Partial migration in European blackbirds : a study on alternative phenotypes

In this dissertation, I describe my work on migratory behavior in a partially migratory population of European blackbirds, Turdus merula. Partial migration, when a population consists of migrants and year-round residents, is a common behavior across the animal kingdom and is an intermediate stage between fixed migratory and sedentary life histories at the population and species level. Therefore, partial migration is commonly used as a model for understanding the evolution of migration. However, the question remains: why do individuals in partially migratory populations migrate?Using a combination of year-round radio tracking and geolocators I observed two distinct migratory periods during the study; one in early autumn, and another during the midst of winter. While blackbirds that migrated in autumn were never observed overwintering within 300 km of the study site, blackbirds that departed in winter were often observed within 40 km. Just prior to autumn migration, migrant blackbirds had higher fat scores than non-migrants and tended to have higher levels of baseline corticosterone, suggestive of anticipatory obligate migration. Sex related differences in migratory behavior are common and are often attributed to differences in reproductive strategies. I show that female blackbirds, independent of age and body size, are more likely to migrate in the autumn than male blackbirds. Unlike autumn migrants, I found no differences in the tendency of males and females to migrate in winter, nor did I find any difference in body size or age of winter migrants.Selection shaping annual cycles requires variation in annual organization among individuals. Despite the significance of partial migration in our understanding of the evolution of migration, little is known about the consistency of individual behavior in the wild. Although individual strategies and timing of migratory movements were consistent over the course of the current study, changes in behavior did occur from autumn migration or residency to the third less defined strategy- “winter escape”. Given this novel result, I propose the ‘Two-Step Model of Partial Migration’ for understanding the proximate control of individual strategies.Variations in the external flight apparatus of birds are beneficial for different behaviors. Long distance flight is less costly with more pointed wings and shorter tails; however these traits decrease maneuverability at low speeds. In order to identify the degree of morphological adaptation by migrants within the population to migration, I compared the wing pointedness and tail length of migrant and resident blackbirds. Contrary to a prediction of adaptive migratory phenotypes, I found no differences between migrants and residents in either measure.The results of my dissertation provide a more complex picture of partial migration in passerines than was previously known. By observing individuals throughout the year, I identified the co-occurrence of sex biased anticipatory migration in the autumn and what appears to be facultative migration in the winter. These findings provide support for different selective pressures on males and females during the evolutionary transition between migratory and sedentary behavior. Consistency in autumn migration behavior indicates potential contribution of endogenous mechanisms of autumn migrants. However, similarity in the flight apparatus of migrants and residents could indicate that morphological characteristics are under similar selective pressures