Date of Thesis

Spring 2023

Description

Birds can cope with energy-shifting challenges by redirecting energy and behavior to maintain energetic homeostasis. Nestlings allocate energy to support their rapid growth and developmental period; thus, their challenges and the subsequent energetic demands can be much different than an adult. At a baseline level, energy is constantly shifting in response to regularly occurring challenges; however, more substantial challenges can initiate the production of glucocorticoid hormones. In birds, the main glucocorticoid is corticosterone. Corticosterone is believed to aid in energy homeostasis by redirecting energy metabolite mobilization. Energy metabolites are intermediaries or end products of cellular metabolism, thus, metabolites like glucose and ketones may be useful indicators of shifts in energy balance. Corticosterone production can also influence changes in behavior which can aid in energy homeostasis. For this two-year study, I utilized a unique biological station on Middleton Island, Alaska, to investigate factors (challenges) that might shift energy balance in 5-day old black-legged kittiwake (Rissa tridactyla) chicks. I took advantage of a supplemental feeding program at the station where some nests (“Fed”) were fed 3 times per day in addition to natural foraging efforts and the remaining nests (“Unfed”) only relied on natural foraging efforts. For part I, I asked how energy expending behaviors and energy intake are related to circulating levels of glucose and ketones. I recorded behavior for an hour and collected blood samples at the end of the hour. I used behavioral data to indicate moments of energy intake and expenditure. I measured circulating glucose and ketone levels as a proxy of energy. I tested three main hypotheses: 1) long-term nutritional status, 2) recent food intake and/or, 3) recent behavior explains circulating metabolite levels. Long-term and short-term feeding status influenced ketone levels. Proportion of time a chick was active had a negative effect on ketone levels in 2021. Proportion of time a chick was aggressive had a positive effect on glucose and ketone levels in 2019 and 2021. For part II, I investigated bi-directional relationships between corticosterone, energy metabolites, and behavior. I repeated the same experimental design as part I but added a corticosterone manipulation after the first video/blood sample, then collected a second blood sample, and started a second one-hour video. I hypothesized that 1) recent or “baseline” energetic state and 2) recent behaviors influence baseline corticosterone levels and 3) elevated corticosterone levels influence energy allocation and 4) redirect behavior. Recent energy metabolites and behaviors did not influence baseline corticosterone levels; however, corticosterone levels were consistently higher in chicks from Unfed nests throughout most of the analyses. There was a positive relationship between elevated corticosterone and subsequent ketone levels. There was a positive relationship between elevated corticosterone levels and proportion of aggression, only in male chicks. Post-restraint feeding counts/second were higher in males than females. Important takeaways were: 1) Ketones mobilize on shorter time scales than previously talked about and should be included in metabolite studies regarding energy mobilization and not just fasting physiology, 2) long-term feeding can change the utilization of energy and energetic pathways, and 3) some results were different between 2019 and 2021 highlighting the effects of varying feeding conditions and the need for multi-year studies to better interpret results.

Keywords

nestling, energy, glucocorticoids, glucose, ketones, environmental endocrinology

Access Type

Masters Thesis

Degree Type

Master of Science

Major

Biology

First Advisor

Dr. Morgan Benowitz-Fredericks

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