Year of Award

2026

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Wildlife Biology

Department or School/College

Wildlife Biology Program

Committee Co-chair

Creagh Breuner, Thomas Riecke

Commitee Members

Paul Lukacs, Angie Luis, Zac Cheviron

Abstract

Environmental variation shapes population dynamics by influencing how individuals acquire and allocate resources. However, the pathways linking environmental conditions to population-level outcomes remain difficult to resolve because they operate across multiple levels of biological organization. Traits such as physiology and morphology mediate these relationships by shaping how individuals respond to challenges and convert resources into survival and reproduction. Physiological traits can change rapidly in response to short-term conditions, whereas morphological traits reflect longer-term processes related to performance and energy balance. Despite their complementary roles, these traits are typically studied in isolation, limiting our ability to understand how environmental conditions propagate through individuals to influence population dynamics.

Glucocorticoid hormones, particularly corticosterone (cort), provide a useful lens for examining these processes because they regulate energy balance and mediate responses to challenges. However, cort is not a single measure; it exists in multiple forms and varies across conditions, energetic states, and sexes, complicating its relationship with fitness. At the same time, morphology influences how individuals interact with their environment by affecting locomotion, energetic demands, and resource acquisition. Yet linking morphology to fitness is challenging because measures of body size often confound structural size and condition, and because the effects of morphology depend on context. Together, these challenges highlight the need for approaches that integrate multiple trait dimensions within a single framework.

In this dissertation, I develop an integrative framework linking environmental conditions to physiology, morphology, and demographic processes using a long-term dataset of mountain white-crowned sparrows breeding in a variable, high-elevation system. This system is characterized by strong interannual variation and a short breeding season, providing a powerful context for examining how conditions shape trait expression and fitness. Chapter 1 evaluates how environmental conditions and energetic state shape corticosterone profiles, clarifying which components best capture biologically meaningful variation. Chapter 2 examines how morphological traits interact with environmental conditions to influence apparent survival, with an emphasis on context-dependent and sex-specific effects. Chapter 3 integrates endocrine physiology with survival and reproduction to assess how trait variation contributes to population dynamics. Together, this work provides a mechanistic understanding of how environmental conditions influence populations through interacting trait pathways.

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