described by Moser and Summers (1987). Sec- ond, counts were used from 40 non-estuarine sites that were covered by WeBS during 1984/85 (the period of the Winter Shorebird Count) and during the five winters 1987/88-1991/92. For each species on each site, the mean January count was calculated for the latter

Population estimates for British waders 9

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Fig. 1. Map of Britain showing the locations of all estuaries considered in this paper. Site code numbers are as follows: 1, Taw/Torridge; 2, Camel; 3, Gannel; 4, Hayle; 5, Fal complex; 6, Fowey; 7, Looe; 8, Tamar complex; 9, Plym; 10, Yealm; 11, Erme; 12, Avon; 13, Kingsbridge; 14, Dart; 15, Teign; 16, Exe; 17, Otter; 18, Axe; 19, The Fleet/Wey; 20, Poole Harbour; 21, Christchurch Harbour; 22, NW Solent; 23, Beaulieu; 24, Southampton Water; 25, Yar; 26, Newtown; 27, Medina; 28, Woot- ton; 29, Brading Harbour; 30, Portsmouth Harbour; 31, Langstone Harbour; 32, Chicbester Harbour; 33, Pagham Harbour; 34, Adur; 35, Newhaven; 36, Rye Harbour/Pett Levels; 37, Pegwell Bay; 38, Swale; 39, Medway; 40, Thames; 41, Crouch/Roach; 42, Dengie; 43, Blackwater; 44, Colne; 45, Hamford Water; 46, Stour; 47, Orwell; 48, Deben; 49/50, Aide complex; 51, Blyth; 52, Breydon Water; 53, N Norfolk Marshes; 54, Wash; 55, Humber; 56, Tees; 57, Blyth; 58, Coquet; 59, Lindisfarne; 60, Tweed; 61, Tyninghame; 62, Forth; 63, Eden; 64, Tay; 65, Montrose Basin; 66, Dee; 67, Don; 68, Ythan; 69, Spey; 70/71, Inner Moray Firth; 72, Cromarty Firth; 73, Dornoch Firth; 74, Loch Fleet; 75, Inner Clyde; 76, Irvine; 77, Loch Ryan; 78, Luce Bay; 79, Wigtown Bay; 80, Fleet Bay; 81, Kirkcudbright Bay; 82, Auchencairn Bay; 83, Rough Firth; 84, Solway; 85, Irt/Mite/Esk; 86, Duddon; 87, Morecambe Bay; 88, Ribble; 89, Alt; 90, Mersey; 91, Dee; 92, Clwyd; 93, Conwy; 94, Lavan Sands; 95, Red Wharf Bay; 96, Dulas Bay; 97, Inland Sea; 98, Cefni; 99, Braint; 100, Foryd Bay; 101, Traeth Bach; 102, Artro; 103, Mawddach; 104, Dysynni; 105, Dyfi; 106, Teifi; 107, Nyfer; 108, Cleddau; 109, Carmarthen Bay; 110, Burry; 111, Swansea Bay; 112, Severn; A, Helford; B, Cuckmere; C, Tyne; D, South Alnmouth; E, Banff; F, Lossie; G, Loch Gilp; H, Loch Gruinart (Islay); I, Loch Indaal (Islay);

J, Hunterston; K, Afan; L, Ogmore.

five years and the percentage change since 1984/85 calculated. The mean percentage change across the 40 sites was then used to adjust the Winter Shorebird Count population estimates. Details of the 40 non-estuarine sites are as follows. Their total length was 243 km (representing c. 2% of the total length of non-estuarine coasts in Britain) but they supported 10% of the waders counted on non-estuarine coasts in the 1984/85 Winter Shorebird Count. Their regional distribution was: Orkney 3, west Scotland 11, north-east Scotland 6, north-east England 14, south England 5, Wales 1. The

geographical bias towards west Scotland and north-east England broadly reflects the distribution of non-estuarine habitats supporting concentrations of waders (Moser & Summers, 1987).

The population estimates for waders which overwinter predominantly inland are derived from the Winter Atlas (organised by the BTO and the Irish Wildbird Conservancy), in which the counters surveyed 3,761 10-km squares in Britain and Ireland during the three winters 1981/82-1983/84 (Lack, 1986). Data also derive from inland WeBS counts.

