The effects of temperature on avian physiology, behavior and development
Sammanfattning: Temperature is a property of profound matter in all aspects of animal life, affecting both individual energy budgets and the environments in which animals reside. Thus, a proper understanding of the effects of temperature variation is instrumental in understanding not only animal form and function, but also the structure and dynamics of wild populations. Accordingly, the aims of this thesis were to investigate how acclimation to ambient temperature and the regulation of body temperature affected various aspects of bird physiology, behavior and development. These efforts focused on two discrete stages of the annual cycle; egg incubation and winter residency. For the incubation work, the impetus was to assess how the maintenance of incubation temperature varied with the demands from incubation, to ultimately assess if egg temperature variation could provide a functional explanation for the trade-off between parental investment in incubation and nestling performance. The objectives of the winter work were to characterize energy conservation patterns manifested through rest-phase hypothermia (a controlled reduction of body temperature at night) and the factors governing its use in resident birds. We found that the demands from incubation constrained parents’ ability to provide an appropriate environment for egg development. Incubating birds must therefore resolve the paradox of minimizing energy expenditure whilst simultaneously maintaining eggs at adequate temperatures. By manipulating incubation temperature in wild nests, we also showed that failure to do so can result in substantial fitness costs to nestlings. Thus, incubation investment is a plastic trait, which is limited in its higher end by the energetic costs of keeping eggs warm and in its lower end by temperature-related constraints on subsequent nestling performance. The winter work suggested that environmental conditions strongly impacted on body temperature regulation in roosting birds. Specifically, we found that the use rest-phase hypothermia was more pronounced in situations where the marginal value of energy conservation was higher, such as in low ambient temperatures or during the short days of mid-winter. However, food availability had an overriding importance in determining the use of energy conservation mechanisms, to the extent that birds avoided using rest-phase hypothermia when food was plentiful. This suggests that nocturnal body temperature was routinely maintained at the highest affordable level. While this idea has sometimes been explained by an increased predation risk in hypothermic birds, work from this thesis suggested that hypothermia may also be avoided to overcome a general physiological dysregulation, e.g. impaired immune function, at low body temperatures. Taken together, we have seen that variation in ambient temperature impacts not only on avian life history evolution, but also on the physiology and behavior of overwintering birds. Hence, this work reiterates the vast importance of temperature variation in the life of animals.
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