Effects of interactions with older males on behavior and reproductive development in first-year male red-winged blackbirdsAgelaius phoeniceus

Abstract

Male red-winged blackbirds Agelaius phoeniceus in their first year normally do not establish territories or breed. To investigate the possibility that interactions with older males might inhibit reproductive development of young males, we compared the behavior of first-year males in the presence and absence of older males. For three months during the first half of the breeding season, we studied mixed- and single-age groups of three males in standardized outdoor aviaries: two groups of three first-year males (FFF groups), two groups of one older and two first-year males (AFF groups), and one group of three older males (AAA group). In all groups one male established clear dominance, invariably the older male in AFF groups. The dominant male in FFF groups displaced subordinates less frequently than did the dominant older male in AFF groups early in the season, but equally frequently later. In May the largest testis weights of males in FFF groups were significantly lower than those of dominant older males in AFF groups. First-year males in the absence of direct interactions with older males can achieve levels of aggressive behavior comparable to those of a dominant older male in similar social environments but the seasonal development of their aggressive behavior occurs later and their testes in May are smaller than dominant adults'. This delay in the seasonal development of aggressive behavior in first-year males, independent of the immediate social environment, suggests that the age-dependent territoriality of males in this species is not strongly regulated by effects of older males on the development of younger males.