Blood Parasites and Dominance in Captive Blackbirds

Evidence of the pathogenicity of haematozoa in wild bird populations is limited, possibly because infected birds alter their behavior to avoid the costs of being parasitized. We tested this hypothesis by examining dominance relationships relative to parasite status in captive Red-winged Blackbirds Agelaius phoeniceus, and Brown-headed Cowbirds Molothrus ater. There was some evidence that uninfected individuals tended to be dominant to infected individuals, but the pattern was variable, even within two studies involving male Red-winged Blackbirds. Dominant parasitized individuals were not consistently larger than the uninfected individuals they dominated, although other, untested asymmetries might have allowed the infected birds to overcome any cost associated with being parasitized.