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THE BTO MIGRATION ATLAS CHRIS WERNHAM After the Swallow, the Arctic Tern is probably one of the best-known avian migrants, famous for its annual journcy of up to 9000 miles each way between its breeding grounds in the northern hemisphere and wintering areas as far south as the Antarctic. Similarly, another of our breeding seabirds, the Manx Shearwater, also travels a long way (about 6000 miles each way) to rieh fishing areas off the east coast of South America during the winter. In addition to that, female shearwaters travel up to 500 miles from their breeding colonies to the Bay of Biscay to feast on sardines during the period of egg formation in spring. We have decidcd to use this amazing seabird migrant as the flagship species for our new project, the BTO Migration Atlas. The BTO and the Ringing Scheme The British Trust for Ornithology has had a long history of studies on bird migration because it is the Organisation responsible for the co-ordination of the British and Irish Ringing Scheme, which has been running since 1909. I am sure that most of you already know about bird ringing but, for those that do not, I will give you a brief introduetion. Ringing involves putting a small metal ring with a unique number and return address onto a bird's leg. Rings are made in a variety of shapes and sizes so that there is a type suitable for almost every species. This activity is carried out by around 2000 highly-trained, licensed ringers all over Britain and Ireland, mostly as a hobby in their spare time. Ringers are only permitted to ring after extensive training, and the welfare of the birds is paramount. When a ringed bird is found dead, or re-caught by a ringer, or seen by an observer because it has colour rings, and reported to the BTO, this is termed a 'recovery'. The unique number on the ring allows the individual bird to be traced. The original motivation for ringing birds was to find out where they went. But now, with new analytical techniques and the power of Computers we can also use ringing information to investigate survival rates, changes in bird numbers and produetivity. Rationale behind the Migration Atlas Fifty years ago only 820,000 birds had been ringed in Britain and Ireland, less than the current annual total in some years, and foreign recoveries were rare and exciting. The map of 'outstanding recovcries' from the 1948 BTO Annual Report showed exciting long distance movements, such as that of a Sandwich Tern to Angola and a Kittiwakc across the Atlantic. A Wheatear recovered in Portugal was the first recovery of any species from the newly-established Fair Isle Bird Observatory. Now, 50 years on, we have much more information available to us: over 25 million birds have been ringed since 1909, resulting in over Vi million recoveries of 260 species. Now the BTO reeeives well over 10,000 recoveries every year. This is why it is now time to collate the information on bird movements from ring recoveries into a Single volume, to 'take stock' and to decide what information we should aim to collect via ringing in the future.
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The Migration Atlas will bc of hugc conservation valuc. Incrcasingly, human pressures are sharpening the need for us to understand thc movcmcnts of birds and the constraints that they may experience during migration, particularly for species of conservation concem. As an example, the Brent Goose is a familiar species in Suffolk because the dark-bellied race winters around the coasts of southern England. The birds seen on thc coasts of East Anglia in winter breed on the Taimyr Peninsula. Those of thc pale-bellicd racc that breed in Greenland and Arctic Canada winter in Ircland and use Iceland as a Staging area in spring and autumn, while those pale-bellied Brents that winter on the north-east coast of England breed in Spitzbergen and Franz Josef Land and their whole population stages on Jutland in spring. For migratory species like these geese and many waders too, it is essential that practical conservationists and policy makers know the routes that they takc and any sites that arc of special importance for their conservation, such as Staging areas. The Migration Atlas will promote the Ringing Scheme and its great conservation value by showing the huge amount of invaluable information that has been collected by ringing so far and what extra information we need to collect in the future. Thc book will show that there are plenty of reasons why it is important for us to keep ringing birds. For example, the Wood Warbiers that breed in the British Isles are thought to winter in similar areas of West Africa to other summer migrants but there is still not a Single sub-Saharan recovery to support this. Even when the seasonal movements of a species are thought to be well established, things may change. For example, several years ago, Chris Mead of the BTO showed that Swallow migration routes had changcd over quite a short time period, so that more were wintering in south-west South Africa (Cape Province) than was previously the case. The increase in Blackcaps from central Europe wintering in Britain in recent years is a further example of such change. As global warming begins to alter the world's climate and Vegetation zones, more such changes in migration pattems. might be expected, and these will need to be monitored, via ringing, in the future. The Migration Atlas has been planned for a number of years, but the project was able to begin in June 1997 as a result o f ' a large grant from the Heritage Lottery Fund and generous matching funding, largely from the ringers themselves and from other BTO members. The book will comprise a number of introduetory and concluding chapters, including the history of bird ringing in Britain and Ireland and its current Scientific Strategy, a brief introduetion to migration ecology and methods of study, detailed methodology, importance of the findings for conservation and recommendations for future data collection. The main part of the project involves putting together analyses and texts for approximately 200 main species, in collaboration with expert invited authors. TOs is where work is focusing at present. Examples of information on movements from recoveries The patterns of movement based on ring recoveries have so far been mvestigated for about 25 species. This presentation now aims to give examples of the fascinating information on bird. movements that can bc gained from ring recoveries, illustrated by some of the species that have been analysed to date.
