Biological Conservation 39 (1987) 153-164

A Revision of Population Estimates for Waders (Charadrii) Wintering on the Coastline of Britain

M. E. M o s e r

British Trust for Ornithology, Beech Grove, Tring, Hertfordshire, HP23 5NR, Great Britain

(Received 19 June 1986; accepted 30 July 1986)

A BSTRA CT

Recent surveys have revealed that approximately 1.3 million waders winter on the British coastline. Dunlin Calidris alpina, oystercatcher Haematopus ostralegus and knot Calidris canutus were the most abundant species, together making up two-thirds of the total More than 80% of the waders were found on estuaries, although these habitats comprise less than one- quarter of the coastline, by length;furthermore, more than 50% of the waders occurred, in mid-winter, on only ten estuaries. The patterns of dispersion varied between species. The estuaries supported all or most of the national populations of black-tailed godwit L. limosa, knot, dunlin, bar-tailed godwit Limosa lapponica and grey plover Pluvialis squatarola, whilst purple sandpiper Calidris maritima, turnstone Arenaria interpres, ringed plover Charadrius hiaticula and sanderling Catidris alba were typically non- estuarine shore species. The winter populations of black-tailed godwit, bar- tailed godwit, knot, oystercatcher and dunlin are concentrated into a few estuaries, while the other species are more dispersed. The implications of these patterns are discussed with respect to estuarine habitat loss. The revised population estimates were similar to those .from earlier studies for .five species, were lower for two species, and were higher for five species. New values are recommended for use in ornithological site assessments, based on the criterion of 1% of the national population. The delimitation of sites, for such ewlluations, is discussed.

I N T R O D U C T I O N

The Atlantic seaboard of Europe and Africa is a wintering ground for almost 7 million waders (Charadrii), according to the most recent estimates

153 Biol. Conserv. 0006-3207/87/$03'50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

154 M. E. Moser

(NOME, 1982). More than 20% of these birds winter on the British coastline, where extensive intertidal flats provide the most northerly wader feeding grounds which remain free from prolonged freezing (Evans, 1984). Wader populations wintering in Britain originate from breeding areas in NE Canada, Greenland, Iceland, N Europe and the USSR (Cramp & Simmons, 1982), and the protection of their wintering areas is therefore of international concern.

Estuaries provide valuable resources for industrial, agricultural and recreational development and such developments have resulted in the loss of extensive areas of intertidal flats during the last century. There is evidence that the number of waders that can use an estuary in winter may be limited by the interference which occurs between individuals when feeding at high densities, causing a reduction in their food intake rates (Goss-Custard, 1985). Birds displaced from intertidal areas which have been lost to development may therefore be unable to settle on the remaining areas, which will already be occupied. Townshend (1985) recorded intense competition for space among grey plovers Pluvialis squatarola at Teesmouth. This resulted in unsuccessful juveniles being displaced by up to 900 km. Habitat loss thus has important implications for the conservation of estuarine wader populations, and there is an urgent need to identify and protect the most important sites.

The evaluation of ornithological sites has been recently reviewed by Fuller & Langslow (1986). For waterfowl, application of the '1% criterion' has been adopted by nations party to the 'Ramsar' Convention (Spagnesi, 1982). This criterion is used to evaluate the importance of wetlands for a particular species by comparing the number of birds using the site with the total numbers of individuals of that species present in the population. Sites holding more than 1% of the population are considered to be important. For international assessments, the total population level refers to that of the East Atlantic Flyway, or Europe. The 1% criterion may also be applied nationally, with the use of national population estimates (Prater, 1981). The reliability of such evaluations depends, at both levels, on the accuracy with which the size of the total populations are known.

In Britain, the most recent population estimates for waders (Prater, 1981) were derived from counts made on estuaries during the period 1969-75. More comprehensive material has been collected during the period 1980-85. This includes the first survey of waders on the non-estuarine coasts of Britain (Moser & Summers, in press), and further extensive counts of waders on estuaries. The aim of this paper is therefore to revise the earlier population estimates and to discuss the conservation implications of the patterns of distribution and abundance of wintering wader populations around the British coastline. The estimates are for Britain only (i.e. exclude

Population estimates of wading birds 155

the whole of Ireland, the Isle of Man and the Channel Islands). Species which winter largely inland, such as lapwing V. vanellus, golden plover Pluvialis apricaria, snipe Gallinago gallinago, jack snipe Lymnocryptes minimus, ruff Philomachus pugnax and woodcock Scolopax rusticola, are not considered.

