A plumage patch signalling nest occupancy is shaped by social environment

The cliff swallow (Petrochelidon pyrrhonota) is a colonial bird that nests in groups of different sizes. Adult cliff swallows have a prominent white forehead patch that we hypothesize functions as a signal of nest occupancy to mitigate costly nest intrusions, especially in crowded nesting colonies. To test this hypothesis, we measured variation in forehead patch area and brightness relative to sex, colony size and date of collection for 241 preserved specimens collected over 43 years. Additionally, we experimentally tested signal function by darkening the foreheads of nesting birds and observing intraspecific interactions. Consistent with a signalling role, we found that females had larger patches than males, patch area increased with colony size, and nests of birds with darkened forehead patches attracted significantly more visits from transient cliff swallows than did nests with no darkened owners. These results suggest that variation in social environment, both in space and time, as well as intersexual differences in incubation behaviour shape the evolution of this conspicuous plumage patch.