I analyzed the occurrence of intermediates between Western (Aechmophorus occidentalis) and Clark's (A. clarkii) grebes, of mixed pairings, and of species composition in populations of Aechmophorus grebes in California and Oregon, USA. Western Grebes comprised 69% of the aggregated total of grebes identified while intermediates represented ~3.5% (41–46 individuals) in the populations investigated. I conclude that numbers of intermediates between purebred parental individuals have increased. Higher percentages of mixed pairings were observed at Lake Almanor; an aggregated 7.9% of nesting pairs were not composed of two purebred grebes of the same species. Statistically, mating remained strongly assortative.
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