Abstract
Mechanisms governing the migratory decisions of birds have long fascinated ecologists and sparked considerable debate. Differential migration, or variation in migration distance, is a common pattern in birds and understanding the causes of differential migration is arguably the most tractable way to answer the more general question of why birds migrate. Overall, this dissertation seeks to improve our understanding of the mechanisms explaining differential migration in birds. In Chapter 1, we use a rapidly advancing tool for diet assessment in wildlife, DNA metabarcoding, and evaluated seven DNA metabarcoding sampling methods to assess the diet of a large avian predator: Buteo lagopus (rough-legged hawk). Beak and talon swabs produced the highest prey detection rates, yielded the greatest prey richness per sample, and contributed the most to an individual’s total prey richness per sampling occasion compared to other sampling methods. While all methods identified similar dominant prey taxa that were consistent with prior diet studies, beak and talon swabs detected greater prey richness at both the individual and population levels. In Chapter 2, we reviewed the differential migration literature which has been slowed by the continued testing of overlapping, non-mechanistic, and circular predictions among a small set of historically entrenched hypotheses. Overall, we describe and refine 12 mechanistic hypotheses proposed to explain differential migration (along with several other special-case hypotheses), seven of which have underlying mechanisms related to food limitation as past research has identified this to be an important driver of differential migration. We also thoroughly reviewed 145 publications to assess the amount of support for 10 critical assumptions underlying differential migration hypotheses and found surprisingly few studies explicitly evaluate assumptions within a differential migration context. We also contend that a thorough comparative analysis that seeks to explain variation in migration distance among species is needed. In Chapter 3, we conduct a differential migration comparative analysis by developing a novel prediction matrix deduced from 8 alternative hypotheses based on migration data from 420 species of migratory birds followed using animal-borne tracking technology. Body mass, seasonal food availability, and adaptations that improve flight efficiency explained significant interspecific variation in migration distance. Moreover, the relationship between body mass and migration distance was not ubiquitous but rather depended on both intraspecific nonbreeding group size and flight mode. Overall, our comparative results support flight efficiency, food limitation, and thermoregulatory-based hypotheses to explain the underlying causes of differential migration in birds. In Chapter 4, we take an intraspecific approach to understanding the causes of differential migration within a single species, Buteo lagopus. We leveraged migration distances from GPS-tracked Buteo lagopus and provide the first empirical evidence in support of a social dominance food maximization hypothesis whereby dominant individuals migrate to higher quality nonbreeding locations with respect to foraging efficiency regardless of migration distance. Within females, the largest and presumably most dominant individuals migrated intermediate distances while smaller, subordinate individuals migrated both longer and shorter distances. We also found support for the social dominance distance minimization hypothesis because more aggressive females migrated shorter distances, although increased aggression at shorter distances may be a consequence of poor body condition. Within males, we found some support for fasting endurance and thermal tolerance hypotheses because body size was negatively correlated with migration distance, particularly among males weighing less than the smallest females. Body size was also negatively correlated with food availability and winter minimum temperatures within both sexes, providing additional mechanistic support for fasting endurance and thermal tolerance hypotheses. Overall, our results suggest differential migration within Buteo lagopus is caused by a combination of competition for nonbreeding resources and constraints on fasting endurance or thermal tolerance. In summary, we found broad agreement that food limitation-based mechanisms explained differential migration both among and within species; while also finding considerable support for flight efficiency-based mechanisms among species. Collectively, this dissertation provides a framework that, together with advances in animal-borne tracking and other technology, can be used to advance our understanding of the causes of differential migration distance, and migratory decisions more generally.