Table 102 The main seasons and proposed means of occurrence of some vagrants to the British Isles Some species occur in more than one category

Transatlantic autumn vagrants Green-winged Teal Anas crecca carolinensis, Ring-necked Duck Aythya collaris, Sora Rail Porzana carolina, American Golden Plover Pluvialis dominica, Semi-palmated Sandpiper Calidris pusilla, Least Sandpiper Calidris minutilla, White-rumped Sandpiper Calidris fuscicollis, Baird's Sandpiper Calidris bairdii, Pectoral Sandpiper Calidris melanotos, Lesser Yellowlegs Tringa flavipes, Solitary Sandpiper Tringa solitaria, Spotted Sandpiper Actitis macularia, Wilson's Phalarope Phalaropus tricolor, American Robin Turdus migratorius, Red-eyed Vireo Vireo olivaceus, Black-and-white Warbler Mniotilta varia, Northern Parula Parula americana, Blackpoll Warbler Dendroica striata, Northern Waterthrush Seiurus noveboracensis, Rose-breasted Grosbeak Pheucticus ludovicianus, Bobolink Dolichonyx oryzivorus, Baltimore Oriole Icterus galbula Possible autumn mirror-image Greenish Warbler P. tochiloides (breeds Finland-Siberia), Arctic Warbler Phylloscopus borealis migrants (breeds northern Scandinavia eastwards), Richard's Pipit Anthus novaeseelandiae, Pechora

Pipit Anthus gustavi, Rustic Bunting Emberiza rustica (breeds Scandinavia eastwards), Little Bunting Emberiza pusilla (breeds Scandinavia eastwards), Yellow-breasted Bunting E. aureola (breeds Finland eastwards)

Possible reverse-direction Grey-tailed Tattler Heteroscelus brevipes (breeds northeast Siberia), Pacific Golden Plover autumn migrants Pluvialis fulva (breeds eastern Siberia and western Alaska), Red-necked Stint Calidris ruficollis (breeds

Siberia), Rose-coloured Starling Sturnus roseusa (breeds Turkey-southern Asia), White's Thrush Zoothera dauma (breeds Siberia), Barred Warbler Sylvia nisoria (breeds mid-Europe eastward), Pallas' Leaf Warbler Phylloscopus proregulus (breeds Siberia), Yellow-browed Warbler Phylloscopus inornatus (breeds Siberia), Dusky Warbler Phylloscopus fuscatus (breeds Siberia), Lanceolated Warbler Locustella lanceolata (breeds Siberia), River Warbler Locustella fluviatilis (breeds central Europe into Siberia), Paddyfield Warbler Acrocephalus agricola (breeds central Asia), Tennessee Warbler Vermivora peregrina (breeds eastern North America), Red-breasted Flycatcher Ficedula parva (breeds southeast Europe eastward), Yellow-browed Bunting Emberiza chrysophrys (breeds Norway eastwards)

Possible transatlantic spring American Coot Fulica americana, Cape May Warbler Dendroica tigrina, Brown-headed Cowbird overshoots Molothrus ater, Cedar Waxwing Bombycilla cedrorum, American Robin Turdus migratorius,

Yellow-headed Blackbird Xanthocephalus xanthocephalus Possible long-distance Eurasian Collared Pratincole Glareola pratincola, Great Spotted Cuckoo Clamator glandarius, Marmora's spring overshoots Warbler Sylvia sarda, Black-eared Wheatear Oenanthe hispanica, Black-headed Bunting Emberiza melanocephala

Other long-distance autumn Olive-backed Pipit Anthus hodgsoni, Siberian Stonechat Saxicola toquata maura, Siberian Thrush vagrants from Siberia Zoothera sibirica, Black-throated Thrush Turdus ruficollis atrogularis, Pallas's Grasshopper Warbler

Locustella certhiola, Lanceolated Warbler Locustella lanceolata, Yellow-browed Warbler P. inornatus, Raddes Warbler Phylloscopus schwarzi aAutumn records only; also occurs in spring.

northwestern sector of North America and first make an easterly flight across that continent, so it is easy to see how they might cross the Atlantic by continuing in the same direction, rather than veering south or southwest for South America. Although most North American vagrants to Europe turn up in autumn, others appear in spring, raising the question whether they may have overwintered in the Old World (Nisbet 1959a).