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Figure 1. Ring recovcries of Sandwich Terns. O Dcad, • re-caught, • rc-sightcd The famous long migration of the Arctic Tern has already been mentioned. Similarly, recovcrics show that the Sandwich Terns ihat breed in the British Isles winter as far south as South Africa Figure 1 shows recovcries of dcad Sandwich Terns, recaptures by ringers and sightings of colour-ringed terns. All these types of rccoveries will be combined in the Migration Atlas. These recovcries can reveal much more about Sandwich Terns than simply where they go in winter. The exaet locations where birds are recovered against the time of year show when the birds are moving and, also, where the 'average' bird spends its winter.
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Difference in latitude (south)
Farne Islands (56N)
0
Bordeaux (45N)
- 1 0 -
-20-
Casablanca (34N)
-30Mauntania (19N)
-40-50 Adult rr-lmmature -60
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Ghana (6N)
Dec
Month of Recovery Figure 2. Timing of the seasonal movements of Sandwich terns brccding in thc British Isles (mean latilude of recoveries by month of recovery). Figure 2 shows the average difference in latitude between the places where Sandwich Terns are ringed during the breeding season and the recovery site, for birds recovered in each month of the year. The top line shows the pattern for birds of breeding age and the lower line for immatures (less than 3 years old). This shows that an 'average' Sandwich Tern, ringed around 56°N on thc Farne Islands, Northumberland, in August, has travelled as far south as Mauritania by October and reaches the average wintering area around Ghana in West Africa by November. Terns of breeding age leave this wintering area in March, reaching the coast of southern Morocco (around Casablanca) by April and their breeding grounds by May. However, immature birds remain south for a longer period, some of the older immatures gradually making their way back to the breeding colonies later in the breeding season and others not retuming until two or three years after they were fledged. A knowledge of the movements of migrant birds is essential for their effective conservation. For example, many Knot are ringed in Britain in thc period August to March each year. They are familiar medium-sized waders that winter on estuaries around the British Isles, and are most well known for their huge flocks which wheel around in perfect synchrony over the mudflats. Recoveries show that the estuaries of Britain are an important wintering area for Knot from two populations: those that breed in Greenland and Arctic Canada and those that breed in northern Scandinavia (Fig. 3). Protection of British estuaries, particularly the lairge concentrations of Knot on the Wash and at Morecambe Bay, is therefore essential for the health of these populations.
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Figure 3. All exchanges of Knot between the British Isles and abroad Similarly, knowledge of movements is essential for the conservation of bird populations that breed in the British Isles. For example, Fulmars have a broad breeding distribution on cliffs around the coasts of Britain and Ireland. Adults of breeding age become pelagic during the winter months, while immature birds roam widely within the North Atlantic Ocean throughout the year. Of particular interest from the conservation perspective are the reports of these Fulmars off Newfoundland, Greenland, Iceland and northern Scandinavia. The latter are almost exclusively immature birds, as the majority of recoveries of adults occur within the North Sea . Many of the recoveries of immatures occur as a result of either accidental capture in fishing nets (notably off Newfoundland) or hunting. It is important that we know about such pressures on our seabird breeding populations, and ring recoveries reveal the areas in which such pressures are operating. Ring recoveries can demonstrate much more than just where a species goes and when. For example, the Sedge Warbier is a familiar small migrant passerine that breeds throughout the British Isles and winters in sub-Saharan Africa. A few years ago, Will Peach of the BTO showed that the annual survival of these birds was closely related to rainfall in the sub-Saharan wintering area in West Africa, low rainfall years, as occurred in the 1970s and 80s, resulting in decreased survival. Ring recoveries demonstrate remarkable differences in wintering areas for Sedge Warbiers from different parts of Britain and Ireland. All sub-Saharan recoveries of Irish-ringed Sedge Warbiers
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Figure 4. Autumn recoveries of Sedge Warbiers ringed in thc British Isles. (excludes two recoveries in Africa) • adults O Ist year have occurred in western Senegal (mostly as a result of ringing expeditions to the Parc National du Djoudj), while most Sedge Warbiers recovered further east in Africa have been ringed in eastern England or Scotland. Recoveries also suggest an interesting difference in migration routes between first year and adult Sedge Warbiers. Dßring the autumn movement south from the British Isles, first year birds appear to be more likely to use inland routes in France and Belgium than adults (Fig. 4). These differences in migration routes and wintering areas are, of course, of biological interest but they are also of importance for conservation of the species. The ring recoveries suggest that inland as well as coastal wetland habitats are important stop-over sites for Sedge Warbiers of different age classes, and that environmental conditions in certain areas of sub-Saharan West Africa may affect the 'health' of populations of Sedge Warbiers breeding in different areas of the British Isles. Most of the examples so far have referred to species which move well away from their breeding areas outside the breeding season. At the start of thc Migration Atlas project, a large number of ringers were consulted for their ideas. Many thought that one of the most difficult challenges would bc presenting recovery information.in an interesting way for resident species that were known to be largely sedentary throughout the year, such as thc House Sparrow. Some even suggested that such species should bc excluded from the book altogether. However, ring recoveries are revealing important and interesting information for these species. Locations of British House Sparrows outside
