Avian thermoregulation across age and seasons
Abstract: Animals constantly need to relate to prevailing environmental conditions and regulate their body temperature accordingly. Body temperature regulation in birds and mammals is inherently linked to energy expenditure and, thus, thermoregulation is a physiological mechanism that could trade-off with other costly processes, with potential long-term fitness consequences. In this thesis, I studied ecological and physiological sources of variation in body temperature across age and seasons, and how variation in thermoregulatory patterns trade-off with other processes within a life-history framework. More specifically, I studied how thermoregulatory development was affected by early life thermal environment and how body temperature was regulated and maintained outside of thermoneutrality. For this purpose, I used small, hole-nesting, passerine birds both during the nestling stage and as adult birds in winter and during breeding.I found that nest environment affected body temperature regulation, short-term growth and development in nestlings but that there also might be long-term effects on apparent survival. More specifically, growing up in a small brood allowed for investment in both growth and thermoregulatory development, compared to nestlings that grew up in large broods. By experimentally increasing nest temperature I was also able to show that nestlings were well equipped to deal with increased nest temperatures, but that it came with a cost in lower body mass gain, and that increased nest temperatures could potentially have positive effects on long-term survival. While working hard to feed their young, the risk of overheating can constrain parental work rate. By experimentally reducing this constraint, I found that females were able to increase innate immune function while reducing work rate, but without compromising nestling development. In winter, small passerines routinely reduce body temperature during the night, thereby lowering energy expenditure. I found that young birds (in their first winter) had both higher metabolic rate and body temperature during the night compared to old birds (in their second winter or older). Such differences could be related to experience-based variation in assessment of predation risk or age-related differences in plumage quality. I also show that handling birds in winter caused a pronounced decrease in body temperature, highlighting the importance of plumage in maintaining body temperature in the cold for these small birds. Overall, these studies demonstrate the need for including age-effects into studies of thermoregulation and energy management in the cold. I have, for the most part, studied avian thermoregulation outside of thermoneutrality, when additional energy is required to maintain body temperature. Given the current changes to our climate, with an expected increased frequency of extreme weather, studies like these can add important knowledge to our understanding of how animals respond physiologically to such changes and how this, potentially, could shape life-histories.
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