METHODS

Sources of data

The population sizes of waders wintering in Britain were assessed from the results of two national surveys organised by the British Trust for Ornithology: the Birds of Estuaries Enquiry (BoEE) and the Winter Shorebird Count (organised jointly with the Wader Study Group). The former survey covers wader populations on estuaries and is carried out monthly, while the latter concerned the wader populations of non-estuarine coasts and was undertaken in a single winter only. Sites considered as 'estuaries' are shown in Fig. 1; they are extensive, often partially enclosed, areas of intertidal mud- and sand-flats. The sea lochs of west Scotland were considered as non-estuarine habitats because of the absence of extensive intertidal flats.

Counts for the Birds of Estuaries Enquiry (henceforth termed 'estuary counts'), have been collected in every year since 1969; details of the methods and results to 1975 are given by Prater (1981). Monthly counts are made on each estuary by teams of experienced amateur ornithologists, on predetermined dates toward the middle of each month. Counts are synchronised between sites to avoid counting twice any birds which might have moved between estuaries. Counts are usually carried out at high tide, when the waders congregate at discrete roosting sites. For each estuary, the count is achieved by visiting all roosts simultaneously. For small roosts, the waders can often be counted individually, whilst on larger roosts the numbers of each species must be estimated.

The Winter Shorebird Count was undertaken between 15 December 1984 and 31 January 1985 with the aim of censusing waders on all coastal habitats not covered by the estuary counts. A full description of the aims, methods and results of this survey is given by Moser & Summers (in press). A single count was obtained for more than 90% of the non-eliffcoastline of Britain; individual counts were co-ordinated for contiguous sections of coast so that errors resulting from movements of birds were minimised (Summers et al., 1984). Counts were carried out within three hours either side of low tide.

156 M. E. Moser

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Population estimates of wading birds 157

Analyses of data

Analyses of estuary counts in this paper are based on the January counts for each site averaged over the five years from 1981 to 1985. January counts were chosen because many species reach their winter peak in this month and because they coincide with international counts. These in turn are organised in January, because movement between estuaries is generally at a minimum in mid-winter. An average count was used for each estuary, rather than taking data for 1985 only, because not every estuary was fully counted in any one year. Only complete counts were used to compute the average. The national totals were calculated by summing, for each species, the average January count on each estuary and adding this total to the count of that species from the survey of non-estuarine shores.

RESULTS

Numbers and dispersion

Summation of the counts for all coastal habitats shows that almost 1.3 million waders winter on the British coastline (Table 1). Twelve species, excluding those which feed predominantly inland (see above), winter on the British coast in numbers exceeding 500 birds; other waders which are regularly recorded in smaller numbers during mid-winter on coastal habitats are avocet Recurvirostra avocetta, spotted redshank Tringa erythropus, greenshank Tringa nebularia, common sandpiper Actitis hypoleucos, green sandpiper Tringa ochropus and little stint Calidris minuta. The total counts for each species on estuarine and non-estuarine habitats are listed in Table 1, with the grand totals for the two habitats combined. Dunlin Calidris alpina was the most numerous species, with the total of 433 000 representing one-third of the total wintering wader population in

Fig. l.--contd. Stour; 47, Orwell; 48, Deben; 49, Ore/Butley/Havergate; 50, Aide; 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, Findhorn/Culbin/Nairn; 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 Sands; 80, Fleet Bay; 81, Kirkcudbright Bay; 82, Auchencairn Bay; 83, Rough Firth; 84, Solway; 85, lrt/Mite/Esk; 86, Duddon; 87, Morecambe Bay; 88, Ribble; 89, AIt; 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.