Far more landbirds seem to cross the Atlantic at mid-latitudes from west to east than from east to west. There are probably two reasons for this. First, the prevailing winds at these latitudes blow from west to east, making it easier for North American birds to be carried to Europe than for European birds to reach North America. Second, many thousands of landbird migrants fly south or south-southeast over the western Atlantic each autumn, as they travel from northeastern North America to Caribbean Islands or to northern South America. Some reach more than 2000 km out from the North American coastline, as shown by ship-born radar, and are thus vulnerable to westerly winds and poor visibility (McClintock et al. 1978). However, no equivalent overwater movement occurs off the western seaboard of Europe, because most birds take the overland route through France and Spain to West Africa. Even the Canary Islands, which lie only 50 km off the West African coastline, receive extremely few individuals from the many millions of migrants that pass southwest each autumn through northwest Africa.

Widespread mist over the sea can also cause birds to fly in the wrong direction (Alerstam 1990b). Once within the mist, migrants can become disoriented, and fly in various directions, as shown by radar. If the mist is sufficiently widespread, birds could end up tens or even hundreds of kilometres from their normal routes, coming down and settling on the first land they encounter, to be recorded as vagrants.

It is not just landbirds that are susceptible to the effects of wind over water. Strong onshore gales offer the best prospects for bringing normally pelagic sea-birds within view of coastal headlands. Counts exceeding 10 000 birds per hour have been achieved at some sites, including rarities. In addition, small seabirds are also frequently blown far off course, sometimes for hundreds of kilometres, while some normally pelagic species, such as Sabine's Gulls Larus sabini and Grey Phalaropes Phalaropus fulicarius, sometimes appear at inland lakes (Elkins 2005). Other small seabird species are occasionally found as 'wrecks' of dead and exhausted birds over land, the Little Auk Alle alle being a frequent victim in Europe (Murphy 1936, Bailey & Davenport 1972, Underwood & Stowe 1984; review Newton 1998a). Juveniles often predominate, and many appear in poor condition, but it is usually uncertain whether poor condition predisposed birds to the effects of strong winds, or whether the winds caused the poor condition through preventing feeding. Not all such wrecks relate to birds on migration, however, because some involve birds near breeding or wintering areas.

Because drift affects mainly birds that are on their normal migration, it usually involves lateral (west or east) displacements, but displacements to the north or south of the usual range occur in extreme conditions. The most striking examples involve seabirds carried by hurricanes hundreds of kilometres beyond their usual range. Such conditions push coastal birds northwards on both the east and west sides of North America, and sometimes far inland. In the Atlantic, such cyclonic storms follow a circular route. They usually begin in equatorial waters off the African coast, travel westward across the Atlantic into the Caribbean or Gulf of Mexico, then swing northward through eastern North America, weakening all the time, to emerge eastward into the Atlantic in the region of Newfoundland. Because winds circle clockwise around a hurricane, they could carry seabirds from either west (on the northern edge) or east (on the southern edge) onto land. As the winds spiral inwards, some seabirds may become caught in the calm of the eye, tend to stay there, and move north with it. Others move ahead of the storm, notably Sooty Terns Sterna fuscata, which on some Caribbean Islands are known as hurricane birds for the warning they provide. Most of the records of tropical seabirds off the coasts of the northern States and Canada occur soon after hurricanes, which would have carried them north. They include White-tailed Tropicbirds Phaethon lepturus, Sooty Terns Sterna fuscata, Black Skimmers Rynchops nigra, Magnificent Frigate Birds Fregata magnificens, and various petrels. Hurricanes may thus be important agents of vagrancy in seabirds (Murphy 1936, Enticott 1999).

It is uncertain how much landbirds are affected by hurricanes, which occur during the autumn migration season, but records abound of offshore islands and ships being inundated with passerine migrants at such times. During one incident on 27 August 1926 in the Gulf of Mexico: 'Birds which seemed to be migrating landbirds, chiefly swallows, filled the air about the vessel and were so thickly strewn on deck that they could be scooped up by the armful' (Murphy 1936). Similarly, on 25 September 1987, hurricane 'Emily' deposited 10 000 Bobolinks Dolichonyx oryzivorus and thousands of Connecticut Warblers Oporornis agilis on Bermuda (Case & Gerrish 1988). While hurricanes may kill many migrating passerines and carry others off course, they are not obviously responsible for the bulk of transatlantic landbird vagrancy (Nisbet 1963).

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