Wader migration in Britain & Ireland: continuing studies in a changing environment

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1 Wader migration in Britain & Ireland: continuing studies in a changing environment John H. Marchant ABSTRACT The special place of Britain & Ireland in wader distributions and flyways has been revealed and illuminated through the efforts of many kinds of birdwatchers and scientists, using methods ranging from simple counting and recording to isotopic analysis and satellite-tracking. Knowledge of some topics is still inadequate or lacking altogether, however, and this may hamper wader conservation.the migration patterns of waders in Britain & Ireland appear to be changing in response to broad-scale changes in climate and landuse, and are likely to continue to do so over future decades.against this background, the conservation of wader populations will depend on continuing studies, using the full range of traditional and advanced techniques. The occurrence of waders in Britain & Ireland Though geographically small, the islands of Britain & Ireland are remarkable in world terms for the numbers and diversity of waders present during the year, including breeding birds, both migrant and resident, passage migrants and winter visitors. Britain & Ireland supports breeding populations of most of the wader species that nest at comparable latitudes across Continental Europe, although several are rare and confined to northern Scotland or southeast England. Flyway studies place the region at a unique T-junction, at which species like Red Knot Calidris canutus and Turnstone Arenaria interpres, migrating from the eastern Canadian Arctic towards wintering areas on eastern Atlantic shores, meet their counterparts on journeys to similar wintering areas from Arctic regions of the Old World. Breeding waders from more than half of the Arctic region are thus regularly encountered as migrants in Britain & Ireland, and are occasionally supplemented by individuals from more remote parts of the Arctic. By virtue of its position on the edge of Europe, the region also attracts vagrant waders from central and southern parts of Europe and Asia, and these add to its overall species richness. Britain is probably richer in wader species than any other area of comparable size, with 77 species currently listed, more than one-third of the global total. It is, however, our wintering wader populations that are most remarkable. On a global scale, there are few places north of 45 N where waders can be found wintering. Hot springs and other isolated patches of unfrozen habitat farther north may hold a few individuals, but large concentrations of waders in the northern half of the northern hemisphere are found only on the western fringes of Europe and North America. The shores of Britain & Ireland, warmed by the Gulf Stream, form the northernmost wintering grounds for many wader species, and the closest suitable wintering places for waders nesting in a broad swathe of the Arctic, from northernmost Canada across Greenland, Iceland and northern Continental 640 British Birds 95 December

2 Europe to the Taimyr Peninsula in northcentral Russia. As well as a uniquely mild winter climate for its latitude, Britain & Ireland has numerous wide estuaries, many of which have a tidal range that is among the world s highest, exposing extensive intertidal feeding grounds for long periods twice a day. How has our knowledge of wader movements been gathered? Much of our knowledge of waders in Britain & Ireland is the result of simple observations of their occurrence and visible migrations. To be of lasting value, these sightings need to be summarised and published, not consigned simply to memories and personal diaries. Thousands of birdwatchers throughout Britain & Ireland provide valuable data for migration studies by submitting records to a county or regional bird recorder. For scarce and rare species, county reports feed through to national reports in this journal and in Irish Birds. Unsystematic, but widespread, bird recording is a good way to determine the seasonal ranges of birds and the sites they use on passage, and can often turn up unexpected information in quick response to changing patterns. Systematic surveys, such as the Wetland Bird Survey, can add a long-term framework to counting and recording, and ensure that not only unusually high counts, but also absences and low counts, contribute fully to the picture. Knowledge of bird movements gleaned from observations is supplemented by other methods of study, the most important being ringing and colour-marking studies, and radio- and satellite-tracking, which increasingly shift the focus of research from the species as a whole to the individual bird (Marchant 2002). Whereas observations and surveys can document the breeding and wintering areas of particular species and the sites they use on passage, they cannot identify the sites used by particular