Explaining Migratory Behaviors Using Optimal Migration Theory
Bird migration is the regular seasonal movements between breeding and nonbreeding grounds. In general, birds that breed in the Northern Hemisphere tend to migrate northward in the spring to take advantage of increasing insect populations and lower predation pressures and fly south when food availability and weather conditions decline. Embarking on a journey that can stretch a thousand miles round trip is a dangerous and arduous undertaking. While en route migrants must stop and feed to replenish their depleted energy reserves, often in unfamiliar locations with unknown predation pressures. They also must react to weather conditions during flight and while on the ground. Additionally, areas of high quality habitats where birds can refuel efficiently and safely may be few and far between. Therefore, it’s not surprising that mortality rates can be higher during migration than at any other period of the year. Behavioral decisions such as where and where to stop, how long to stay, and when to leave all involve costs and benefits with an ultimate goal to balance the costs and benefits on order to achieve a successful and efficient migration. Optimal migration theory, aims to explain how migrants balance behavioral and physiological parameters of migration that minimize total time spent on migration, total energy expended, or mortality risk. The eventual result of these optimization pressures is thought to be a gradient of behavioral strategies that optimize different combinations of the three currencies: time, energy, and risk. I investigated how migratory behaviors of North American songbirds in the autumn balance the three currencies. More specifically I 1) explored how stopover site selection varies across migratory strategies at the landscape (Chapter 2) and habitat-patch (Chapter 3) scale; 2) investigated the importance of wind for the evolution and maintenance of migratory routes (Chapter 4); and 3) explored how selection of wind conditions for migratory departure affects overall behavioral strategies (Chapter 5). With this research, I hope to further our predictive abilities of migratory behaviors under various environmental and geographic situations using an optimal migration framework.