How Natural Selection Shapes Birds' Tails

Abstract

Birds differ widely in the shape and size of their tails, yet until recently nearly all attempts to understand this morphological diversity have focused exclusively on the role of sexual selection for elaborate tail ornaments. In contrast, here we use aerodynamic models and comparative data to examine the extent to which natural selection can explain observed variation in avian tail morphology. A tail reduces a bird's overall lift-to-drag ratio (L/D), but the tail is important in maintaining stability over a range of flight speeds and in generating lift to help with turning and slow flight. As predicted, we found that large, open-country birds selected for high L/D have relatively short tails. Conversely, birds that need high maneuverability in order to feed aerially or avoid collisions in cluttered environments have longer tails. One prediction not supported by our data is that tails should also be elongated in species that need supplementary lift in order to wind-hover. Tail shape can also be important,...