PARENTAL PROVISIONING IN A VARIABLE ENVIRONMENT - EVALUATION OF 3 FORAGING CURRENCIES AND A STATE-VARIABLE MODEL

We estimated the reproductive success of black terns (Chlidonias niger ) based on three optimal foraging currencies (maximizing the net rate of energy intake, daily delivery rate, and efficiency, respectively) a nd a state variable model. There was a broad range of capture interval s (the time required for the parent to capture a single prey) when the flight speeds predicted by the three currencies were so high that the y resulted in daily provisioning costs which parents could not fully r ecover through self-feeding. Whenever the efficiency currency produced higher estimates of reproductive success, parents lost comparatively less weight than when they foraged as rate-maximizers. If parents did not experience any weight loss, the net rate and efficiency currencies made equivalent fitness projections. However, both of these currencie s provided lower fitness returns than daily delivery rate at longer ca pture intervals. There were a number of capture intervals when estimat es of reproductive success from the state variable model and at least one of the foraging currencies were equal. Provisioning behaviour unde r the state variable model was much more flexible and parents were the refore able to reduce their self-feeding rate on days when food was pa rticularly scarce, thereby increasing the total delivery to the nest. This resulted in higher fitness returns than was possible under the fo raging currencies. Our results suggest that efficiency-maximizing is m ore likely to provide fitness returns that are equivalent to the state variable model in comparison with the rate-maximizing alternatives, F urthermore, only the efficiency currency and the state variable model made predictions of flight speed that were similar to speeds measured in black tern parents provisioning young at natural nests.