Nest spacing in relation to settlement time in colonial cliff swallows.

How colonial animals space their nests in relation to conspecifics may provide clues as to whether coloniality provides net benefits or occurs only because breeding sites are limited. We examined how nearest-neighbour distance varied in relation to settlement time in the highly colonial cliff swallow, Petrochelidon pyrrhonota, comparing observed nearest-neighbour distances to those expected if birds spread out to maximize nest spacing. Cliff swallows generally settled closer to each other than required by the available substrate, and clustered their nests closer in large colonies than in small ones. The first settlers at a colony site spaced themselves further apart than later arrivals but did not maximize nearest-neighbour distances. The first arrivals maintained greater nest spacing throughout the season than did birds that arrived later. Colony size and amount of nesting substrate had no effect on initial settlement distances of the first arrivals, but eventual nearest-neighbour distances declined with colony size. First arrivals may gain less from nesting with conspecifics and thus are less likely to cluster their nests than later arrivals, which may often be young or naïve birds that gain more from the social benefits of colonial nesting. The results are consistent with the presumed social advantages cliff swallows receive from coloniality and do not support the hypothesis that colonies result from nesting site limitation. Copyright 2000 The Association for the Study of Animal Behaviour.