individuals, or any differential use by particular races or sex or age classes unless these groups can be readily Wader migration in Britain & Ireland separated in the field. This requires some form of individual marking. Widespread catching and individual marking of birds with numbered rings began in Britain & Ireland in 1909 and currently produces around 11,000 ring-recoveries each year from around three-quarters of a million birds ringed (Clark & Wernham 2002). The development of the cannon-net in the 1960s led to large increases in the numbers of shore waders caught for ringing and, consequently, to a great surge of data on wader movements. The contribution made by the few thousand amateur bird ringers in Britain & Ireland to the study of bird migration, in terms both of personal time and expense and of information gained, is quite remarkable. The typical ring-recovery, say from a bird ringed and later found dead, provides just two fixed points in time and space in the life of that particular individual. Other sites the bird may have visited, and the route and timing of the movement between the two points, are unknown. Consequently, interpreting a single ring-recovery may be difficult, especially if neither ringing nor recovery location indicates where the bird may have been hatched or was breeding. Since recovery rates for waders are often less than 3%, multiple reports of a single individual are rare. The addition of a unique marking that can be read in the field, however, such as a combination of colour-rings or a numbered darvic, has enabled the collection of multiple reports for many individuals of an increasing range of species. Waders, with their long legs and preference for open habitats, are especially well suited to colour-ring studies Turnstones Arenaria interpres, langlesey, July Richard Chandler British Birds 95 December

3 Wader migration in Britain & Ireland Whereas ringing itself requires government licensing and a long period of training, reporting a colour-ring sighting is a valuable contribution that any birdwatcher armed with notebook and telescope can make to ringing studies. The development of radio-tracking has enabled yet more detail to be gathered about the lives of individual birds. Unlike any visual methods of study, radio-tracking can be used to fix the position of individuals at night as well as by day. For example, nocturnal studies of Woodcocks Scolopax rusticola have revealed an unexpected use of pasture for feeding. In most radio-tracking studies, birds carrying transmitters have been trailed by researchers carrying directional aerials and other reception equipment, either on foot or in a vehicle. In others, a set of receivers fixed in strategic positions, for example around an estuary, record the positions of birds in a study area automatically. The geographical limitations of groundbased radio-tracking can now be overcome by satellite-tracking, through which global surveillance of individual birds is now a possibility. Detailed routes have been published for individual wildfowl (geese and swans) and raptors on migration to or from Britain & Ireland, and selected individuals can be tracked on the Internet almost in real time. As satellite-transmitter technology improves, we can look forward to its use on smaller birds, including waders, addressing many of the currently unanswered questions about wader migration. Unless costs fall by several orders of magnitude, however, it is likely that satellite-tracking will continue to focus on only a small number of individuals, and that a number of assumptions will have to be made to relate the results from these individuals to the movements of the species or population as a whole (Wernham & Baillie 2002). A complete picture of a species movements emerges only when the results of all available study methods are combined, with relatively few ring-recoveries or satellite tracks for individual birds set against a more general background knowledge of the movements of the species, derived most straightforwardly from simple observation and recording. Ringing as a tool for studying wader migration The BTO s Migration Atlas (Wernham et al. 2002) seeks to bring together all that is known about bird movements involving Britain & Ireland. The results from bird ringing are discussed, alongside those from other information sources, to give a full and rounded picture of the seasonal migrations and other movements of the birds that breed, winter, or pass through the region. Of 188 species covered in depth, 26 are waders; a further eight wader species are dealt with in somewhat less detail. Over 1,000 ring-recovery maps have been newly prepared for the Migration Atlas. Some update previously published analyses, but many present information that is completely new. It is these ringing results that have typically provided the first evidence to link breeding and wintering areas for British & Irish bird populations, sometimes