Abstract
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 facilitate 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.