Sexual Dimorphism and Population Differences in Structural Properties of Barn Swallow (Hirundo rustica) Wing and Tail Feathers.

Péter L Pap,Diego Rubolini,Csongor I Vágási,Nicola Saino,Lőrinc Bărbos,Gergely Osváth,José Miguel Aparicio,Orsolya Vincze,Piotr Matyjasiak,Anders Pape Møller,Alexandre Roulin

doi:10.1371/journal.pone.0130844

Abstract

Sexual selection and aerodynamic forces affecting structural properties of the flight feathers of birds are poorly understood. Here, we compared the structural features of the innermost primary wing feather (P1) and the sexually dimorphic outermost (Ta6) and monomorphic second outermost (Ta5) tail feathers of barn swallows (Hirundo rustica) from a Romanian population to investigate how sexual selection and resistance to aerodynamic forces affect structural differences among these feathers. Furthermore, we compared structural properties of Ta6 of barn swallows from six European populations. Finally, we determined the relationship between feather growth bars width (GBW) and the structural properties of tail feathers. The structure of P1 indicates strong resistance against aerodynamic forces, while the narrow rachis, low vane density and low bending stiffness of tail feathers suggest reduced resistance against airflow. The highly elongated Ta6 is characterized by structural modifications such as large rachis width and increased barbule density in relation to the less elongated Ta5, which can be explained by increased length and/or high aerodynamic forces acting at the leading tail edge. However, these changes in Ta6 structure do not allow for full compensation of elongation, as reflected by the reduced bending stiffness of Ta6. Ta6 elongation in males resulted in feathers with reduced resistance, as shown by the low barb density and reduced bending stiffness compared to females. The inconsistency in sexual dimorphism and in change in quality traits of Ta6 among six European populations shows that multiple factors may contribute to shaping population differences. In general, the difference in quality traits between tail feathers cannot be explained by the GBW of feathers. Our results show that the material and structural properties of wing and tail feathers of barn swallows change as a result of aerodynamic forces and sexual selection, although the result of these changes can be contrasting.

Highlights

Morphology of wing and tail feathers of birds may vary depending on aerodynamic forces to which they are subject and on their function in intra- and inter-sexual signaling [1,2,3,4,5,6]
Our results show that the material and structural properties of wing and tail feathers of barn swallows change as a result of aerodynamic forces and sexual selection, the result of these changes can be contrasting
Wing feathers are exposed to strong aerodynamic forces during flight, and, flight style might determine the structure of these feathers [6,7]

Summary

Introduction

Morphology of wing and tail feathers of birds may vary depending on aerodynamic forces to which they are subject and on their function in intra- and inter-sexual signaling [1,2,3,4,5,6]. The predicted consequence of the difference between sexes in tail length is sexual dimorphism in the structural components of these feathers If such sex-dependence is driven by dimorphism in tail length, the difference between males and females in tail length may solely explain the simpler structure of long rectrices. If the sexual difference in feather traits persists even after controlling for dimorphism, sex related variation in tail length does not solely explain the sexual dimorphism in feather structure This may show that males do not fully compensate for elongation of the tail

Methods

Results

Conclusion