Should I Stay or Should I Go? Alternative migratory strategies in partially migratory western bluebirds (Sialia mexicana)
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Partial migration, which occurs when only some animals in a population migrate, is ubiquitous. However, despite numerous studies investigating how partially migratory systems are controlled, we know relatively little about what determines whether an individual migrates. In this thesis, I investigate ecological factors and individual characteristics shaping alternative migratory strategies in a partially migratory population of western bluebirds (Sialia mexicana). First, I ask whether sex, age, or body size are related to variation in migration behaviour. Using hydrogen isotopes in claw tissue to infer migratory strategy, I show that young bluebirds, particularly young females, are less likely to migrate than older birds, and that bluebirds mate assortatively by migratory strategy. I also demonstrate that some bluebirds switch strategies between years, indicating that migratory behaviour is not under strict genetic control. Second, I ask whether individual differences in diet could lead to variation in migratory strategy. Using nitrogen and carbon isotopes in feathers, I show that the diet of resident males during fall moult differs from that of resident females and migrants of both sexes. Responses to experimentally presented novel foods suggest this variation could be due to differences in behaviour: resident males are less hesitant to eat novel insects than migrant males, but the opposite is true of females. Finally, I ask whether two personality traits, boldness and aggression, differ with migratory strategy. I show that residents are bolder than migrants, but there is no difference between groups in aggression. However, resident bluebirds from BC are more aggressive than resident bluebirds from California. My results indicate that social influences, variation in diet, and personality traits may all play a role in determining whether bluebirds migrate. They also imply that different factors may shape migratory strategies in males and females. Overall, my research emphasizes the necessity of examining a wide variety of ecological factors and individual characteristics when attempting to determine how partially migratory systems are controlled. The flexibility of these systems suggests they are likely to shift as a result of global environmental change. To better predict their responses, an understanding of factors driving variation in migratory strategy is essential.