Abstract

Hummingbirds present a unique combination between extremely high life costs and a number of efficient adaptations to fuel these demands. In addition to cognitive abilities, territorial hummingbirds display aggressive behaviors that allow for access to better food resources. In year-round territorial species, male-male territorial aggression is similar between breeding and non-breeding seasons; however, the endocrine mechanisms underlying control of territoriality during these distinct seasonal periods may differ. In many species, testosterone (T) triggers increased aggression during the breeding season whereas territoriality in the non-breeding season can be regulated by circulating the biologically inert sex steroid precursor dehydroepiandrosterone (DHEA) and converting it to T in target tissues. The seasonal hormonal regulation of hummingbird territorial behavior has heretofore been unknown. Our goal was to assess seasonal changes in sex steroids, territorial aggression levels, and body condition during reproductive and non-reproductive seasons in hummingbirds. To validate the use of cloacal fluid (CF) for the study of sex steroids, steroid levels in plasma and CF were correlated in Sephanoides sephaniodes. During the reproductive season, Calypte. anna, Archilochus alexandri, and Selasphorus rufus males showed high levels of T that were positively correlated with aggression, but the relationship between T and body condition was not consistent across species. As expected, T levels in females were significantly lower than in males in all seasons, however still detectable. During the non-reproductive season, CF DHEA of Calypte anna was high and positively correlated with aggressive behaviors and body condition. Our results suggest that hummingbirds display aggressive behaviors that could be linked to different hormones during the breeding and non-breeding seasons.