Experimental evidence for a phenotypic trait as an age-dependent intrasexual social signal between familiar individuals

Signals of social status can function to alert conspecifics of an individual's dominance, social rank and fighting ability. Such signals are beneficial in resource-limited systems because they can act to reduce conflict between competitors, especially when individuals are unfamiliar with each other. Here, we investigate the social communication function of a conspicuous plumage characteristic of a Neotropical migrant songbird, the black-throated blue warbler, Setophaga caerulescens, after breeding territory establishment when individuals are familiar with neighbours. To do this, we experimentally manipulated males' white wingspots, a putative signalling trait, and subsequently observed their behaviour. We found that males with experimentally eliminated wingspots had more frequent close-range interactions within and at their territory boundaries with conspecific males and had more agonistic interactions involving aggressive vocalizations compared to control males, although the effects differed according to age class. However, frequency of close-range interactions with female conspecifics was not influenced by wingspot manipulation. Finally, wingspot manipulation did not influence males' territorial song rates or responses to a simulated intruder, indicating no change in individual self-perception. Our study provides evidence that, even when individuals are familiar with each other and on established territories, social status signals continue to convey important information.