The Role of Plumage Polymorphism in Dominance Relationships of the White-throated Sparrow

Abstract

Abstract We studied dominance behaviors of captive winter flocks of White-throated Sparrows (Zonotrichia albicollis) in central Pennsylvania. Preliminary tests with a small group of birds indicated that there might be differences in behaviors between color morphs if age and sexual differences were controlled. Studies of two larger groups produced considerable variation in results between the groups. One group displayed striking correlations between dominance ability and color. In this group white adult males dominated others more than tan adult males did, whereas tan immature males, adult females, and immature females were more often dominant than were white immature and female birds. The second group did not display such striking differences. Tan adult and immature females were more dominant than white females, as in the first group, but not significantly. Immature males displayed the opposite trend from the first group-white morphs were more dominant than tans. Adult males in the second group showed no clear trend. We also found differences in dominance between tan and white females that appeared to depend on season, white birds dominating tan birds in a small group in the spring, a reversal of relationships documented in the fall. Within age-sex classes, dominant females in both large groups tended to be duller in plumage brightness (scaled as an index) than were subordinate females. In one of the two large groups, duller immature males were more dominant than brighter ones, while brighter adult males tended to be more dominant than duller ones. We suggest that the relationships between immature male plumage and dominance may be influenced by the morph of adult males of specific dominance status in the flock. Spring plumage indices were more often correlated with fall dominance differences than were fall plumage conditions, suggesting a genetic or causal relationship between dominance and plumage rather than a proximate (status signaling) function. Recorded morph frequencies from this study and the literature support the hypothesis that selection is balanced between the two morphs and is dependent on the advantages or disadvantages of aggressiveness within a sex.