BLOOD PARASITES AND DOMINANCE IN CAPTIVE BLACKBIRDS

Evidence of the pathogenicity of haematozoa in wild bird populations i s limited, possibly because infected birds alter their behavior to avo id the costs of being parasitized. We tested this hypothesis by examin ing dominance relationships relative to parasite status in captive Red -winged Blackbirds Agelaius phoeniceus, and Brown-headed Cowbirds Molo thrus 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. Dominan t parasitized individuals were not consistently larger than the uninfe cted individuals they dominated, although other, untested asymmetries might have allowed the infected birds to overcome any cost associated with being parasitized.