10 J. T. Cayford, R. J. Waters

Population change Comparing the new population estimates presented in this paper with those of Moser (1987) provides a crude measure of how wader numbers have changed during the period. However, population indices calculated using the multiplicative index model described by Underhill (1989) and Underhill and PrOs-Jones (1994), based on data from estuarine and a few non-estuarine sites, provide a more accurate measure. Using this model, missing values are inputted and each observa- tion is modelled as the product of three independent factors: year, site and month. Consistency intervals are calculated using bootstrap procedures. Indices were based on 1000 bootstrap samples for the period 1970/71-1993/94, using 1972/73 as the base year (index = 100), thus updating the population trends previously described for the period 1970/71-1990/91 by P~s-Jones et al. (1994). The December-February data were used to calculate standard winter index numbers.

Analyses of data Traditionally, the importance of estuaries has been assessed largely, but not exclusively, using January counts of waterfowl averaged over a five-year period (Prater, 1981; Moser, 1987; Davidson et al., 1991). This is because January is the month when the winter count is at its highest for most wader species, between-site movements are generally at a low level for most species, and also when coverage of sites is most complete (Salmon et al., 1989; Underhill & PrOs-Jones,

1994). Furthermore, in an international context, most winter wader counts are undertaken in January. In this paper, January counts at each site were averaged over the five winters and the sum of these average counts taken to represent the total British wader population.

RESULTS

PolNdation esfinmtes for estuaries and ~ coasts The revised population estimates for the 16 wader species which overwinter predominantly on Britain's estuaries are presented in Table 1. The revised estuarine total of c.1.3 million represents a 27% increase from the 1981-1985 estimated total (Moser, 1987). A small part of this increase can be explained by the fact that 12 sites, previously defined as non-estuarine (and which hold approximately 4000 waders in total), have been re-defined as estuaries since the previous estimate was made. Comparison of the new estimates with those presented by Moser (1987) suggests that all estuarine wader species (with the exception of bar-tailed godwit) have increased in number (Table 1). Dunlin (507,000), knot (280,000) and oystercatcher (266,000) are the most numerous estuarine species (Table 1).

Population estimates for the wader species which regularly occur on Britain's non-estuarine coasts are also presented in Table 1. Figure 2 compares the proportions of 11 species present on the 40 sample sites with those recorded in the Winter Shorebird Count

Table I. Total numbers of waders counted on British estuaries 1981--85 and non-estuarine coasts 1984-85 compared with revised estimates in 1987/88-1991/92

Estuaries Non-estuarine coasts

1981-1985 Revised % 1984/85 Revised % estimatC estimate change estimatC estimate change

Oystercatcher 215 000 266 000 +24 65 500 93 700 +43 Avocet b - - 1 270 . . . . Ringed plover 6 640 9 630 +45 16 400 19 000 +16 Grey plover 19 200 41 900 +118 2 050 1 310 -36 Knot 218 000 280 000 +28 4 830 11 100 +130 Sanderling 4 560 5 440 +19 9 150 17 800 +95 Purple sandpiper 540 720 +33 15 600 20 600 +32 Dunlin 409 000 507 000 +24 24 000 25 200 +5 Ru~ - - 180 . . . . Black-tailed godwit 4 770 7 410 +55 - - - - - - Bar-tailed godwit 55 900 46 700 -16 4 910 5 760 + 17 Curlew 48 100 73 900 +54 43 100 41 300 -4 Spotted redshank b - - 90 . . . . Redshank 54 300 79 700 +47 21 100 34 300 +63 Greenshank b - - 190 - - - - 130 - - Turnstone 9 780 13 100 +34 34 700 51 300 +48 Total 1 045 790 1 333 230 ca +27 241 340 321 500 c +33

Figures have been rounded: >100 000 to the nearest thousand; >10 000 to the nearest hundred; <10 000 to the nearest 10. aMoser (1987). bAvocet, ruff, spotted redshank and greenshank were excluded by Moser (1987). CExcludes green sandpiper (28) and common sandpiper (22). dThe totals should be treated as minima because they exclude counts of golden plover and lapwing which winter mostly inland (see Table 3).


