Overwinter population estimates of British waterbirds Andrew J. Musgrove, Graham E. Austin, Richard D. Hearn, Chas A. Holt, David A. Stroud and Simon R. Wotton Abstract In total, over 12.5 million waterbirds occur in Britain during the winter. Estimates of the numbers of non-breeding waterbirds visiting Britain are important for the birds’ conservation, both for status assessments and for the identification and designation of nationally and internationally important sites. This paper collates data from a wide range of sources, principally for the period 2004/05 to 2008/09, and produces estimates for 92 different species or populations, some using novel analytical methods developed by the authors. For 15 species or populations, formal estimates of wintering numbers are presented for the first time. The estimates demonstrate that species such as Avocet Recurvirostra avosetta , Gadwall Anas strepera and, especially, Little Egret Egretta garzetta have increased substantially in the last decade, while others, such as Greenland White-fronted Goose Anser albifrons flavirostris , Dunlin Calidris alpina and Common Pochard Aythya ferina , have declined markedly. Introduction breeding grounds as food availability dwin - Overwintering waterbirds constitute one of dles with the onset of winter, and a post- the most distinctive and significant elements breeding migration to Britain can involve a of the British avifauna. A very high propor - shorter journey than might otherwise be tion of many populations moves to Britain required. Second, despite the fact that for at least part of the non-breeding season, (outside protected areas) much of the British the numbers involved being especially countryside is seriously degraded and inhos - notable in comparison with the relatively pitable to wildlife, Britain does retain sub - small proportion (about 2.5%) of the land stantial areas of wetland habitats, in area of Europe that Britain occupies (Moser particular its estuaries, which are highly pro - 1987). Such concentrations of birds are due ductive in terms of invertebrate food, and to a number of factors. First, because of the newly created inland wetlands, such as influence of the Atlantic Ocean and the Gulf flooded gravel-pits. Third, Britain’s geo - Stream, Britain’s overwinter temperatures graphic position places it along an important remain markedly higher than those of other migratory route (the East Atlantic Flyway) regions at a similar latitude. Species that between northern breeding grounds and breed at more northerly latitudes (west to important wintering areas farther south such Canada and Greenland and east to Scandi - as Iberia, Morocco and the Banc d’Arguin in navia and Arctic Russia), or to the east in Mauritania. Finally, there has been a general northern Europe and Russia, vacate their reduction in hunting pressure for many of 364 © British Birds 104 • July 2011 • 364 –397 Overwinter population estimates of British waterbirds g n i l p i T d i v a D 205. One of the most memorable of British winter birding experiences is surely the dawn flight of Pink-footed Geese Anser brachyrhynchus leaving their roost, in this case at Snettisham, Norfolk, December 2008. Pinkfeet numbers have increased steadily in recent years, the most recent estimate being 360,000 in autumn 2009. these populations, an increase in provision of and the Convention on Biological Diversity, nature reserves and the implementation of among others. In addition, as a member of other positive conservation policies, all of the European Union, the UK is bound by which help to create favourable wintering Directives 2009/147/EC and 92/43/EEC conditions (Tubbs 1977, 1991; Owen et al. (generally referred to as the Birds Directive 1986). and Habitats Directive, respectively). Among Despite these positive aspects, however, other requirements, these international Con - there are increasing causes for concern. Mon - ventions and European Directives oblige the itoring schemes such as the Wetland Bird UK to monitor its waterbirds and provide for Survey (WeBS) suggest that, following their conservation (for further details, see decades of generally increasing numbers of www.jncc.gov.uk/page-1359 ). waterbirds, many species are now declining, The monitoring of waterbirds in Britain is in some cases rapidly (Calbrade et al. 2010). typically undertaken by collating sample Conversely, numbers of other species con - counts to derive interannual population tinue to increase. In addition to the scientific trends, i.e. the relative numbers of a species desire to understand such changes, and a present in one year compared with the moral argument to reverse declines if they numbers in another. However, there are cases are due to human factors, there is also a legal where it is necessary to know not just relative requirement to investigate these changes in numbers, but population sizes as well. These population size and the factors underlying include the application of criteria supporting them. The UK is a signatory to the Conven - the Ramsar Convention; Criterion 6 says that tion on Wetlands of International Impor - a wetland is considered internationally tance especially as Waterfowl Habitat (better important if it regularly holds at least 1% of known as the Ramsar Convention), the Con - the individuals in a biogeographic population vention on the Conservation of Migratory of one species or subspecies of waterbird. Species of Wild Animals, the African- Knowledge of population size is also impor - Eurasian Migratory Waterbird Agreement, tant when assessing threat and extinction risk, British Birds 104 • July 2011 • 364 –397 365 Musgrove et al. in understanding resource use and interspe - numbers of waterbirds overwintering in cific competition, and in assessing the poten - Britain. Such assessments have previously tial effects of avian diseases. It should be been undertaken approximately once every noted that population estimates are required ten years, the most recent being those at both national and international levels; described by Kershaw & Cranswick (2003) waterbird monitoring data from Britain are for wildfowl and by Rehfisch et al. (2003) for thus passed on to the International Waterbird coastal waders. Both of those papers relied Census and so contribute to the regular largely on data from the period 1994/95 to Waterbird Population Estimates publications 1998/99. The aim of this paper is to update (e.g. Wetlands International 2006). these estimates (and the corresponding 1% Given the need to work with reasonably thresholds for determining site importance), up-to-date estimates, and given that many principally by making use of new data species are showing clear changes in recent but also by considering new analytical status in Britain, it is timely to reassess the approaches for particular species. Scope Geographic area This assessment covers Great Britain, including the offshore islands of England, Scotland and Wales (such as Shetland, Orkney, the Outer Hebrides, Anglesey, the Isles of Scilly and the Isle of Wight). However, the Isle of Man and the Channel Islands are omitted. Estimates of non-breeding waterbirds in Northern Ireland are traditionally considered alongside those in the Republic of Ireland; the latest all-Ireland estimates were published by Crowe et al. (2008). For birds making use of the sea, the estimates presented here attempt to stick broadly to birds visible from the r e l d shore, although for n a h a number of species, C notably Red-throated d r a h Diver Gavia stellata c i R and Common Scoter 206. Black-tailed Godwits Limosa limosa , Snettisham, Norfolk, April Melanitta nigra , concen - 2010. In the early part of the winter at least, over 95% of Black-tailed trations of birds located Godwits and almost all Bar-tailed Godwits in Britain occur on estuaries, by aerial survey farther so WeBS counts of these two species produce a clear and accurate offshore have also been picture of totals and trends. These show a substantial decline in wintering Bar-tails in recent years (the current estimate is 38,000); included as discussed moreover, the Bar-tailed to Black-tailed ratio has now fallen from in individual species 4:1 to less than 1:1 in a single decade. accounts. 366 British Birds 104 • July 2011 • 364 –397 Overwinter population estimates of British waterbirds Timescale and season clear demarcation between seasons, examina - Most data used in the analyses relate to tion of WeBS data suggests that, for a useful counts made in the five winters between estimate of the size of the wintering popula - 2004/05 and 2008/09 (although more recent tion, data for November to March only counts were available for several species). The should be considered for Little Stint C. overwinter period, for the purposes of the minuta , Whimbrel Numenius phaeopus , estimates presented in this paper, is defined as Common Sandpiper Actitis hypoleucos , September to March for the majority of Spotted Redshank Tringa erythropus and species. It should be noted that this differs Greenshank T. nebularia . Therefore, estimates from the period used for some previous esti - for these five waders have been calculated mates, for example Rehfisch et al. (2003), in based on the period November to March. which wader estimates were all based on data collected during November to March. Wader Species and populations covered counts for September and October were Previous papers presenting British popula - omitted from previous estimates owing to tion estimates of waterbirds have tended to concerns about passage birds inflating the focus on either wildfowl or waders, owing to winter numbers. However, among the more the manner in which monitoring has histori - numerous species, only Ringed Plover cally been organised in Britain. This paper Charadrius hiaticula and Sanderling Calidris represents the first occasion that new esti - alba show very pronounced peaks in numbers mates have been presented for both of these on passage compared with those during the main groups together, in addition to esti - winter, and these typically occur in August mates for divers (Gaviidae), grebes (Podici - and May (which are omitted from the pedidae), cormorants (Phalacrocoracidae), analyses here).