158 M . E . Moser

TABLE 1 Numbers of Waders Counted on the Estuarine and Non-estuarine Coasts of Britain

in Winter, 1980-85. (Figures, excluding the totals, have been rounded up as follows: > 100000 to the nearest

thousand; > 10000 to the nearest hundred; < 10000 to the nearest ten.)

Non-estuarine Per cent on

Estuaries coast Totals estuaries

Oystercatcher 215 000 65 500 279 500 76.9 Ringed plover 6 640 16 400 23 040 28"8 Grey plover 19 200 2 050 21 250 90"4 Knot 218 000 4 830 222 830 97"8 Sanderling 4 560 9 150 13 710 33-3 Purple sandpiper 540 15 600 16 140 3'4 Dunlin 409 000 24 000 433 000 94.5 Black-tailed godwit 4 770 0 4 770 100.0 Bar-tailed godwit 55 900 4 910 60 810 91 '9 Curlew 48 100 43 100 91 200 52.7 Redshank 54 300 21 100 75 400 72.0 Turnstone 9 780 34 700 44 480 22'0 Totals 1 045 790 240 340 1 286 130

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Fig. 2. Dispersion patterns of waders (all species combined) on the British estuaries (N = 112). The histogram shows the ranked average January wader total (1981-85) for each estuary, while the curve expresses these figures as the cumulative percentage of the estuarine total. NB All wader species, including lapwing, golden plover, etc. recorded on the January

counts for each estuary, are included in this figure.

Population estimates of wading birds 159

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160 M.E. Moser

Britain. Oystercatcher Haematopus ostralegus (279 500) and knot Calidris canutus (222 830) together accounted for a further 40% of the total.

More than 80% of the 1.3 million waders were found on the estuaries, although these habitats comprise less than one quarter of the 17 186 km coastline of Britain (Moser & Summers, in press). There were marked differences in the importance of the two habitats for each species (Table 1): all species occurred on both habitats, except for black-tailed godwit L. limosa, which was restricted to estuaries; more than 90% of the knot, dunlin, bar-tailed godwit Limosa lapponica and grey plover also occurred on estuaries, whilst purple sandpiper Calidris maritima, turnstone Arenaria interpres, ringed plover Charadrius hiaticula and sanderling Calidris alba were most abundant on non-estuarine shores.

The populations were distributed unevenly between the 112 estuaries treated here (Fig. 1). More than 50% of the waders occurred on the top ten and more than 90% on the top 40 British estuaries (Fig. 2). There were marked differences in the patterns of the individual species (Fig. 3). The British populations of black-tailed godwit, bar-tailed godwit and knot were concentrated into a very few estuaries only, with more than 50% of their total numbers occurring on the top five sites for that species (note that the most important estuaries are not necessarily the same for each species). The estuarine populations of oystercatcher and sanderling were also concentrated into a few sites, although substantial numbers also occurred on non-estuarine shores. Grey plover and dunlin, although confined mainly to the estuaries, were dispersed over a large number of sites. The remaining species were relatively well dispersed within estuaries and occurred in good numbers on non-estuarine shores.

DISCUSSION

The total of 1.3 million waders recorded on the British coastline during this study compares closely with just over 1"4 million, which can be calculated from the data given by Prater (1981). Some large discrepancies are, however, apparent when these earlier estimates are compared with the revised figures for individual species (Table 2). Differences may be attributed either to real population changes which have occurred since the earlier studies, or to the more comprehensive coverage of the recent material. Changes of less than 20% have occurred for oystercatcher, curlew, black-tailed godwit, knot and purple sandpiper. The population estimates for redshank and dunlin have decreased by 25% and 22% respectively. In both cases the changes correspond with long-term declines in the winter population level, as measured by the BoEE January

Population estimates of wading birds 161

TABLE 2 Previous (Prater, 1981) and Revised Estimates of the Population Levels of Waders

Wintering on the British Coastline. (The revised Values are the totals from Table 1 of this paper (rounded to 2 significant figures) and should be used in all evaluations of the national importance of wintering sites for

waders, based on the '1% criterion'.)

