Morphological Correlates of Dominance in Captive Male Red-Winged Blackbirds

Abstract

Male birds of polygynous species are thought to be larger than females because large size is advantageous in competition for mates (Selander 1958, 1972); however, the mechanism by which large size confers such an advantage is not always clear. In Red-winged Blackbirds (Agelaius phoeniceus), females are strongly influenced by territory quality in making their choice of mates (Searcy 1979a). If large size gives a male an advantage in aggressive encounters with other males, and winning such encounters is important in securing and defending a territory, then a simple mechanism exists by which large size could lead to higher mating success for males. Thus, the relationship between body size and dominance in conflicts in male Red-winged Blackbirds is of evolutionary importance. I report here on the results of an aviary study designed to examine this relationship. A second objective of this study was to examine the relationship between dominance and epaulet size and color. Plumage characteristics may be used to signal social status in birds (Rohwer 1975, 1977); for example, correlations between dominance and plumage coloration exist in Harris' Sparrows (Zonotrichia querula; Rohwer 1975) and White-throated Sparrows (Z. albicollis; Harrington 1973). Blackening the epaulets of territory-owning male Redwinged Blackbirds causes many to lose their territories (Peek 1972, Smith 1972). Epaulets of first-year males are smaller and more orange than those of adults, and firstyear males are generally subordinate (Wiley and Harnett 1976). These observations suggest that features of the epaulet signal dominance status, although other interpretations are possible. My study was designed to determine whether dominance and epaulet characteristics are correlated in male Redwinged Blackbirds.