A complex plumage pattern as an honest social signal

Abstract

Although birds are highly patterned animals, the efficacy and information content of complex colour signalling by combining multiple feathers has rarely been investigated. We examine a melanin-dependent plumage pattern in the red-legged partridge, Alectoris rufa , in a captive breeding facility in Spain. Both males and females are striking in that the majority of the entire side of the body is covered with a bold brown, black and white vertical pattern. During agonistic displays the feathers are aligned in nearly perfect, continuous dorsoventral ‘flank bands’. We report on the unusual morphology and developmental complexity of the pattern, including the number and anatomical location of the display feathers. We investigate whether the area of any of the three colours of individual feathers was a signal of a bird’s quality. Black alone varied significantly with age, sex and mass scaled to body size in both sexes, as well as the heterophil/ lymphocyte ratio of females. Birds in poor condition appear not to be able to pigment the feather completely to its edge. The negative correlation between the black areas of the display feathers of mated pairs and the negative correlation between male ornament size and reproductive success suggest various behavioural associations with colour. Plumage patterns provide many more potential opportunities for signalling an individual’s quality than overall colour alone, and they have the advantage that they can be turned off and on at the appropriate time through behavioural modification.