Previous Revised Per cent estimate estimate change

Oystercatcher 300 000 280 000 - 7 Ringed plover 12 000 23 000 + 92 Grey plover 10000 21 000 + 110 Knot 250000 220000 - 12 Sanderling 10 000 14 000 + 40 Purple sandpiper 18 000 16 000 - 11 Dunlin 550 000 430 000 - 22 Black-tailed godwit 5 000 4 800 - 4 Bar-tailed godwit 45 000 61 000 + 36 Curlew 100 000 91 000 - 9 Redshank 100 000 75 000 - 25 Turnstone 25 000 45 000 + 80

population indices (Salmon & Moser, 1985). Populations of the remaining five species listed in Table 2 have all increased. For grey plover and bar- tailed godwit this corresponds with increases in the BoEE January population indices (Salmon & Moser, 1985). Both species are largely confined to estuaries where count coverage has been consistently good; the observed change can therefore be attributed to a real increase in the numbers of birds wintering in Britain. (For bar-tailed godwit, the increased values have occurred as a result of birds seeking refuge in Britain from severe January weather on the Continent. These short-term immigrations inflate the population estimates over and above the normal winter levels.) The January population indices for ringed plover, turnstone and sanderling have shown no such increases (Salmon & Moser, 1985). The higher population estimates presented here must therefore be due to the location of unexpectedly large numbers on the previously unsurveyed non-estuarine coasts. All three species occur in much larger numbers on these habitats than on estuaries (Table 1).

Most waders are restricted in winter to coastal habitats, where they congregate onto relatively small areas of suitable habitat (mainly intertidal flats). This has implications for their conservation because damage to only a few sites could seriously threaten entire populations; the results of this study allow prediction of those species populations most at risk from estuarine developments. Black-tailed godwit, knot, dunlin, bar-tailed

162 M. E. Moser

godwit, grey plover, oystercatcher and redshank are most at risk because high proportions of their populations winter on estuaries, the habitat which is most threatened by development. The populations of black-tailed godwit, bar-tailed godwit and knot are concentrated into a very few estuaries (see Fig. 3) and the national populations of these species would therefore be particularly sensitive to damaging developments at those sites. Substantial numbers of the remaining species occur on non-estuarine shores which are generally less threatened from development, although they may suffer chronic pollution from oil-spills (Heubeck & Richardson, 1980). The birds on these habitats are also more dispersed and therefore less likely to be affected by localised habitat loss, except in areas of particularly high density.

Site evaluation

The '1% criterion' for the evaluation of ornithological sites has proved particularly effective for the identification of important wetlands for wintering waders and wildfowl (Fuller & Langslow, 1986), because of the clumped dispersion of their populations. The revised population estimates presented in Table 2 provide the most accurate and up-to-date assessment of the size of the British wader populations. These values should be applied in all future national site evaluations for waders, until further revisions are published. These values apply to winter populations (November-March) only. Certain limitations of this system are discussed by Fuller & Langslow (1986) and it should be noted that there are several other features which may contribute to the importance of a site, which have not been treated in the present analysis. For example, a site may be important outside the winter period as a moulting area or migration staging post. Also, it may support a particularly high density or diversity of species, or a concentration of individuals from a particular sub-population of a species.

The British coastline is endowed with many estuaries which support rich wader populations. These estuaries differ greatly in size, and many are separated by no more than a few kilometres, often by open coastal habitats which also support good numbers of waders. There may therefore be difficulties in defining the limits of sites for ornithological evaluations. The intertidal areas of most estuaries are relatively discrete, the boundary with the much narrower non-estuarine coast being easily recognised. Even on large estuaries, such as the Firth of Forth (Symonds et al., 1984), Humber (Tasker & Milsom, 1979) and Thames (K. Osborne, pers. comm.), movements of waders occur throughout the estuary on a daily basis, although the extent to which this occurs differs between species (Symonds et al., 1984). Daily movements of waders between estuaries are much less

Population estimates of wading birds 163

frequent, although they may occur under certain circumstances (e.g. when roosting sites are limited). Estuarine sites for waders can therefore be identified as contiguous areas of intertidal flats, over which regular movements of waders occur on a daily basis. Such estuaries should be considered as individual sites, with the possibility of combining adjacent sites should there be evidence of regular inter-estuary movements of waders. The definition of sites on non-estuarine shores is more complex. Few stretches support large concentrations of waders and these are usually distinguishable from the other parts of the coast which are relatively barren. More studies are required of the movements of waders on these habitats and between non-estuarine shores and adjacent estuaries before further refinements can be made to the procedure for defining such sites.

