INITIATION OF MIGRATORY AND EMERGENCY LIFE HISTORY STAGES IN NOMADIC FINCHES: INVESTIGATING CUES AND ENDOCRINE MECHANISMS

Adam J. Becker

doi:10.7273/000006478

An animal’s annual cycle is made up of sequential life history stages (LHSs), such as migration, breeding, molt, and hibernation, which are temporally repeated. Different LHSs can have drastically different demands and the endocrine system is an important mediator of the transition from one LHS to another. Additionally, when exposed to unpredictable and adverse conditions, any animal may enter an emergency life history stage (ELHS), where behavioral and physiological responses are directed away from the current LHS and directed towards escape and recovery. Migration is a LHS that is widespread in the animal kingdom, though the nature of migration can vary considerably. One way to conceptualize variation in migratory movements is on a gradient from those that are rigid (obligate) to those that are flexible (facultative) in their timing and direction. Androgens, specifically testosterone, have been shown to be important mediators for the initiation of obligate spring migration in birds; however, we know little about whether testosterone plays a similar role in more flexible migrations. In Chapter 2, I test the hypothesis that testosterone stimulates physiological preparation and/or behavioral readiness for migration during the spring nomadic migration of pine siskins (Spinus pinus), a form of facultative migration. First, I test whether testosterone supplementation stimulates earlier or greater expression of spring migratory physiology and behavior. Second, I test whether inhibition of testosterone’s effects inhibits expression of spring migratory physiology and behavior. I found that testosterone supplementation stimulated the expression of migratory behavior and inhibition of testosterone’s effects led to inhibition of behavior. However, neither supplementation nor inhibition treatments in these experiments affected physiological preparation for migration. Thus, the results of this chapter suggest that testosterone stimulates behavioral readiness to migrate, but not physiological preparations for migration, in nomadic pine siskins. Given testosterone’s apparent effect on migratory behavior, Chapter 3 sought to elucidate the mechanism by which testosterone exerted this effect. Testosterone is able to influence physiology and behavior through two main pathways, either by activation of androgen receptors (ARs) or via aromatization into estradiol and then activation of estrogen receptors (ERs). But little work has been done to determine which of these mechanisms is involved in the transition to a spring migratory state in any bird species. In Chapter 3, I tested whether testosterone’s pathway of action to stimulate a spring migratory state was through ARs, aromatase and then ERs, or if both were necessary for initiation of a migratory state. To do this, I conducted another experiment with pine siskins during the spring nomadic period in which birds received either a control treatment, AR inhibition alone, aromatase inhibition alone, or AR inhibition and aromatase inhibition in combination. In this study, only the aromatase inhibition group displayed migratory physiology and behavior. I interpret these results as providing support for the hypothesis that effects of testosterone on migratory behavior and physiology are mediated by activation of ARs. Whereas migration is one means by which animals cope with changing environmental conditions, an ELHS can also be a means to cope with an adverse and unpredictable environmental change. In Chapter 4, I examined how animals respond to wildfire smoke, a relatively unpredictable and potential adverse event. To do this, I collected behavioral and physiological data from red crossbills (Loxia curvirostra) housed in outdoor aviaries before, during, and after exposure to naturally occurring smoke from distant wildfires. One group of birds was fitted with back-mounted accelerometers to collect detailed data on activity. A second group was periodically captured, and body condition measurements and blood samples were taken. I found that daily activity patterns and heterophil/lymphocyte ratios both varied significantly in association with smoke exposure, suggesting that birds initiated an ELHS. It also appears that exposure to cold smoke from wildfires negatively affected body condition through decreased body mass and hematocrit levels, potential indicators of overall health. Overall, my dissertation provides insight into the initiation of the spring migratory and emergency life history stages of nomadic finches.

INITIATION OF MIGRATORY AND EMERGENCY LIFE HISTORY STAGES IN NOMADIC FINCHES: INVESTIGATING CUES AND ENDOCRINE MECHANISMS

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