30

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Fig. 2. The proportions of wader species in the selected non- estuarine sample compared with the 1984/85 Winter Shore- bird Count national figures. Perfect estimation is defined by the line. OC, oystercatcher; CU, curlew; TT, turnstone; DN, dunlin; RK, redshank; RP, ringed plover; PS, purple sandpiper; KN, knot; SS, sanderling; GV, grey plover; BA,

bar-tailed godwit.

which covered the entire coastline. Despite the geographic bias towards western Scotland and north-east England, the sample sites support a representative community of waders: (Winter Shorebird Count = 1.37 + 0.83 sample; r = 0.84, n = 11, p <0.001) in which the regression slope is not significantly different from 1. The species which deviate most from the Winter Shorebird Count totals are curlew, which are under-represented on sample sites (7.3% vs 17.9%), and dunlin, which are over- represented (17.2% vs 9-9%). Because 95% of Britain's wintering dunlin occur on estuaries, this bias will not significantly affect the overall population estimates. Figure 3 shows the change in the populations of each species on matched non-estuarine sample sites expressed as a percentage of the Winter Shorebird Count data. The results suggest that non-estuarine populations have increased for nine of the 11 species (Fig. 3). The most

Oystercatcher Ringed plover

Grey plover Knot

Sanderling Purple sandpiper

Dunlin Bar-tailed godwit

Curlew Redshank Turnstone

- ' - I

J • I I • I

' , = , ,

H

-100 - 5 0 0 50 100 150 200 Percent

Fig. 3. Changes in the populations of waders on 40 selected non-estuarine sites counted between 1987/88 and 1991/92 shown as the % change from the counts made on the same

sites in 1984/85. Mean + I SE.

marked changes have been increases for knot (+ 130%), sanderling (+95%) and redshank Tringa totanus (+63%) (Table 1). It should be noted, however, that the error bars are large for knot and sanderling (Fig. 3); this is unimportant in the case of knot which has a relatively small population on non-estuarine coasts but is more serious in the case of sanderling. The mean percentage changes in the non-estuarine populations of each species presented in Table 1 were used to calibrate the Winter Shorebird Count data. The new estimate of 322,000 waders on non-estuarine coasts represents a 33% increase on the 241,000 estimated by Moser (1987) (Table 1). Oystercatcber (93,700), turnstone (51,300) and curlew (41,300) are the most numerous non-estuarine coastal species.

Black-tailed godwit, avocet Recurvirostra avosetta, knot, dunlin and grey plover are largely confined to estuaries, whereas purple sandpiper, turnstone, sanderling and ringed plover mainly occur on rocky coastlines and sandy bays (Fig. 4).

The national population totals for coastal habitats have been calculated for each species by taking the sum of the new estuarine and the non-estuarine totals. The number of waders wintering on Britain's coast has apparently increased from c.1-3 million (in 1981-1985) to c. 1.65 million (in 1987/88-1991/92) (Table 2). Note that these are minimum total estimates because they do not include golden plover and lapwing which utilise many estuaries, though the bulk of their populations occur in other habitats.

Population trends based on estuarine and non-estuarine coastal data Comparing the sum of the new estuarine and non-estuarine totals for each species presented in Table 2 with the estimates for 1981-1985 suggests that most wader species have increased substantially during the last decade. Only one species, bar-tailed godwit, has declined The largest increases have occurred in grey plover

Purple sandpiper • Turnstone Sanderling i

Ringed plover Greenshank

Curlew Redshank

Oystercatcher Bar-tailed godwit

Dunlin Knot

Grey plover Black-tailed godwit

Avocet I I I I i

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% Estuarine

Fig. 4. The percentages of the total British coastal population of waders estimated to occur on estuaries.


Table 2. Combined population estimates for waders wintering on estuaries and non-~'='tuartne coasts in Britain

Total Revised % 1981-1985 a total change

Oystercatcher 279 500 359 000 +28 Avocet - - 1 270 - - Ringed plover 23 040 28 600 +24 Grey plover 21 250 43 200 +103 Knot 222 830 291 000 +31 Sanderling 13 710 23 200 +69 Purple sandpiper 16 140 21 300 +32 Dunlin 433 000 532 000 +23 Ruff - - 180 - - Black-tailed godwit 4 770 7 410 +55 Bar-tailed godwit 60 810 52 500 -14 Curlew 91 200 115 000 +26 Spotted redshank - - 90 - - Redshank 75 400 114 000 +51 Greenshank - - 310 - - Turnstone 44 480 64 400 +45 Total 1 286 130 c 1 653 460 bc +29

Figures have been rounded: > 100 000 to the nearest thousand; >10 000 to the nearest hundred; <10 000 to the nearest 10. aMoser (1987). bExcludes green sandpiper (28) and common sandpiper (22). 'The totals should be treated as minima because they exclude estuarine and non-estuarine coastal counts of golden plover and lapwing which winter mostly inland (see Table 3).

