Longdistance Dispersal

Ring recoveries of dispersed birds tend to occur in all directions from the natal area, but fall off exponentially with increasing distance (Chapter 17). On this basis, while most birds disperse relatively short distances, occasional individuals are likely to disperse much further than others, beyond the regular range. Such dispersing individuals would be expected to occur in any direction from the regular range, and not just in the zone between the breeding and wintering range. They would also be expected to occur mainly in the post-breeding period, before autumn migration. Records fitting such a pattern have been obtained for the Scissor-tailed Flycatcher Muscivora forficatus which breeds in the southern prairie region of North America and winters on southern Caribbean islands. Individuals of this distinctive species have been seen in almost every North American State and in Canada from British Columbia to Nova Scotia, mainly in late summer after breeding but also in spring. These birds evidently move long distances in almost any direction from their breeding areas. In the British Isles, dispersal vagrants are most likely to derive from species whose nearest breeding areas lie in western continental Europe, such as Tawny Pipit Anthus campestris, Melodious Warbler Hippolais polyglotta, Woodchat Shrike Lanius senator and Ortolan Bunting Emberiza hortulana. Individuals of these species occur mainly in late summer, but also at other times (see Figure 10.5 for Ortolan Bunting).

Some waterbirds that breed early in the year in subtropical and Mediterranean latitudes disperse mainly northwards after breeding, sometimes for hundreds of kilometres, as many southern wetlands dry out. Several heron and ibis species participate in these movements, including Great White Herons Ardea alba, Squacco Herons Ardeola ralloides and Black-crowned Night Herons Nyctocorax nycticorax, as also do Brown Pelicans Pelecanus occidentalis, Little Blue Herons Egretta caerulea, Snowy Egrets Egretta thula, Black-necked Stilts Himantopus mexi-canus and Bald Eagles Haliaeetus leucocephalus in North America (Townsend 1931). This northward dispersal seems more marked in North America than in Europe, possibly because of the greater areas of wetland habitat remaining in southern parts of the continent, and their larger populations of waterbirds. Some seabirds also disperse northwards after breeding, resulting for example in the late summer appearance of occasional Blue-footed Boobies Sula nebouxii and Magnificent Frigate Birds Fregata magnificens at the Salton Sea in southern California, along

Ortolan Bunting


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