Causes of interspecific variation in avian migration distance

Migration is an awe-inspiring behavior employed by a diverse suite of taxa throughout the animal kingdom. Avian migration is arguably the most well-known animal behavior and birds vary widely in how far they migrate, but the ecological mechanisms underlying interspecific variation in migration distance remain unclear. We developed a novel set of predictions deduced from seven mechanistic hypotheses proposed to explain interspecific variation in avian migration distance. We then used a Bayesian phylogenetic comparative analysis to test predictions from these seven hypotheses based on migration data and species traits from 446 species of migratory birds throughout the world based on data from animal-borne tracking technology. Body mass, seasonal food availability, and morphological/behavioral traits that improve flight efficiency explained significant interspecific variation in migration distance after controlling for absolute breeding latitude (which was positively correlated with migration distance). Specifically, we found migration distance was negatively correlated with body mass and seasonal food availability but positively correlated with hand-wing index (a measure of wing elongation and pointedness) and soaring flight. Moreover, the negative relationship between body mass and migration distance was not ubiquitous but rather depended on flight mode and intraspecific nonbreeding group size, such that species employing flapping flight during migration and those overwintering in smaller intraspecific groups had the strongest negative relationships between body mass and migration distance. Within group-living birds, smaller bodied species may gain a foraging or thermoregulatory advantage that shortens migration distances, while larger bodied species may gain an energetic advantage by using flock formations during migration that facilitates longer migrations to more suitable nonbreeding areas. From a life history perspective, maximum annual reproductive investment was negatively correlated with migration distance, while minimum annual reproductive investment and adult annual survival were not correlated with migration distance. Overall, our results support flight efficiency, food limitation, and thermoregulatory-based hypotheses as causes of interspecific variation in migration distances in birds. Our results also help to refine the numerous and often ambiguous mechanisms underlying the negative relationship between body size and migration distance and refute several hypotheses previously proposed to explain interspecific variation in avian migration distance.