A C K N O W L E D G E M ENTS

The Birds of Estuaries Enquiry is organised by the British Trust for Ornithology (BTO) and co-sponsored by the BTO, the Nature Conservancy Council and the Royal Society for the Protection of Birds. The Winter Shorebird Count was an associated project undertaken for the BTO and the Wader Study Group. Fieldwork for both surveys is carried out entirely by volunteers, some of whom have contributed counts for over 15 years. The appearance of this paper is an acknowledgement of their enormous contribution to ornithology and the conservation of coastal birds and their habitats.

I am very grateful to Mrs D. Smallwood-Keating and Miss C. Ray for processing much of the data used in this paper, and to Dr M. W. and Mrs A. Pienkowski (formerly of the Department of Zoology, University of Durham) and their assistants from the Manpower Services Scheme for their help with checking computer printouts. Mrs E. Murray kindly drew the figures. I am grateful to Dr R. W. Summers for his role as joint-organiser of the Winter Shorebird Count and to Dr R. J. O'Connor, Dr M. W. Pienkowski, Mr A. J. Prater and Dr R. W. Summers for their comments on an earlier draft of this manuscript.

REFERENCES

Cramp, S. & Simmons, K. E. L. (eds) (1982). The birds of the Western Palaearctic, 3. Oxford, Oxford University Press.

Evans, P. R. (1984). The British Isles. In Coastal waders and wildfowl in winter, ed. by P. R. Evans, J. D. Goss-Custard and W. G. Hale, 261-75. Cambridge, British Ornithologists' Union.

164 M.E. Moser

Fuller, R. J. & Langslow, D. R. (1986). Ornithological evaluation for wildlife conservation. In Wildlife conservation evaluation, ed. by M. B. Usher, 248-69. London, Chapman & Hall.

Goss-Custard, J. D. (1985). Foraging behaviour of wading birds and the carrying capacity of estuaries. In Behavioural ecology: Ecological consequences of adaptive behaviour, Symp. Br. ecol. Soc., 25th, 169-88. Oxford, Blackweli.

Heubeck, M. & Richardson, M. G. (1980). Bird mortality following the Esso Bernicia oil spill, Shetland, December 1978. Scott. Birds, 11, 97-107.

Moser, M. E. & Summers, R. W., Wader population on the non-estuarised coasts of Britain and Northern Ireland: results of the 1984-85 Winter shorebird counts. Bird study, in press.

NOME (1982). Wintering waders on the Banc d'Arguin. Report of the Netherlands Ornithological Mauritanian Expedition 1980. Wadden Sea Working Group, Communication No. 6.

Prater, A. J. (1981). Estuary birds of Britain and Ireland. Calton, Poyser. Salmon, D. G. & Moser, M. E. (1985). Wildfowl and Wader Counts, 1984-85.

Slimbridge, Wildfowl Trust. Spagnesi, M. (ed.) (1982). Atti della Conferenza sulla conservazione delle zone umide

di importanza internazionale specialmente come habitat degli uccelli acquatici, Cagliari 24-29 novembre 1980. Supplemento alle Ricerche di Biologia della Selvaggina, 8,

Summers, R. W., Corse, C. J., Meek, E. R., Moore, P. & Nicoll, M. (1984). The value of single counts of waders on rocky shores. Wader Study Group Bulletin, 41, 7-9.

Symonds, F. L., Langslow, D. R. & Pienkowski, M. W. (1984). Movements of wintering shorebirds within the Firth of Forth: Species differences in usage of an intertidal complex. Biol. Conserv., 28, 187-215.

Tasker, M. & Milsom, T. P. (1979). Birds of the Humber Estuary. Peterborough, Nature Conservancy Council (unpublished report).

Townshend, D. J. (1985). Decisions for a lifetime: Establishment of spatial defence and movement patterns by juvenile grey plovers Pluvialis squatarola. J. Anim. Ecol., 54, 267-74.