in counter-intuitive ways. For example, it would surely be anticipated that summer recoveries of Red Knots ringed in Britain & Ireland would be in the Russian Arctic, where so many of our waders originate, whereas, in fact, almost all recoveries are in Greenland and northeast Canada (fig. 1). A single recovery mapped in Siberia is an indication that nominate-race (Siberian) Red Knots also occur here, but the ringing evidence suggests that these birds are, in fact, remarkably rare in Britain & Ireland (Boyd & Piersma 2001). Most surprising is the relatively recent discovery that many Red Knots which leave British estuaries in spring for stopover sites in northern Norway are bound for Greenland, rather than for Siberia (Piersma & Davidson 1992). Bar-tailed Godwits Limosa lapponica are Palearctic nesting birds, found between Norway Fig. 1. The locations in June of 81 Red Knots Calidris canutus ringed or recovered in Britain & Ireland. 642 British Birds 95 December

4 and western Alaska. The larger, dark-backed race baueri, which occurs in the eastern half of this range, has not yet been seen in Britain & Ireland. Ring-recoveries indicate that birds from Wader migration in Britain & Ireland Fig. 2. The recovery locations and all included exchanges of Bar-tailed Godwits Limosa lapponica between Britain & Ireland and abroad.those abroad in the breeding season (five, red) and winter (34, blue) are differentiated from those abroad during other, or unknown periods (100, grey). throughout the range of the western, nominate race occur in Britain & Ireland, and suggest that this is an important winter destination or stepping stone on passage for the entire population of this race, not just for a section of it (fig. 2). Many individuals that pass through British and Irish estuaries spend the winter on the West African coast. The Turnstone is found nesting on almost all Arctic coasts, and around the Baltic Sea. The ringrecoveries from those birds which occur in Britain & Ireland in autumn identify the Baltic as a major source of passage migrants but suggest that perhaps none come from farther east (fig. 3a). By contrast, recoveries from winter ringing link Britain & Ireland exclusively to the breeding grounds in Greenland and northeast Canada (fig. 3b). Ringing has thus established that the Turnstones which British and Irish birdwatchers see include birds both of Baltic and of Nearctic origin, but that these groups Fig. 3. (a) Locations during the breeding season (41, red) and in winter (108, blue), and movements of over 20 km, of Turnstones Arenaria interpres present in Britain & Ireland during autumn. (b) Locations during the breeding season (36, red) and spring (50, grey), and movements of over 20 km, of Turnstones present in Britain & Ireland during winter. a b British Birds 95 December

5 a b c Wader migration in Britain & Ireland Fig. 4. Exchanges of Common Snipe Gallinago gallinago between (a) northern Britain (16), (b) southern Britain (56), and (c) Ireland (51), and abroad. have quite different seasonal patterns of occurrence. The Migration Atlas has also found many cases where different subpopulations occur simultaneously in Britain & Ireland, but with different regional distributions. For example, the regional recovery maps for Common Snipe Gallinago gallinago show a clear regional sepa- ration between visitors from Iceland and those from Continental Europe (fig. 4). Icelandic birds (race faeroeensis) apparently occur exclusively in Ireland and in westernmost Britain, south to Scilly (although passage through the Northern Isles is also suspected). Continental visitors, on the other hand, are found throughout Britain, but especially in the south, and in Ireland too, alongside Icelandic birds in some cases but with a more easterly distribution. Within populations, ringing also demonstrates that there are differential movements between breeding and wintering areas for different age and sex classes. For waders breeding in Britain & Ireland, such differential movements have been detected between adults and immatures for six out of seven species that were tested (Siriwardena & Wernham 2002). To conserve migratory populations effectively, should that be required, a detailed understanding of the birds locations at all seasons is essential. A good knowledge is also needed of any differential movements made by the species, since a conservation programme that benefited only one sex or age class of the population would be unlikely to succeed. Ringing is far from being just about ring-recoveries, however. Much emphasis has been placed in recent years on its other roles in monitoring populations and assessing productivity and survival. The capture of birds for ringing is the basis for the collection of valuable biometric data from live birds, and for the more advanced migrationstudy techniques such as radio- and satellitetracking, the use of genetic markers, and studies that infer breeding or moulting locations from the ratios of stable isotopes. Ringing is also likely to remain the most cost-effective and population-representative approach to migra- 644 British Birds 95 December

