Secondary sexual traits, parasites, and polygyny in red-winged blackbirds, Agelaius phoeniceus

Data from a 3-year study of red-winged blackbirds (Agelaius phoeniceus) were used to test the hypothesis that parasites (in this case, haematozoa) reduce male fitness and cause diminished expression of secondary sexual traits, which, in turn, are used by females to select parasite-free males as mates. There was no evidence indicating a fitness cost to being parasitized because parasitized males were as likely as unparasitized males to acquire a territory and to survive from one year to the next. Similarly, parasitized and unparasitized females did not differ with regard to how early they started nesting, how many eggs they laid, or their year-to-year survival. Secondary sexual traits, particularly intrasexual aggression, did reliably (>80%) reveal the parasite status of males. Plumage and morphological traits also allowed discrimination of parasitized and unparasitized females. However, apparent mating patterns were unrelated to either the males' or the females' parasite status. Only if genetic analyses reveal that unparasitized males actually realize higher productive success will these results potentially provide support for the parasite hypothesis of sexual selection.