Signalling with a cryptic trait: the regularity of barred plumage in common waxbills.

Sexual signals often compromise camouflage because of their conspicuousness. Pigmentation patterns, on the contrary, aid in camouflage. It was hypothesized that a particular type of pattern-barred plumage in birds, whereby pigmented bars extend across feathers-could simultaneously signal individual quality, because disruptions of these patterns should be perceptually salient at close range and help assess plumage condition. Here we show that common waxbills (Estrilda astrild), which have extensive barred plumage, have more regular patterns as adults than as juveniles, and that adult males have more regular patterns than females. Both these differences are indicative of sexual signalling in species with conventional sex roles. More regular barred plumage was related to better body condition in adult males. Colour ornamentation traits were also related to aspects of quality, either the same as barred plumage (body condition) or a different one (good feather development), supporting both the 'redundant message' and the 'multiple message' hypotheses for the coexistence of multiple sexual signals. Although receiver responses to the regularity of barred plumage were not studied here, research on other species has shown that barred plumage can mediate social interactions. We conclude that using barred plumage as a signal of quality helps circumvent the functional compromise between camouflage and communication.