(+ 103%), sanderling (+69%), black-tailed godwit (+55%), redshank (+51%) and turnstone (+45%), though the trend for sanderling should be interpreted cautiously in view of the imprecise estimate of population change (Fig. 3). Comparison of the percentage changes for populations on estuaries and non-estuarine coasts (Table 1) shows that the pattern of population change has been rather different in the two habitats for several species. In particular, the population increases for grey plover, curlew and dunlin on estuaries have not occurred to the same extent on non-estuarine coasts. Conversely, the estimated increases in the populations of knot, bar-tailed godwit and sanderling have

been much greater on non-estuarine coasts than on estuaries.

The indices developed by Underhill are presented for selected species for the period 1970/71-1993/94 to place the population trends suggested by the five-year datasets in a temporal context (Fig. 5). The results confirm that estuarine populations of grey plover, turnstone and black-tailed godwit have indeed increased as indicated by the five-year datasets. A full account of recent population trends is provided by Pr)s-Jones et aL (1994).

Inland population estimates For those wader species for which >10% of the national population is thought to occur in inland habitats, estimates are given in Table 3. These are likely to be less accurate than those for species which mainly occupy coastal habitats where count data are more frequent and spatially comprehensive. In particular, there is considerable uncertainty about the sizes of the large wintering populations of golden plovers, lap- wings and especially snipe. These three species winter mainly inland although golden plover and lapwing occur in large numbers at some coastal sites but these are probably used in conjunction with inland feeding areas.

Revised national population estimates The total population estimates presented in this paper are the sum of (1) the revised estuarine total; (2) the revised non-estuarine coastal total; (3) the inland wetland and non-wetland habitat total (Table 4). These allow 1% thresholds to be proposed for assessing national importance in site evaluations. Note that direct comparisons of the revised British population estimates with the most recent East Atlantic Flyway population estimates (Smit & Piersma, 1989) is invalid because the latter derive from an earlier period (1975-85). Nonetheless, it is clear from Table 4 that Britain holds especially high proportions of the international populations of ringed plover, knot, redshank and turnstone.

x

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Year

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Fig. 5 (a)-(g). Winter index numbers for seven species of waders on British estuaries 1970/71-1993/94 [see Underhill & PrOs-Jones (1994) for methods].

DISCUSSION

The data presented in this paper have re-affirmed the national and international importance of Britain's wintering wader populations and emphasised the importance of long-term monitoring programmes such

as WEBS. The expanding number of threats to estuaries and the waterfowl populations they support continues to reinforce the need for accurate, up-to-date information on the distribution and abundance of waterfowl upon which conservation actions can be based. The new population estimates presented in this paper are

14 £ T. Cayford, R. J. Waters

Table 3. Revised British population estimates for wintering waders in which >10% of the polmintinn is estimated to occur inland

Coastal Inland Total population % occurring estimate a estimate b estimate inland

Golden plover (68 000) (182 000) (250 000) 73% Lapwing (219 000) (1 281 000) (1 500 000+) 85% Ruff 180 520 700 74% Jack snipe c (30) - - - - - - Snipe d (3 000) - - - - - - Spotted redshank 90 30 120 25% Greenshank 320 60 380 50% Green sandpiper 30 720 750 96% Common sandpiper 20 100 100 80%

Figures excluding the revised population estimate have been rounded: >100 000 to the nearest thousand; >10 000 to the nearest hundred; <10 000 to the nearest 10. Estimates in parentheses are very approximate. The estimate for lapwing should be treated as a minimum.

aCoastal estimates are based on WeBS data. bBased mainly on information in Lack (1986). 'Winter populations of jack snipe probably lie in the range 10 000 - 100 000 (Lack, 1986). dWintering snipe in Britain number 'many hundreds of thousands' (Lack, 1986).

based on the most complete data available and therefore represent the best current estimates of the wintering populations of wader species in Britain. It is acknowledged, however, that for some species these figures almost certainly underestimate the actual number of waders present because of incomplete coverage. Despite the fact that waders are one of the most thoroughly monitored groups of animals in Britain, knowledge of their numbers and distributions on non- estuarine coasts and inland habitats is relatively poor compared with estuaries. The precision of the estimate of population change was poor for sanderling on non-estuarine coasts, which are especially important for this species. This emphasises the need to improve the level of monitoring on non-estuarine coasts to allow population changes to be thoroughly assessed and so that the importance of sites can be reviewed on a regular basis.

