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

Abstract

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