A plumage patch signaling 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
signaling 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 behavior shape the evolution of this
conspicuous plumage patch.