Meeting Abstract
Animals compete for resources that are required for breeding and self-maintenance such as nesting sites, mates, and food. High levels of competition can be a source of stress for individuals, as suggested by a many studies showing increased stress hormones (glucocorticoid levels) when densities are high. For example, in tree swallows, Tachycineta bicolor, females can compete vigorously for nest boxes and there is evidence that in dense populations, females exhibit increased levels of glucocorticoids (CORT). Previous studies however did not distinguish between the effect of population density, settlement patterns, and the frequency of aggressive encounters. Here we present an experiment where we manipulated the distances between nest-boxes after settlement to manipulate the frequency of aggressive interactions between females while keeping the population density constant. Specifically after settlement, we moved a randomly selected group of boxes closer together and another group farther apart. We carried out behavioral observations to estimate the frequency of aggressive interactions and obtained blood samples from females both before and after the manipulation to look at changes in CORT. Most aggressive interactions were observed at the settlement period, before the manipulation. We found no effect of the nest-box distance manipulation on female CORT levels or the frequency of aggressive interactions. We discuss these results in the context of how population density may affect stress hormones in natural populations.
