Nonbreeding Distribution In Closely Related Species

Examples can be found of closely related migratory bird species that: (a) occupy completely (or largely) different regions in both breeding and non-breeding seasons (e.g. European Pied Flycatcher Ficedula hypoleuca and Collared Flycatcher F. albicollis in Eurasia-Africa); (b) occupy the same region in the breeding season but different regions in the non-breeding season (e.g. Meadow Pipit Anthus pratensis and Tree Pipit A. trivialis in Eurasia-Africa); (c) occupy different regions in the breeding season but the same region in the non-breeding season (e.g. Tawny Pipit A. campestris and Red-throated Pipit A. cervinus in Eurasia-Africa); or (d) occupy the same regions in both breeding and non-breeding seasons (e.g. Redwing Turdus iliacus and Fieldfare T. pilaris over much of their ranges in Eurasia, or Sedge Warbler Acrocephalus schoenobaenus and Reed Warbler A. scirpaceus over most of their ranges in Eurasia-Africa). Where species occupy the same range, they usually differ markedly in habitat or feeding habits, as indicated above.

Allopatric (or 'alloheimic') winter distributions are found in several groups of closely related migratory species. For example, among the Old World Hippolais warblers, the Melodious Warbler H. polyglotta, Olivaceous Warbler H. pallida, Upcher's Warbler H. languida and Icterine Warbler H. icterina occupy similar habitats in mainly different parts of Africa, while the Olive-tree Warbler H. olivetorum overlaps only with the Icterine, but is much larger (so presumably takes different foods). In the New World, closely related species of several songbird genera occur together in breeding areas, yet show range segregation in the non-breeding season, including species of Wilsonia, Vermivora, Dendroica and Oporornis warblers, Piranga tanagers, Catharus thrushes, and Empidonax, Tyrannus and Myiarchus flycatchers. Despite the many examples, it is impossible to prove that their segregation into geographically distinct wintering areas is a result of competition, but this remains the most likely explanation (Greenberg 1986). Also, in those that show geographical segregation, it is sometimes only the central parts of the range that are separated, with extensive overlap elsewhere. There are thus varying degrees of geographical segregation.

Some closely related species, which overlap in breeding range and diet, separate geographically in winter, with one staying and the other migrating. One possible explanation is that food supplies, which are sufficient for both species in summer, can support only one in winter, so to avoid competition the other leaves. An example is provided by the Herring Gull Larus argentatus and Lesser Black-backed Gull Larus fuscus, which in Europe share much of the same breeding range, but the latter winters, on average, much further to the south. The fact that the Herring Gull stays behind may be linked with its feeding more on land-based food supplies than the Lesser Black-backed Gull, which feeds more on fish at sea. Another example is provided by the Common Stonechat Saxicola torquata (resident over much of Europe) and the Whinchat S. rubetra (long-distance migrant between Europe and Africa).

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