Physiological basis of aerobic capacity and workload ability in birds

Many behaviours exhibited by free-living animals that are crucial for survival and reproduction involve elevated levels of activity or “workload”. Individuals with higher workload ability should be able to cope with the high metabolic demands imposed by these behaviours better and consequently would have higher fitness. High workload could also result in costs such as impaired reproduction or reduced survival. However, the underlying physiological mechanisms that allow individuals to have higher workload ability, and the mechanisms underlying costs of high workload remain poorly understood. This thesis took an exercise perspective and investigated the physiological basis of aerobic capacity and workload ability in birds, using both a comparative, phylogenetic approach, as well as various laboratory-based experimental approaches. In surveying the literature, we identified several potential common physiological markers underlying individual variation in exercise performance and costs of exercise. We also found that hematological traits co-vary with life-history variables, and to a certain extent, energy metabolism in birds at the interspecific level. Additionally, we provided experimental evidence for physiological responses to flight at high attitude and showed that the relationship between hematocrit and flight performance is dependent on altitude. Lastly, we provided experimental evidence for behavioural and physiological adjustments to high workload and demonstrated that physiological adjustments to high workload can negatively impact reproduction. Taken together, this thesis uncovered several physiological mechanisms underlying workload ability and costs of high workload in birds. Future work should consider and integrate multiple physiological systems when studying the physiological basis of workload ability, and more generally, life-history trade-offs in animals. Ultimately, we hope that the knowledge we gain from this thesis can be used to complement studies in free living animals and aid in the design of field experiments.