An unexpected age-related pattern in feather growth contributes to age-dependent ornament expression in a passerine bird
Abstract
The length of the ornamental tail rectrices has been shown to be an important condition-dependent sexually selected trait in various bird species. Similarly, the shape and depth of the tail fork may represent a costly sexual signal that enhances individual attractiveness but compromises flight manoeuvrability.
Avian flight feathers, including ornamental rectrices, generally become longer in successive years. In this study, we analysed age and sex differences in the length of central tail feathers and the depth of the tail fork in 141 Barn Swallows observed repeatedly over consecutive years.
As expected, tail fork depth was mainly determined by the length of the outer ornamental tail streamers and to some extent to the non-ornamental central feathers, with individuals with deeper forks having longer outer streamers and shorter central tail feathers. Central tail feathers were sexually dimorphic and shorter in males than females.
We also found an unexpected pattern of shortening of central tail feathers with increasing age that contributed to an age-related increase in tail fork depth in both sexes. The shortening of central tail feathers was achieved by the reduction of daily feather growth rates and not by feather abrasion.
To the best of our knowledge, our study provides the first evidence of age-related continuous shortening of flight feathers in an avian species and suggests a possible sexual role of central tail feather length in Barn Swallows.