Testing four hypotheses to explain partial migration: balancing reproductive benefits with limits to fasting endurance

Abstract

Seasonal migration is ubiquitous in animals, and yet its underlying cause(s) remain poorly known. Species exhibiting short-distance altitudinal migration and intraspecific variation in migratory behavior (partial or differential migration) are ideal study systems for examining the selective pressures that affect individual migratory decisions. We used an individually marked population of yellow-eyed juncos, breeding along a 1000-m elevational gradient and migrating up and down that gradient, to examine the morphological, behavioral, and reproductive traits associated with migratory behavior. We tested the four most well-known hypotheses proposed to explain partial migration: the thermal tolerance, fasting endurance, dominance, and arrival time hypotheses. Our results indicate that: (1) limits to juncos’ fasting endurance constrain their ability to overwinter at high elevations, in support of the fasting endurance hypothesis, (2) differences in body size mediate fasting ability and are associated with variation in migratory behavior and overwinter apparent survival, (3) migratory behavior interacts with reproductive success, in partial support of the arrival time hypothesis, and (4) additional mechanisms that are not captured by the four well-known hypotheses might better explain individual variation in migratory behavior. Less migratory females achieved greater nesting success the following breeding season. Among males, nesting success influenced migratory tendency the following winter. Successful males may either migrate to a more benign winter climate without paying reproductive costs, or high levels of parental effort might physiologically constrain their ability to overwinter in harsh climates. Migration is a costly behavior, so migration must yield fitness benefits, at least for some individuals. Species in which migratory behavior varies among individuals in the same population provide an under-utilized opportunity to understand the costs and benefits that mediate individual migratory decisions. By marking and tracking a population of yellow-eyed juncos, we found that larger individuals were less likely to migrate, and this held true even within a sex class (males). Smaller individuals were not only more likely to migrate but also had lower apparent survival, and these differences were driven by size-related differences in fasting ability. Less migratory females accrued reproductive benefits, but males with the highest reproductive success were not less likely to migrate as predicted by the arrival time hypothesis. Our study demonstrates that migratory decisions balance the potential reproductive benefits and survival risks of overwintering on high-elevation breeding grounds.