Drift By Wind

The influence of wind on the occurrence of birds outside their regular range is evident to any bird-watcher. Drift by crosswinds is one of the commonest ways in which migrants turn up as vagrants in places off their normal routes, and often several species from the same region appear simultaneously. Almost every autumn, on days with easterly winds, migrants that would normally pass down the western seaboard of continental Europe turn up on the east side of Britain (Lack 1960a , Evans 1968), and in periods with prolonged easterly winds, migrants that breed as far away as Siberia may appear. In contrast, at times of prolonged westerly winds, vagrants from North America turn up.

In clear weather, drift may not even be noticed, but if the birds meet mist or rain, 'falls' of migrants occur, leading to concentrations in particular localities, especially sea-coasts. In one incident on 11 October 1982, an estimated 15 000 Goldcrests Regulus regulus, 4000 Robins Erithacus rubecula and other birds settled on the Isle of May off eastern Scotland (Zonfrillo 1983). In another incident in

September 1965, an estimated 15 000 Common Redstarts Phoenicurus phoenicu-rus, 8000 Northern Wheatears Oenanthe oenanthe, 4000 Pied Flycatchers Ficedula hypoleuca, 3000 Garden Warblers Sylvia borin and many others suddenly appeared on one 3.2 km stretch of coast in eastern England (Davis 1966). Over a somewhat wider area, at least half a million birds of 78 species were estimated. Many of these birds seemed exhausted, and others were washed up dead on beaches. When the cloud broke and the wind veered, the majority of survivors disappeared, presumably having re-oriented and continued their journeys. Some individuals of these species normally pass over southeast England in migrating from Scandinavia towards Iberia, but in easterly winds the numbers are greatly swollen, and in rain large numbers alight and wait for conditions to improve. This type of weather-induced displacement and fall is likely to affect adults as well as immatures, but not necessarily in similar proportions.

The same occurs under easterly winds in spring, when species that do not breed in Britain, such as Red-spotted Bluethroats Luscinia svecica and Icterine Warblers Hippolais icterina, but which are heading for Scandinavia, may be deflected across the North Sea. Again, large numbers of birds may sometimes be involved, including rarer species with more easterly distributions, such as Red-throated Pipit Anthus cervinus and Rustic Bunting Emberiza rustica. At least one species, the Red-footed Falcon Falco vespertinus, is much commoner as a drift migrant in Britain in spring than in autumn, probably because it is a loop migrant whose spring migration route through Europe lies far to the west of the southward autumn route.

Arrivals of vagrants from more distant areas are likewise correlated with particular weather systems (Elkins 2005). For example, the appearance of Siberian birds in western Europe is associated with the occurrence of prolonged fine anticyclonic weather with easterly winds over the western half of Eurasia in September. The numbers of Siberian species seen in Britain thus vary greatly from year to year, with unusually large numbers in the autumns of 1975, 1982, 1992 and 2003 (Baker 1977, Howey & Bell 1985, Elkins 2005). In contrast, the appearance of North American birds in western Europe follows prolonged westerly gales at mid-latitudes. Westerly jet stream winds over the Atlantic can reach speeds of 250 km per hour, potentially bringing migrants over 5000 km from Canada to Britain in less than 24 hours. Almost all the North American vagrants that have appeared naturally in Europe are long-distance migrants which carry substantial body fat, as are the European species that have appeared in North America.

Again, the numbers of transatlantic migrants recorded in Britain vary greatly from year to year, depending on weather conditions, with unusually large numbers recorded in 1976, 1982, 1985 and 1995. They include passerines, shorebirds and waterfowl, as well as gulls and terns, with more than 60 species in total having been recorded (Table 10.2). The most frequent passerines include Gray-cheeked Thrush Catharus minimus, Blackpoll Warbler Dendroica striata, Red-eyed Vireo Vireo olivaceus and Rose-breasted Grosbeak Pheucticus ludovicianus, and the most frequent shorebirds are the Pectoral Sandpiper Calidris melanotus and White-rumped Sandpiper C. fuscicollis. Most of these species have a strong westeast component in the initial part of their autumn migration. Some of the North American wader species that occasionally cross the Atlantic (such as Pectoral Sandpiper Calidris melanotos and Baird's Sandpiper Calidris bairdii) breed in the

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