Date of Award


Degree Type


Degree Name



Biological Sciences

First Advisor

Love, Oliver

Second Advisor

Vézina, François


Birds; Energetic physiology; Fat stores; Hormones; Life history stages; Muscle growth




Animals living or breeding in highly seasonal temperate and polar ecosystems require stored energy (i.e. lipids and protein) in the form of somatic reserves to prepare for predictable energetically demanding stages within their annual cycle, such as hibernation, migration, reproduction, or overwintering. However, the physiological mechanisms underlying fat and muscle gain in free-living vertebrates are not fully known. Nonetheless, research in mammals and poultry have identified a number of energy-regulating hormones that mediate metabolic (peripheral) and behavioural (central) effects on lipid and protein stores. Here I extensively reviewed the mechanistic advances on energy-regulating hormones in birds, and then integrate concepts from mammalian studies to design a conceptual framework for field-testing in avian systems. To test this, I then used a comparative approach to examine the temporal and stage-related variation in circulating levels of fat- (baseline corticosterone) and muscle-promoting (testosterone and insulin-like growth factor-1 or IGF-1) hormones before spring migration in captive male snow buntings (Plectrophenax nivalis) and breeding in free-living female common eiders (Somateria mollissima). Baseline corticosterone did not appear to signal for fat deposition in premigratory buntings, while slight and rapid elevations in eiders may stimulate fattening and may fuel reproduction, respectively. Elevated testosterone in buntings may mediate skeletal muscle growth as the variation temporally matched muscle scores, where elevations in female eiders are potentially more important for breeding behaviours. In both species, the down-regulation of IGF-1 may represent a fat-sparing action, where the up-regulation may be for visceral organ remodeling. My results demonstrate potential for hormone pleiotropy on complex phenotypes, and my thesis collectively highlights the value of examining naturally circulating hormone levels as foundational information on phenotypic changes across a broad range of birds with stages in highly seasonal ecosystems.