6 Wader migration in Britain & Ireland tion studies in the foreseeable future, but its value will be greatly enhanced if validated against the more novel and expensive technologies, and used in combination with them. Remaining unknowns As well as summarising known patterns, the Migration Atlas draws special attention to what is not yet known about the movements of British and Irish birds. The Atlas will, hopefully, inspire new studies specifically designed to fill some of the remaining gaps in our knowledge, perhaps by using new techniques or in some cases by the redirection of existing studies (Wernham & Baillie 2002). Tables 1 and 2 present summaries of current knowledge of movements for British and Irish wader species. The symbols in these tables indicate a simple categorisation of the relative state of knowledge of each topic, based largely on the information available to each species-text author of the Atlas and on a crude assessment of the ring-recovery data. The data are extracted and simplified from comprehensive tabulations by Wernham & Baillie (2002), who provide more information on how the tables were constructed. Among regular breeding waders in Britain & Ireland, information on movements is particularly poor for Dunlin Calidris alpina, while among wintering populations, perhaps the worst known are those of Greenshank Tringa nebularia and Green Sandpiper T. ochropus; rare species, even less well known, are not tabulated. Whereas ringing data allow differences between age groups to be tested for seven breeding and four wintering wader species, no species had enough recoveries of known sex to test for differences between the sexes (Siriwardena & Wernham 2002). Differential movements between the sexes are remarkably poorly known for waders, despite the often-considerable size and structural differences between the sexes which make differential movements especially likely among this group of birds Common Snipe Gallinago gallinago, Kent, September Changes in migration patterns: the need for continuing study Most of the existing knowledge of bird movements has been gathered only during the last 100 years or so. Over a similar period into the future, most scientists expect to see an unprecedented rise in global temperatures, with farreaching consequences for the global environment and for terrestrial plant and animal communities everywhere. Likely consequences for the UK include a rise in average annual temperatures of between 2 C and 3.5 C by the 2080s and a sea-level rise of cm in southeast England over the same period (Hulme et al. 2002). Because waders so often live in extreme habitats, make journeys that are close to the limits of their physiological capabilities, and rely heavily on traditional stopover sites on migration, they are likely to be more susceptible than most birds to global climate change. In the UK, changes in winter wader distribution and numbers have already occurred in response to the warmer winters of recent decades, and further change will occur as coastlines and sediment distribution change with rising sea levels (Austin et al. 2001). Table 1 also indicates that some change has already occurred in the wintering grounds of waders which breed in Britain & Ireland. Oystercatchers Haematopus ostralegus and Eurasian Curlews Numenius arquata made significantly fewer long-distance movements after their median recovery year than before, and are thus apparently becoming less migratory, whereas Northern Lapwing Vanellus vanellus has shown the opposite tendency (Siriwardena & Wernham 2002). Our Richard Chandler British Birds 95 December

7 Wader migration in Britain & Ireland understanding of temporal change is very poor, however, for waders wintering in Britain & Ireland (table 2). Northward and uphill movements of vegetation zones during the present century are predicted to reduce drastically the extent of treeless Arctic tundra where many waders nest (Zöckler & Lysenko 2000). The prospect of Arctic waders contending with the compounded problems of shrinking breeding areas and ongoing disruption to their patterns of migration is extremely worrying for their long-term conservation. Future bird-migration studies will, therefore, not simply be a question of plugging the gaps in existing knowledge. There will also need to be a constant re-evaluation of what we think we already know, to ensure that the information remains valid. Such an effort will require the deployment of the full range of study skills, from simple bird recording to the most advanced technology, and the keen participation of volunteer birdwatchers and ringers as well as professional ornithologists. Acknowledgments Dr Christine Wernham and Dr Tony Prater kindly provided comments on a draft of this article. Footnote