VOL. LU JULY No. 7 1959 BRITISH BIRDS WADER MIGRATION IN NORTH AMERICA AND ITS RELATION TO TRANSATLANTIC CROSSINGS By I. C. T. NISBET IT IS NOW generally accepted that the American waders which occur each autumn in western Europe have crossed the Atlantic unaided, in many (if not most) cases without stopping on the way. Yet we are far from being able to answer all the questions which are posed by these remarkable long-distance flights. Why, for example, do some species cross the Atlantic much more frequently than others? Why are a few birds recorded each year, and not many more, or many less? What factors determine the dates on which they cross? Why are most of the occurrences in the autumn? Why, despite the great advantage given to them by the prevail­ ing winds, are American waders only a little more numerous in Europe than European waders in North America? To dismiss the birds as "accidental vagrants", or to relate their occurrence to weather patterns, as have been attempted in the past, may answer some of these questions, but render the others still more acute. One fruitful approach to these problems is to compare the frequency of the various species in Europe with their abundance, migratory behaviour and ecology in North America. If the likelihood of occurrence in Europe should prove to be correlated with some particular type of migration pattern in North America this would offer an important clue as to the causes of trans­ atlantic vagrancy. In this paper some aspects of wader migration in North America will be discussed from this viewpoint. SOURCES OF INFORMATION The basic source of information on the distribution of North 205 206 BRITISH BIRDS [VOL. LII American birds is the Fifth A.O.U. Check-List (1957). Brief accounts of migration routes of some species are given by Bent (1927-29), Pough (1951) and Snyder (1957); these have been supplemented by numerical data drawn from the state and regional avifaunas listed at the end of this paper, and by unpublished information and comments supplied privately by friends. British records are quoted from The Handbook (Witherby et ah, 1941), later information being derived largely from records published and abstracted in this journal. No claim is made to accuracy in detail, since much of the information used is unpublished or un- quantitative, and the author is not competent to judge the accuracy of many of the records of the rarer species. Minor errors in the data used will not impair their statistical validity. I am greatly indebted to Messrs. H. H. Axtell, J. Baird, J. A. Hagar and C. S. Robbins for their invaluable assistance in the preparation of this paper. AUTUMN MIGRATION PATTERNS IN NORTH AMERICA At the risk of oversimplification, the waders of eastern North America can be divided into six classes on the basis of their autumn migration patterns there. Spring migration will be discussed in a separate section. CLASS I. Coastal breeding birds of temperate and subtropical regions, usually with fairly short migrations. These include the following species, listed with their northern limits in summer and winter: American Oystercatcher* (39°N. to 35°N.), Eastern Piping Plover (si°-34°), Wilson's Plover (38°-28°), Eastern Willet (45°-37°) and Black-necked Stilt (32°-23°). CLASS II. Temperate and subarctic breeding species of the interior ("prairies", etc.), which occur east to the Atlantic coast (more commonly towards the south) on autumn passage. These include: Western Piping Plover, Long-billed Curlew (increasing but still rare on the Atlantic coast), Western Willet, Marblerf Godwit (rare), American Avocet (rare but increasing) and Wilson's Phalarope (rare). CLASS III. Temperate and subarctic species of inland habitat, which occur commonly on migration near the Atlantic seaboard but are less common on the coast itself: Killdeer, American Wood­ cock, Wilson's Snipe, Upland Plover, Spotted Sandpiper, Solitary Sandpiper. CLASS IV. Arctic or subarctic species of eastern or central Canada, which migrate in large numbers down the Atlantic coast and include the most numerous species there: Hudsonian Whimbrel, Greater and Lesser Yellowlegs, Short-billed Dowitcher, Least, Pectoral and Semipalmated Sandpipers. A few of these *Scientific names are given in an appendix; subspecific names are used where relevant to the discussion. VOL. LII] WADER MIGRATION IN NORTH AMERICA 207 species, notably the Pectoral Sandpiper and the Lesser Yellow- legs, also occur in large numbers in fresh-water habitats inland. A number of Holarctic species (e.g. Ringed and Grey Plovers, Turnstone, Knot, Dunlin and Sanderling) also belong to this group, but fall outside the scope of this paper. CLASS V. Western arctic species whose main migration route is through the centre of the continent, but which occur in greater or lesser numbers on the