Functional relationship between group size and population density in Northwest Atlantic seabirds

Abstract

In many bird societies, group size results from the fusion and fission of existing groups. Group size dynamics should ideally be investigated in a population-level context since population density is likely to influence the rate of encounter between individuals and thus the value of joining or leaving groups. However, few studies have documented the shape of the functional relationship between group size and population density. The functional relationship can be of 2 types: (1) a strictly increasing function reflecting random joining and leaving, and (2) a saturating function indicating a preference for a range of group sizes so that a large increase in population density results primarily in more groups rather than larger groups. Using at-sea survey data from 2006 to 2010 in the Northwest Atlantic, I documented a saturating functional relationship between group size and population density in 5 species of seabirds and a strictly increasing function in 2 other bird species. The results suggest that upper limits to group sizes may be common in seabirds, perhaps reflecting feeding competition in larger groups. © Inter-Research 2011.