Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

Creagh W. Breuner

Commitee Members

Zachary A. Cheviron, Thomas P. Hahn, Bret W. Tobalske, Heather E. Watts


body condition, corticosterone, migration, movement ecology


University of Montana


Migration allows animals to use resources that vary in time and space. The predictability of this resource variation underlies different migratory strategies. Obligate migrants, with predictable movements, use resources that covary with the seasonal phenology of temperature or rainfall patterns. Facultative migrants, with unpredictable timing and destination of movements, rely on unpredictably variable resources. In this dissertation I explore the physiological changes birds undergo in preparation for obligate vs. facultative migratory flight. Hormones facilitate rapid organismal responses to environmental and internal information, making them an ideal physiological system to understand mechanisms controlling migratory behavior. I focus on corticosterone (CORT), a metabolic hormone that underlies activity patterns and feeding behavior.

Among obligate migrants, I aim to clarify CORT’s role in refueling at and departing from stopover sites. Chapter 1 develops a Stopover-CORT hypothesis, synthesizing current literature on CORT physiology, body condition, and refueling rate during the alternate phases of obligate migration: fuel catabolism (flight) and fuel accumulation (stopover). Chapters 1 and 2 together test this hypothesis in the field. We find that CORT reflects body condition among birds arriving at a spring stopover site and predicts departure among fall migratory birds. We use optimal migration theory to explore observed differences between spring and fall migratory physiology.

Among facultative migrants, I aim to understand how birds respond physiologically and behaviorally to changes in food availability, which is proposed to be the proximate cue initiating departure. In captivity, Chapter 3 tests hypotheses about how body composition, CORT, and activity patterns respond to experimentally manipulated changes in food availability. We find that Pine Siskins, a nomadic, facultative migrant, do not prepare for movement, instead relying on an escape strategy to abandon resource-poor areas. In the field, Chapter 4 tests these same hypotheses and finds that siskins need sufficient—but not large—fuel stores to initiate departure. Body condition and food availability interact to inform siskins’ movement decisions.

Collectively, this dissertation explores the physiology underlying predictable and unpredictable migratory movements. We find that facultative movements are physiologically distinct from obligate movements, though energetic condition can influence both movement types. More broadly, these studies contribute to our understanding of migratory behavior, how it varies with both large-scale resource fluctuations and local food availability, and the role of corticosterone in mediating migratory physiological changes.



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