Atlantic coast in autumn: American Golden Plover (immatures), Long-billed Dowitcher, Stilt Sand­ piper, Western Sandpiper and Buff-breasted Sandpiper (rare on the coast). Baird's Sandpiper, although breeding east to north­ west Greenland, also has an overland migration route and rarely wanders east to the Atlantic coast. CLASS VI. Western or central arctic species whose autumn migration is considered to be largely offshore from eastern Canada to South America; these occur with varying degrees of abundance in the north-eastern United States but are rare in the south-east: American Golden Plover (adults), Eskimo Curlew, Hudsonian Godwit and White-rumped Sandpiper. Many Hudsonian Whimbrel also use this route, as does the Nova Scotia population of Eastern Willets, while some Pectoral Sandpipers and Lesser Yellowlegs probably use a shorter over-water route from the eastern United States to the West Indies. Each of these categories is somewhat arbitrarily defined, and some species may qualify for inclusion in more than one. For example, Pough (1951) suggests that most of the species in Class V use the over-water route of Class VI, at least in small numbers, but he gives little evidence except their sporadic occurrence at Bermuda. A more detailed analysis of the migration routes of North American waders would require more data than are at present available. Quantitative information. To supplement the above accounts with comparative numerical data, Table I has been constructed. This gives the size of some of the larger flocks or concentrations of each species which occur nowadays in four well-watched areas: the coasts of Massachusetts, New Jersey and Maryland, and the shores of Lake Erie near Buffalo, New York. The figures should on no account be used as quantative measures of relative abundance, since they have been collected in widely different ways, but they do give a rough idea of the comparative status of the various species in different parts of the eastern United States. Historical changes in abundance, which have been profound in many species, are indicated briefly in the list for Massachusetts, but are less thoroughly known else­ where. For comparison, the total numbers of British records of each species are also included in the table, and these will be discussed in the next section. 208 BRITISH BIRDS [VOL. LII TABLE I—HIGH COUNTS OF AMERICAN WADERS ON AUTUMN MIGRATION IN VARIOUS AREAS, WITH TOTAL NUMBER OF AUTUMN AND WINTER RECORDS IN THE BRITISH ISLES Data from Massachusetts and Maryland are the highest numbers normally expected in the more favoured coastal areas, those from New Jersey and Lake Erie are the maximum numbers observed in sample counts during 3 and 15 years respectively. Published data from Bailey (1955), Griscom and Snyder (1955), Urner and Storer (1949) and Stewart and Robbins (1958); unpublished estimates by J. Baird (Massachusetts), C. S. Robbins (Maryland) and H. H. Axtell (Lake Erie). Massa­ New Mary­ Lake Britain Class Species chusetts Jersey land Erie to 1940 since 1940 I American Oystercatcher — r 1 h — — — Piping Plover (subsp.) 20 be 75 10 h — — — Wilson's Plover — 1 2 li — — — Eastern Willet — e — e 100 f — — — Black-necked Stilt — — — — — — II Long-billed Curlew — b 1 — — — — Marbled Godwit 2 b 2 5 — — — Western Willet ro 5>f — e 1 — — 'American Avocet — — 1 — — — Wilson's Phalarope 1 2 — — — 1 III 'Killdeer 60 be 53 5<> g <>37 8 4 'Wilson's Snipe 12 a 10 log 1 e 1 e American Woodcock 5" 1 g — g — — — 'Upland Plover 4 a 45 — 15 1 e 'Spotted Sandpiper 6 a 48 12 f>7.? 5 — e 'Solitary Sandpiper 8 3 2 6 5 4 IV *Hudsonian Whimbrel 50 1.4H 75 11 . 2 *Short-billed Dowitcher 1,500 be 2,600 75 12 — — *Lesser Yellowlegs 1,500 c 500 '5" "3 13 c. 30 'Greater Yellowlegs 250 d qoo 25 14 4 5 'Least Sandpiper 100 300 300 253 4 1 'Pectoral Sandpiper 40 1.15 40 160 62 c. 90 Semipalmated Sandpiper 20,000 4,000 1,500 i,348 1 2 V Long-billed Dowitcher iS 40 — e 24 f 11 f *Baird's Sandpiper 1 — 20 5 2 Western Sandpiper 12 '52 75 5 — Stilt Sandpiper 10 22 25 6 — 1 Buff-breasted Sandpiper 2 1 — 1 18 10 VI 'American Golden Plover 40 bd 38 5 360 4 e 1 e 'Eskimo Curlew — b 8 — Hudsonian Godwit 20 bd 2 1 1 'White-rumped Sandpiper 50 d 35 10 10 '4 l3 NOTES—a: has decreased in recent years. b: decreased greatly in nineteenth century. c: has increased in last 50 years. d: has increased in last 25 years. e: probably overlooked. f: arbitrarily listed as this species, although it is not yet officially admitted to the British list, g: more common in winter, h : more common in summer. * denotes a species that has been recorded in Greenland. VOL. LII] WADER MIGRATION IN NORTH AMERICA 209 RECORDS IN GREAT BRITAIN In Table I the British records are separated into those made before 194Q (largely specimen records) and those made subsequently (largely sight records).