Meeting Abstract
Tail feather (rectrix) morphology is highly variable among birds. This variation is driven both by the role of the tail as a functional part of the aerial locomotor apparatus and as a display structure. Indeed, the most spectacular tail feathers are the result of sexual selection for elaborate rectrices used for display. Previous work has shown that tail fan shape is correlated with the morphology of the underlying caudal skeleton that supports the rectrices, with forked- and graduated-tailed taxa taxa exhibiting characteristic pygostyle shapes. This study tests whether this general pattern holds in species with sexually dimorphic tail feathers in an effort to understand whether evolution in rectricial morphology is linked with phenotypic changes in the caudal skeleton. Feather and skeletal morphology was quantified in selected dimorphic passeriform species and closely related monomorphic species representing a diversity of tail shapes and body sizes. The dimorphic species examined include White-Rumped Shama (Muscicapidae), Scissor-Tailed Flycatcher (Tyrannidae), Pin-Tailed Whydah (Viduidae), Boat-Tailed Grackle, (Icteridae). Skeletal morphology was quantified using a combination of linear and geometric morphometics. Permutational MANOVA was used to assess differences in skeletal morphology between males and females of each taxon. None of the examined taxa exhibit sexual dimorphism in pygostyle shape. In just one taxon, Pin-Tailed Whydah, males have a significantly larger pygostyle surface area than females. These results suggest that evolution of large tail feathers does not require changes in the axial skeleton to support this heavy and aerodynamically costly structure. As such, caudal skeletal morphology in these taxa appears more influenced by phylogeny and locomotor function than by its relationship with tail fan shape.
