Longitudinal patterns

Many migratory species show consistent east-west patterns in distribution year-round. Most long-distance migrants with wide west-east breeding ranges in Europe also extend in winter from west to east across Africa. Those species that breed only in western Europe (e.g. Melodious Warbler Hippolais polyglotta) winter mainly in the western half of Africa; and those species that breed only in eastern Europe or Asia winter mainly in the eastern half of Africa (e.g. Masked Shrike Lanius nubicus). There are exceptions, however, such as Lesser Whitethroat Sylvia curruca and Red-backed Shrike Lanius collurio, in which the birds from western Europe migrate via eastern Europe to eastern Africa, where they join more eastern breeders.

Similarly, in the New World, species that breed only in western North America winter only on the western side of Central America or beyond (e.g. Hammond's Flycatcher Empidonax hammondii, Townsend's Solitaire Myadestes townsendi, Grey Vireo Vireo vicinior), while others that breed only in eastern North America winter only in the eastern side of Central America and the Caribbean Islands or beyond (e.g. Yellow-throated Vireo Vireo flavifrons, Northern Parula Parula americana, Prairie Warbler Dendroica discolor). As in Eurasia, however, some species with a wide longitudinal spread in breeding distribution concentrate in winter in either eastern or western parts of a potential wintering range (e.g. Rusty Blackbird Euphagus carolinus, which breeds from coast to coast across boreal North America but winters entirely in southeastern parts of the continent). These species form exceptions to the more or less parallel migrations shown by most species with transcontinental breeding ranges.

Closely related migrant species tend to segregate not only from one another in their breeding or wintering areas, but also from similar species that are resident year-round in those areas. Among Eurasian-Afrotropical migrants, chats, wheatears and shrikes generally occur in African habitats devoid of resident species, and segregate from one another, while various silviid warblers overlap broadly with residents, but forage in different parts of the habitat (Moreau 1972). In the New World, such patterns are commonplace, and ecological segregation is often evident from detailed study. For example, 20 different warbler species occur in Jamaica, two being present year-round and the other 18 in winter only (Lack & Lack 1972). These various species show marked ecological differences from each other: one climbs branches and twigs, one feeds from hanging dead leaves and twigs, one lives in thick herbage, and one fly-catches. Of the five ground-feeders, four are separated by the type of terrain, while two that inhabit the same terrain are separated by feeding method - one probes and the other picks. Of the 11 leaf-gleaners, the two resident species are separated largely by habitat, the rest mainly by feeding method, the parts of the tree in which they forage, or the type of leaf from which they take insects. Hence, even where a relatively large number of similar species occur in the same geographical area, each differs ecologically from all the others in a way that would be expected to lessen competition for food between them.

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