Variability in size of groups in communal roosts: influence of age-class, abundance of individuals and roosting site

Abstract

Many birds use communal roosts, but how they use them is generally poorly known. For conservation purposes, and to better understand communal roosting behaviour, it is important to know which factors produce variability in group sizes at roosts. I studied the effect of age-class, abundance of individuals and roost site selection on the patterns of variability in group size in the Andean condor (Vultur gryphus). I fitted a variance-mean relationship known as Taylorâ??s Power Law (TPL) to the number of individuals that roosted daily in seven communal roosts, and made temporal (in each roost) and spatial (among roosts) comparisons of the relationship. The abundance of individuals in a roost fitted well to TPL both temporally and spatially. The variability in the abundance of individuals was significantly influenced by the roost selected, independent of the effect of the mean number of individuals. Moreover, the effect of the roost site on the variability of abundance of individuals was stronger than the effect of age-class. Accordingly, I highlight the importance of the roost site in modulating group size variability of communally roosting individuals. This understanding of the patterns of variability in communal roosts could aid in prioritizing sites for the conservation of a species.