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thc brecding season and movements of grcater than 20 km betwcen brecding and non-brceding localions show that the great majority of recovenes occur within 20 km of the ringing site but, interestingly, birds ringed at Spurn Bird Observatory on the east coast (mainly in autumn) may be found brecding over 100 km inland. This suggests that some birds may move from mland to use coastal habitats in winter. Similar findings are suggested for Wrens. Previous intensive research based on ringing showed that Wrens move into lowland reedbeds, where they do not breed, during the winter months, and that individual Wrens may use the same reedbeds in consecutive winters. Ring recoveries suggest that the Wrens that breed in upland areas, such as the Pennincs and the Welsh hĂźls, move to lowland and coasal areas during the winter However, the recovery map also shows very clearly how little we know about the movements of upland-breeding Wrens and also about the Wrens which breed in Ireland. This is exactly how the Migration Atlas will identify gaps in our knowledge, so that ringers can help to fill these gaps in the future. Problems with ring recoveries Finally, some of the problems involved when describing bird movements using ring recoveries should be mentioned. It would be easy to look at this map of Swallow recoveries abroad (Fig. 5) and to conclude either that no Swallows die while crossing the Sahara or that Swallows do not cross the Sahara at all. Of course neither of these interpretations is true. Swallows are not recovered in
Figure 5. Recoveries abroad of Swallows ringed in thc British Isles
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the Sahara because of the low human population density, so that ihc chanccs of a dead bird being found and reported to the B T O are extremely slirn. Any Interpretation of movemenls based on recovery patterns must bc modified because these recoveries always depend on the presence of people to find and report birds, or ringers to re-catch birds, or observers to check for colour rings. For this reason, invited authors for individual species accounts in the Migration Atlas are mostly active ringers, as well as experts on the biology of their species. This means that they will be familiar with the interpretation difficulties They will bring in other sources of information on bird movements, such as observations of seasonal changes in distribution, counts of visiblc migrants, results from radar studies and other intensive research to give a rounded picture of the movements of their species. Many of the interpretation problems will be covered in an important introduetory chapter of the book, which will summarise differences in recovery chances across geographica! areas and through time for all species and look at how the way that birds are recovered may influence their chances of recovery. For example, 5 9 % of Cormorant recoveries for which we know the cause of death are attributed to hunting and 24% to accidental capture in fishing nets. In order to correctly interpret movements of Cormorants from these recoveries we need to k n o w whether Cormorants are more likely to be hunted o caught in nets, and therefore reported to the BTO, in some places than in others. Such causes of death are more likely to complicate the interpretation of recovery patterns than, say, deaths due to natural lllness, because hunting and capture in fishing nets can vary both in space and time, for example due to changes in the regulations governing fishing net use. In an introduetory chapter of the Migration Atlas, the ways in which factors such as hunting pressure, accidental capture in fishing nets and oil pollution have varied geographically and through time will be examined comprehensively for the first time. It should then be possible to assess how these changes may influence any interpretation of movements, based on recovery Information from almost 200 species, for species like the Cormorant. Conclusion The aim of this presentation was to give an overview of the B T O Migration Atlas project, the types of information the book will contain and lts great value for the practical conservation of the birds that frequent the British Isles throughout the year. The book will demonstrate how essential ringing is for understanding the seasonal movements of both well-known long-distance migrants and traditionally more sedentary species whose movements would bc difficult to detect if individual birds could not be identified and located throughout the year. A knowledge of all these types of movement is essential if we are to conserve our birds successfully. The Migration Atlas should reach publication in the year 2000, to form a fitting tribute to the great value of bird ringing and guide future research based on ringing at the Start of a new millennium. Chris W e m h a m British Trust for Omithology The Nunnery Thetford IP24 2PU E-mail: chris.wernham@bto.org
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