STEROID HORMONE LEVELS ARE RELATED TO CHOICE OF COLONY SIZE IN CLIFF SWALLOWS

One hypothesis to explain the extensive variation in colony size seen in most taxa is that individuals sort themselves among groups based on phenotypic characteristics that correlate with their performance in groups of different sizes. We investigated how baseline levels of the steroid hormones, corticosterone and testosterone, were associated with choice of colony size and the likelihood of moving to a different site in later years in colonially nesting Cliff Swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, USA, in 2000-2004. We sampled hormone levels of birds caught at colonies and, using mark-recapture, monitored their movement and choice of colony size through subsequent years in the study area. Maximum likelihood estimation and multistate model fitting (with program MARK) revealed that birds with baseline corticosterone levels above the average for the colony and time of sampling were less likely to choose a colony of the same size or larger in a later year than were birds with corticosterone levels below the average. This result held for Cliff Swallows in both fumigated (parasite-free) sites and colonies naturally infested with ectoparasites. Relative baseline corticosterone level was unrelated to the likelihood of movement between different colony sites, and corticosterone level measured after birds were held for 60 minutes was unrelated to either colony-size choice or the probability of movement. Males whose testosterone levels were above the average for the colony and time of sampling were more likely to choose a colony of the same size or larger in a later year than were ones whose testosterone levels were below the average, but the opposite pattern was found for females. The results indicate that steroid hormone level is a predictor of whether a Cliff Swallow will settle in a relatively small or large colony, and support the hypothesis that variation in colony size reflects, in part, a distribution of birds