Environmental effects, thermal energetics, and avian life history variation

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Mothers play a critical role in shaping the physiology of their offspring, via hormonal signals during embryonic development and/or via adjustment of postnatal care. The major goal of my proposed research is to use wild and captive birds to understand whether mothers prime their offspring in an adaptive way, to cope with predicted environmental change. My students and I will address three general questions: (1) If mothers experience heat waves prior to egg laying, are their nestlings better able to cope with high temperatures in the future? (2) Do mothers adjust levels of thyroid hormones deposited in their eggs as a mechanism to prime nestlings for future temperatures? (3) Is the ability of adult birds to feed their nestlings limited by the risk of overheating, and can the temperature at which overheating occurs be increased by maternal cues during development? To test question 1, we will expose captive birds to simulated heat waves prior to egg laying. We will then raise nestlings in environments that "match" the temperature their mother experienced or are "mismatched." We predict that if mothers can prime a nestling's thermal physiology via prenatal cues, then "matched" nestlings will have faster growth and will be better able to cope with heat waves than those that experienced "mismatched" temperatures. To test questions 2 and 3, we will study wild tree swallows. We will experimentally increase thyroid hormone levels in eggs and raise nestlings in experimentally cooled environments. Thyroid hormones are known to increase growth rates and metabolism. We predict that nestlings exposed to thyroid hormones as embryos will display lower levels of physiological stress when exposed cold. Because mothers may deposit different amounts of hormone into eggs, we will also study whether this may be a mechanism by which mothers adjust the size hierarchy and competitive advantage of individual nestlings. To test question 3, we will experimentally increase the ability of breeding birds to dissipate heat and measure flight activity using an automated radio-telemetry system. We predict that parents with an increased capacity to dissipate heat will increase their activity and nestling feeding rates. To test whether thermal limits may be modified by the rearing environment, we will train birds from Objective 1 to exercise in the heat. We predict that offspring whose mothers experienced heat waves prior to egg laying will maintain higher activity in the heat but may experience physiological costs of high activity. Overall, this research will help understand whether mothers are able to adjust the capacity of their offspring to cope with predicted temperature change and identify possible hormonal mechanisms. The proposed research will provide key physiological data that can be used by researchers and managers when predicting the capacity for populations and species to cope with climatic change.