to tables 1 & 2 1. Routes and timing of migration: refers to both spring and autumn migrations. (-S) = information moderate for autumn migration but poor for spring 2. Winter movements: refers to both within-winter and betweenwinter ring-recoveries. (-W) = information moderate for between-winter movements but poor for within winter movements Table 1. Summary of current knowledge of the movements of British and Irish breeding populations of waders, based on subjective assessments by species-text authors for the Migration Atlas (Wernham et al. 2002).The codes (good), (moderate) and (poor) reflect the relative quality of information available on each topic. Adapted and abridged from Wernham & Baillie (2002). Species Juvenile Natal Breeding Routes Wintering Sex Age Regional Migration Summering Temporal movements dispersal dispersal and area differences differences differences strategy areas of change in timing of immatures wintering migration 1 areas Oystercatcher Haematopus ostralegus - Stone-curlew Burhinus oedicnemus (-S) Little Ringed Plover Charadrius dubius (-S) Great Ringed Plover Charadrius hiaticula - Dotterel Charadrius morinellus European Golden Plover Pluvialis apricaria - Northern Lapwing Vanellus vanellus Dunlin Calidris alpina Ruff Philomachus pugnax Common Snipe Gallinago gallinago (-S) - Woodcock Scolopax rusticola (-S) - Black-tailed Godwit Limosa limosa Whimbrel Numenius phaeopus Eurasian Curlew Numenius arquata (-S) - Common Redshank Tringa totanus - - Greenshank Tringa nebularia Common Sandpiper Actitis hypoleucos 646 British Birds 95 December

8 Wader migration in Britain & Ireland References Austin, G. E., Rehfisch, M. M.,Viles, H. A., & Berry, P. M Impacts on coastal environments. In: Harrison, P. A., Berry, P. M., & Dawson,T. P. (eds.), Climate change and nature conservation in Britain and Ireland: modelling natural resource responses to climate change (the MONARCH project). UKCIP Technical Report, Oxford. Boyd, H., & Piersma,T Why do few Afro-Siberian Knots Calidris canutus canutus now visit Britain? Bird Study 48: Clark, J., & Wernham, C Ringing in Britain and Ireland. In: Wernham, C.V., Toms, M. P., Marchant, J. H., Clark, J. A., Siriwardena, G. M., & Baillie, S. R. (eds.), The Migration Atlas: movements of the birds of Britain and Ireland. London. Hulme, M., Jenkins, G. J., Lu, X.,Turnpenny, J. R., Mitchell,T. D., Jones, R. G., Lowe, J., Murphy, J. M., Hassell, D., Boorman, P., McDonald, R., & Hill, S Climate change scenarios for the United Kingdom: the UKCIP02 scientific report. Tyndall Centre for Climate Change Research, University of East Anglia, Norwich. Marchant, J Learning about bird movements: methods of study. In: Wernham, C.V.,Toms, M. P., Marchant, J. H., Clark, J. A., Siriwardena, G. M., & Baillie, S. R. (eds.), The Migration Atlas: movements of the birds of Britain and Ireland. London. Piersma,T., & Davidson, N. C. (eds) The Migration of Knots. Wader Study Group Bulletin 64 (suppl.). Siriwardena, G., & Wernham, C Synthesis of the migration patterns of British & Irish birds. In: Wernham, C.V., Toms, M. P., Marchant, J. H., Clark, J. A., Siriwardena, G. M., & Baillie, S. R. (eds.), The Migration Atlas: movements of the birds of Britain and Ireland. London. Wernham, C., & Baillie, S The future of migration studies for bird conservation science. In: Wernham, C.V.,Toms, M. P., Marchant, J. H., Clark, J. A., Siriwardena, G. M., & Baillie, S. R. (eds.), The Migration Atlas: movements of the birds of Britain and Ireland. London. Wernham, C.V.,Toms, M. P., Marchant, J. H., Clark, J. A., Siriwardena, G. M., & Baillie, S. R. (eds.) The Migration Atlas: movements of the birds of Britain and Ireland. London. Zöckler, C., & Lysenko, I Water Birds on the Edge. First circumpolar assessment of climate change impact on arctic breeding water birds. World Conservation Monitoring Centre, Cambridge. Table 2. Summary of current knowledge of the movements of British and Irish wintering populations of waders. Sources and conventions are as for Table 1. Species Winter Routes and Breeding Sex Age Regional Migration Temporal movements 2 timing of origins differences differences differences strategy change in migration 1 movements Oystercatcher Haematopus ostralegus Great Ringed Plover Charadrius hiaticula European Golden Plover Pluvialis apricaria Grey Plover Pluvialis squatarola (-W) Northern Lapwing Vanellus vanellus (-W) Red Knot Calidris canutus Sanderling Calidris alba Purple Sandpiper Calidris maritima Dunlin Calidris alpina Ruff Philomachus pugnax Jack Snipe Lymnocryptes minimus (-W) Common Snipe Gallinago gallinago Woodcock Scolopax rusticola Black-tailed Godwit Limosa limosa Bar-tailed Godwit Limosa lapponica Eurasian Curlew Numenius arquata (-W) (-S) Common Redshank Tringa totanus Greenshank Tringa nebularia Green Sandpiper Tringa ochropus Turnstone Arenaria interpres John H. Marchant, BTO, The Nunnery, Thetford, Norfolk IP24 2PU British Birds 95 December