Geographical Variation in the Nestling Coloration of Parasitic Cowbirds

Abstract

Cowbird nestlings show significant geographical variation in rictal flange color. A total of 24 cowbirds from the southwest, where the race Molothrus ater obscurus breeds, had yellow flanges whereas 73 from the remainder of the US and Canada, where the races M. a. artemisiae and M. a. ater breed, had white flanges. There is little or no intralocality variation. Nestling cowbirds with yellow and white flanges apparently are accepted and fed by hosts regardless of the color of the rictal flanges of the host's own young, but possibly the quality of host care received by a nestling cowbird is influenced by its rictal flange color and host responses are the selective pressure responsible for the geographical variation. Character displacement in nestling coloration is suggested by the fact that M. a. obscurus is the only race of M. ater that has yellow flanges and it is also the only race that is sympatric with another cowbird, M. aeneus, a species that apparently has white flanges. Character displacement could be caused by competition for host species and is further indicated by other lines of evidence.