Natural Resources, School of

 

Date of this Version

2005

Citation

Ecology, 86(11), 2005, pp. 2904–2915

Comments

Copyright 2005 by the Ecological Society of America. Used by permission.

Abstract

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
with inherently different neuroendocrinological characteristics.