Comparison of the new population estimates presented in this paper with those of Moser (1987) provides good evidence that wintering numbers have increased for most wader species in Britain during the last decade. Apparent increases in population may simply reflect a continuing improvement in the coverage at some sites. The counts on which population estimates are based may also vary because of systematic and ran- dom/chance events which cause under- or over-estimation. On the other hand, genuine changes in populations may be the result of improved conditions on the breeding grounds (leading to enhanced productivity), increased survival over winter, and/or an international redistribution of the population in response to an increase in the relative quality of British wintering sites. The suggestion that population changes are linked to density-dependent factors which limit the maximum number of waders that an estuary can support (Evans, 1976; Goss-Custard, 1985) is not without

its critics. Evans and Pienkowski (1984) suggest that population regulation in shorebirds is illusory and that severe weather acting unpredictably on both breeding and wintering grounds prevents populations from ever reaching 'carrying capacity' (Brown, 1969; Fretwell & Lucas, 1969). The theoretical construct of 'carrying capacity' has proved useful, however, in developing mod- els which aim to predict whether birds displaced from an estuary as a result of a development scheme can be accommodated on other estuaries (Goss-Custard et al., 1994).

The results presented here accord with the analysis of Pr)s-Jones et aL (1994) that demonstrated marked population increases in grey plover and black-tailed godwit, and only one wader species (bar-tailed godwit) showing a moderate decline in Britain. The considerable variation in population trends between species shown both here and by PrOs-Jones et aL (1994) suggests that no single factor can explain why numbers have increased in Britain for most species during the last decade. Wintering populations of several wader species increase after years of high lemming abundance on the breeding grounds which has the effect of decreasing predation of wader nests (Martin & Baird, 1988; Mason, 1988; Underhill et aL, 1993), and it is likely also that long-term population change in some species may be influenced by changes in mortality from hunting following changes in hunting laws (Tubbs, 1991; Ferns, 1992). The rapid growth in the wintering grey plover population has been attributed to a shift in winter distribution, the removal of hunting pressure (Tubbs, 1991) or improved conditions on the breeding grounds resulting in an absolute increase in population levels with birds occupying estuaries in order of prefer- ence (Moser, 1988; PrOs-Jones et al., 1994). In the case of dunlin, the marked decline in numbers which took place during the 1970s and early 1980s has been linked

Population estimates f o r British waders

Table 4. British and East Atlantic Flyway wintering wader population estimates (Scientific names follow Voous, 1977)

15

Revised British British 1% East Atlantic population population Flyway

estimate threshold a population estimate b

Oystercatcher Haematopus ostralegus 359 000 3 600 874 000 Avocet Recurvirostra avosetta 1 270 10 67 000 Ringed plover Charadrius hiaticula 28 600 290 48 000 c Golden plover Pluvialis apricaria 250 000 2 500 - - Grey plover Pluvialis squatarola 43 200 430 168 000 Lapwing Vanellus vanellus 1 500 000+ 20 000 d - - Knot Calidris canutus 291 000 2 900 345 000 e Sanderling Calidris alba 23 200 230 123 000 Purple sandpiper Calidris maritima 21 300 210 50 000 Dunlin Calidris alpina 532 000 5 300 1 373 000 r Ruff Philomachus pugnax 700 - - - - Jack snipe Lymnocryptes minimus 10 000-100 000 - - - - Snipe Gallinago gallinago >> 100 000 - - - - Black-tailed godwit Limosa limosa 7 410 70 66 000 g Bar-tailed godwit Lirnosa lapponica 52 500 530 115 000 h Curlew Numenius arquata l 15 000 1 200 348 000 Spotted redshank Tringa erythropus 120 - - 6 50ff Redshank Tringa totanus 114 000 1 100 109 001Y Greenshank Tringa nebularia 380 - - 19 000 i Green sandpiper Tringa ochropus 750 - - - - Common sandpiper Actitis hypoleucos 100 - - 39 000 i Turnstone Arenaria interpres 64 400 640 67 000 k

aBritish 1% population thresholds have been rounded to the nearest 100 for species with populations >100 000 and to the nearest 10 for species with populations <100 000. 1% thresholds are not given for species with fewer than 1,000 wintering individuals in Britain or for snipe and jack snipe because their population sizes are unknown.

