Understanding the elements that shape the ontogeny of migration

To understand animal behavior we must understand how behavior develops across ontogeny. Certain behaviors may be innate, especially those that are truly vital such as behaviors for foraging and eating. But, the acquisition of some behaviors is externally influenced by factors such as stress or social context. Animals can also modify their behavior through learning by forming new memories and retrieving them later. Learning can be asocial through trial-and-error and experience, but it can be socially influenced directly (teaching) or indirectly (e.g. through local enhancement). The ontogeny of behavior has received an increasing amount of attention, but its study remains challenging and answers about how behavior develops remain elusive.

In this dissertation, I use seasonal, long-distance migration as a model to examine how behaviors change over the course of an animal’s lifetime. Migration often exposes animals to risks and energetic costs because they are traveling through inherently unsuitable habitats. Animals migrate anyway because it allows them to track seasonal resources and improve their breeding success. Thus I expected that individuals that repeat their migrations and have the opportunity to gain experience should improve in their ability to accomplish energetic efficiency. I addressed this at three scales: the scale of the migratory corridor, the scale of individual routes, and the scale of movements made over minutes.

I leveraged a unique data set comprising a decade of GPS tracking efforts focused on white storks (Ciconia ciconia) breeding in southwestern Germany. White storks are soaring-gliding, collective migrants; they travel in loose flocks using collective searching to locate thermal uplifts. I asked what features of the landscape define the flyway for white storks, whether they select their routes to maximize availability of uplift and social partners, how they fly in thermals, and how each of these changes as storks age.

I found that white storks of all ages move in ways that will reduce their energetic expenditure, but adult storks are restricted by the breeding season. That restriction shifts their migratory timing. Accordingly, older storks shift the scale of their route selection from the immediate, hourly conditions to arrival times that are days or weeks in the future. This leads them to experience less supportive conditions than they did as juveniles, but they cope with these conditions, even with fewer other migrants around them.

This dissertation demonstrates how the evolutionary legacy of a species, embedded in its morphology and environment of evolutionary adaptedness, interacts with individual experience and motivation to shape the development of animal behavior. Balancing energy, opportunities, mortality, and reproduction depends on the value of each currency in the specific context for a given behavior. In this case, the context of age alters both the ability to balance different currencies and the value of those currencies and thus alters how individuals should behave to increase their fitness.