Relationships of Steroid Hormones and Polygyny to Territorial Status, Breeding Experience, and Reproductive Success in Male Red-Winged Blackbirds

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Abstract
We collected 354 blood samples from territorial and nonterritorial male Red-winged blackbirds (Agelaius phoeniceus) to determine plasma levels of testosterone (T) and corticosterone (B) of a polygynously breeding bird, and to determine the potential influences of steroid hormone levels on obtaining breeding territories. In contrast to monogamous species, we found no peak in average T levels early in the breeding season and a five-week-long high plateau in T levels during the period when most nests were initiated. These endocrinologic characteristics may result from the long breeding season and resource defense polygyny of Red-wings. Territory owners had higher T levels during the breeding season than adult floaters. Subadult floaters had the lowest levels. Also, territory owners generally had the highest circulating levels of corticosterone during most of the breeding season. This implies greater energetic demands and stress on owners than on floaters. During early April the T levels of males for which we had complete breeding and territorial history information correlated significantly with their annual fledging success, and males with more breeding experience tended to have higher peak T levels. There was also a positive correlation between B levels in early April and eventual fledging success. Circulating levels of steroid hormones may affect territory ownership and reproductive success in this species.