bBased on Smit and Piersma (1989). Unless stated to contrary in footnotes c-k, the totals are for the entire East Atlantic Flyway. Consult Smit and Piersma (1989, Table 12) for suggested international 1% thresholds. No attempt has been made to estimate international populations of golden plover, lapwing, ruff, jack snipe, snipe and green sandpiper. Note that Smit and Piersma's estimates are based on data for the period 1975-85 and are in need of updating (Rose.& Stroud, 1994).

CThis total is for north-west Europe and the Mediterranean; a further 195 000 ringed plovers made up of two subspecies C. h. hiaticula and C h. tundrae winter in west Africa (Smit & Piersma, 1989).

dThe 1% threshold has been set at the conservative level of 20 000 because of the considerable uncertainty about the size of the wintering population.

eThis total is for the Nearctic knot C c. islandica which winters mainly around the North Sea. The Siberian knot C c. canutus winters mainly in west Africa with an estimated population size of 512 000 (Smit & Piersma, 1989).

fThis total is for C a. alpina, the race which winters in north-west Europe and in the Mediterranean. Estimates for the other dunlin races are C a. arctica 15 000 (winters west Africa), C. a. schinzii (Icelandic) 800 000 (winters west Africa), C a. schinzii (temperate) 21 000 (winters Mediterranean) (Smit & Piersma, 1989).

gThis total is for L. I. islandica which winters almost entirely in north-west Europe. L. l. limosa overwinters in tropical Africa and the population size is unknown (Smit & Piersma, 1989).

h This total is for the European wintering population; a further 707 000 bar-tailed godwits of this race winter in west Africa (Smit & Piersma, 1989).

i These totals are for coastal wintering populations in Europe and Africa; large numbers winter inland in Africa. J This total is for the north-west European wintering population. A further 177 000 redshank winter in southern Europe and west

Africa, these being mainly T. t. totanus. Note that there is considerable overlap of winter distribution between the two subspecies. Both T. t. robusta and T. t. totanus overwinter in north-west Europe (Smit & Piersma, 1989). Note that the revised British population now exceeds the East Atlantic Flyway estimate because the latter relates to an earlier period (1975-85).

kThis total is for north-west Europe, a further 32 000 turnstones winter in west Africa (Stair & Piersma, 1989).

to the spread of the invasive intertidal cord grass Spartina anglica, with numbers declining most on estuaries where the spread of the grass was greatest (Goss- Custard & Moser, 1988). For some species, changes in numbers may result from movements triggered by severe weather or favourable breeding conditions. PrOs- Jones et al. (1994) suggest that the widely fluctuating

levels shown by bar-tailed godwit are influenced by immigration of birds to Britain from the Wadden Sea during severe winter weather and that knot have benefited from improved conditions on the breeding grounds. Clearly, further research is required to estab- lish the relative importance of the various contributory factors which influence year-to-year changes in numbers.


ACKNOWLEDGEMENTS

The Wetland Bird Survey (WEBS) is administered and funded by the British Trust for Ornithology, The Wild- fowl & Wetlands Trust, the Royal Society for the Pro- tection of Birds and the Joint Nature Conservation Committee (on behalf of the Countryside Council for Wales, the Department of the Environment for North- ern Ireland, English Nature and Scottish Natural Her- itage). We acknowledge the extraordinary contribution made by thousands of volunteers who provided all of the data on which this paper is based. Carol Powley helped prepare the data, Sophie Foulger helped prepare the manuscript and Su Gough drew the figures. We are grateful to Rob Fuller and David Stroud in particular and to Nigel Clark, John Goss-Custard, Jeff Kirby, Ken Norris, Ken Smith and the referees for providing very constructive comments.

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Documents

Population estimates for waders Charadrii wintering in Great Britain, 1987/88–1991/92