The effects of energetic and physiological state on the foraging decisions of birds
Animals make many decisions throughout their lives and there is good reason to suggest that these decisions are related to their energetic state. However, there is a need for more experimental data linking animals' behavioural decisions directly with their energetic state. Using behavioural assays and measurements of energetic state, I conducted a number of experiments that investigated the relationship between an animal's foraging decision-making and their energy levels. In Chapter 2, I examined the effects of energetic state on the rationality of foraging decisions of European starlings (Sturnus vulgaris). I found that there were no breaches of rationality and that there were no relationships with energy reserves. In Chapter 3, I investigated risk-sensitive foraging behaviour and its relationship with temperature and behaviour in rufous hummingbirds (Selasphorus rufus). However, the birds were indifferent to risk and there was no relationship with any of the variables that were included in the analysis. The remaining four experimental chapters dealt with problems related to the state-based consumption of chemically defended prey in European starlings. The results from these experiments confirmed many long-held assumptions regarding the state-based consumption of chemically defended prey. They also confirmed some recent predictions of stochastic dynamic programming models, which were less obvious. Generalist predators, such as European starlings, can also adapt to new food sources quickly and develop sophisticated behaviours that may mediate their intakes of dangerous compounds. The results of this thesis show that a bird's state can play an important role in mediating the foraging decisions that they make, especially regarding chemically defended prey. More generally, these results have important implications for how the dynamic systems of foraging behaviour and energetic state are viewed, and demonstrate a path as to how these different areas can be integrated.