Morphological Differentiation of Brown-Headed Cowbirds in the Okanagan Valley, British Columbia

We investigated morphological variation in Brown-headed Cowbirds Molothrus ater in the Okanagan Valley, southern British Columbia, Canada. We found that cowbirds were significantly differentiated in body measurements (wing, tail, tarsus, and culmen lengths) from both western subspecies, M. a. artemisiae and M. a. obscurus. The pattern of differentiation in body shape suggests Okanagan Valley cowbirds are not hybrids of the two above-mentioned subspecies, contra the situation in northern California. Rather, the Okanagan birds appear to have been present and isolated there for sufficient time for morphological differentiation to have occurred. This finding calls into question the hypothesis that many cowbird hosts do not recognize the parasite's eggs because of the recency of arrival of the cowbird in western North America.

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