Natural selection on wing and tail morphology in the Pacific Swallow

Abstract

Severe climates provide excellent opportunities to study natural selection in a study population, particularly when surviving and non-surviving individuals are directly observed. Classic examples include the effects of cold, rainy weather on hirundines that can forage only when weather conditions allow flying insects to be active. Previous studies have demonstrated the effects of natural selection on wing and tail morphologies. However, these studies focused on breeding grounds, thus it was difficult to distinguish foraging costs, mating efforts, and migration costs. In the current study, we compared wing and tail morphologies between individuals that survived and those that died during a severe, rainy winter in the Pacific Swallow, Hirundo tahitica, a short-tailed non-migratory hirundine. We found that survivors had longer wings than non-survivors. In addition, survivors had shorter tails than non-survivors after controlling for the correlation with wing length. No significant differences in the keel or bill sizes were found between the two groups with and without controlling for other variables, indicating that selection on body size or foraging apparatus should be negligible. These results indicated that selection favors Pacific Swallows with relatively short tails during foraging, which provides an insight into the evolution of the diverse tail length in hirundines.