Avian assessment of risks: Balancing the threat of starvation and predation during reproduction (Turdus migratorius, Buteo regalis)

In species with parental care, such as birds, foraging parents must allocate their time such that the risk of starvation and predation to themselves and their young is minimized. Risk of nestling starvation or predation generally decreases if a parent increases foraging or nest attendance. However, the potential cost to adults associated with these behaviors is increased risk of predation. Haw parents balance this trade-off between their own survival and that of their young (i.e., current versus future reproduction) is a central issue of life history theory. I employed both an experimental and observational approach to investigate if avian parents face a trade-off in the allocation of time spent minimizing starvation versus predation during reproduction. First, I experimentally manipulated nestling starvation risk and adult predation risk for an American Robin (Turdus migratorius) population. As predicted, parents whose young had a lower starvation risk had a stronger response to an increased risk of nestling predation. Further, I found that experimentally weighted females tended to respond to the increased risk of their young, rather than minimize their own risk. This result was consistent for both nest defense intensity as well as time allocation to nest attendance. I also collected observational data for three years from a Ferruginous Hawk (Buteo regalis) population to determine the influence of food availability on parental behavior and the rate of juvenile hawk starvation and predation. I predicted that Ferruginous Hawks would increase their nest defense intensity and nest attendance as food availability increased, but they did not. Ferruginous Hawks have a much longer lifespan than American Robins; hence risky behavior that minimizes the probability of predation is likely unwarranted for this species because failure of a single breeding attempt does not reduce their lifetime reproductive performance as much as injury or mortality would. The results of my study demonstrated that avian parent behavior in response to offspring starvation and predation risk is variable, and the degree to which parental behavior can be predicted will depend in part on the species' life history traits. Nevertheless, it is clear that behavioral choices, such as whether to be nest attentive or not, have important implications for reproductive success.