Evolution of short tails and breakdown of honest signaling system during a severe winter in the Pacific swallow Hirundo tahitica

Abstract

In dynamic selection regimes, an honest signal that indicates high quality in favorable environments might be negatively related to quality in unfavorable environments, though empirical evidence is scarce. Here, we studied the relationship between plumage ornaments and physiological state in the Pacific swallow Hirundo tahitica on Amami Oshima Island in 2 years with contrasting weather conditions: a once-in-a-century severe winter and a mild winter in the following year. Previous studies have shown that, during the severe winter, many Pacific swallows, particularly those with long tails, died whereas no carcasses were found in the next year, indicating relaxed selection during mild years. Here we showed that after the severe winter, the offspring generation had shorter tails than the parental generation, which could not be explained by age or yearly difference in tail length, indicating the evolution of short tails. Moreover, the relationship between tail length and body condition differed between years: longer-tailed swallows had a better body condition in the mild winter, whereas the reverse tendency was found in the severe winter. A similar relationship was found between plasma corticosterone level, an indicator of physiological stress, and tail length (i.e., swallows with longer tails tended to have lower corticosterone levels during the mild winter while the reverse pattern was found during the severe winter). In contrast, another plumage ornament, pheomelanin pigmentation, was positively linked to body condition during the two study years and had no detectable relationship with the plasma corticosterone levels. Differences in the relationship between specific plumage traits and physiology may help to explain the diversity of tail length and plumage coloration in hirundines.