Mechanisms underlying small-scale partial migration of a subtropical owl

Partially migratory populations provide excellent opportunities to study why animals migrate. Three hypotheses have been proposed to explain the variations in individual migratory decisions: the body-size hypothesis, the arrival-time hypothesis, and the dominance hypothesis. These hypotheses were developed for long-distance migrants breeding in the temperate region; whether the same mechanisms affect small-scale partial migration in the subtropical zone remain to be examined. In this study, we analyzed the condition dependence of the small-scale migratory behavior of Lanyu scops owl (Otus elegans botelensis) and examined the costs and benefits of alternative strategies. We found that migrants gained more weight than residents during the nonbreeding season. Resident males were more likely to breed the following year than migrant males, and resident females were more likely to breed successfully than migrant females. Our results showed that male owls tended to be resident, supporting the arrival-time hypothesis. The sexual difference in migratory tendency does not support the body-size hypothesis, which predicts the lighter weighted male to be more migratory than the heavier females. Also, within the same sex, migratory tendency was not related to body mass. The patterns that younger owls and nonbreeders were more migratory than older ones, and breeders are consistent with the dominance hypothesis. However, Lanyu scops owl does not demonstrate winter food competition as assumed by the dominance hypothesis. We propose that the difference in the probability of breeding between dominant and subordinate owls lead to dominance-dependent partial migration.