Species and pseudospecies: the structure of crossbill populations
1992
Knox, A. G. (Alan G)
The holarctic crossbills Loxia have often been regarded as one of the classic examples of avian panmixia, despite a large number of named races, clines and other geographic variation. There are also reports of two or more ‘subspecies' nesting sympatrically, without interbreeding. Crossbills feed almost exclusively on conifer seeds. Eruptions occur at times of cyclical cone crop failures; the birds involved may then breed in new areas for one or more years. Rather than being nomadic in their movements, to explain the clinal and/or area effects, it is suggested that erupting birds are reasonably faithful to core breeding areas, to which some subsequently return. Genetic continuity within a species is maintained through adjoining or overlapping core breeding areas. Although normally connected by intermediates, some populations apparently do not interbreed when they come together temporarily during irruptions. At such times, they behave as separate species, to which the term pseudospecies is applied. Mechanisms promoting rapid speciation and founder effects are discussed.
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