Hybridization as an Agent of Competition between Two Bird Allospecies: Ficedula Albicollis and F. Hypoleuca on the Island of Gotland in the Baltic

Abstract
Two closely related flycatcher species, Ficedula hypoleuca Pall. and F. albicollis Temm., have largely allopatric distribution ranges in Europe. However, they are sympatric on 2 islands in the Baltic, where, in contrast to the rest of northwestern Europe, F. albicollis is much more abundant than F. hypoleuca. A considerable rate of hybridization was found which may prevent F. hypoleuca from building up a large and viable population and thereby to outcompete F. albicollis from its only bastion in northwestern Europe, i.e., the Baltic islands, where it is probably a relict from a period with circumstances more favorable than at present. Should F. albicollis evolve the faculty of species discrimination, so that hybridization would no longer take place, this would mean the end of its population on the Baltic islands, although the change would enhance individual fitness.

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