White tail spots in breeding Barn Swallows Hirundo rustica signal body condition during winter moult

The determinants and function of pigmentation of feathers and other tissues have been the focus of a large number of studies, particularly with respect to socio‐sexual communication. However, many birds exhibit depigmented white spots or bars on their feathers whose function is poorly understood. Here we assess whether white feather spots reflect phenotypic condition at the time of moult by investigating the covariation between spot size or shape and condition‐dependent feather growth rate, as gauged by width of the growth bars on the tail feathers of Barn Swallows. We found that feathers with higher growth rates had larger, less rounded white spots. In addition, variance in spot perimeter for a given spot area was larger in males than in females. This study is the first to provide evidence that features of white markings on feathers directly reflect body condition at the time of moult and can therefore reliably signal phenotypic quality in the context of socio‐sexual communication. In addition, the study highlights the potential communication function of the shape and not just the size of colour signals.