Nest decorations: an 'extended' female badge of status?

Extended phenotypes as signals are widely distributed among animal taxa. For example, many bird species build eye-catching nests or structures, which can potentially mirror the quality or ability of the builder. Rock sparrow, Petronia petronia, nests are usually overly decorated with feathers belonging to different species. Feather carrying in this and other species seems to play a role beyond their supposed thermoregulatory function, that is, to provide insulation to eggs and developing chicks. In this study, we documented for the first time this intriguing pattern of behaviour in the rock sparrow and experimentally tested its potential role as a sexually selected or status signal by means of a feather supplementation experiment carried out in two distinct populations from Italy and Spain. We found that females were responsible for feather carrying, laid larger clutches and provisioned their young at a lower rate in those nests with experimentally added feathers. Decorated nests sustained fewer intrusions by floater individuals and were defended with greater intensity by both parents than control nests, which supports the role of nest ornamentation as a status signal to conspecifics. Presence of experimental feathers did not significantly increase the frequency with which males provisioned their young but males tended to desert their brood less often and spent more time guarding the brood in experimental nests, indicating that feather presence may also play a role in an intersexual context. Overall, our results allow us to exclude the thermoregulation hypothesis as a likely explanation for the occurrence of these decorations and provide partial evidence for the idea that feather carrying conveys information to the partner and potential competitors. Our study thus supports the notion that nonbodily traits serving a direct (naturally selected) function can also evolve a signalling component.