Functional Responses of Birds of Prey: Biases Due to the Load-Size Effect in Central Place Foragers

Functional responses of birds of prey have been estimated from analyses of prey delivered by a forager to sedentary consumers, usually mate or offspring located in a nest. Large items of prey captured by these single-prey loaders are more often carried to the nest and less often consumed at capture sites than small ones. Therefore, analyses of prey delivered at the nest will bias any estimate of functional response by the forager, which is the response of interest in a consideration of effects on prey population dynamics. Some simple graphical models are presented to illustrate these biases. The main predictions of the models are: 1) If alternative prey are smaller than the primary prey, the functional response will be underestimated, because the composition of prey types will vary less with the abundance of the primary prey type among items delivered than among all items captured. 2) If alternative prey are larger than the primary prey, the functional response will be overestimated, because the composition of prey types will vary more with the abundance of the primary prey among items delivered than among all items captured. Data on allocation of primary and alternative prey captured by kestrels Falco tinnunculus and rough-legged buzzards Buteo lagopus at high and low abundances of the primary microtine rodent prey support these predictions. The results from studies on functional responses of avian predators are reinterpreted in light of the models' predictions. Some of the conclusions from these studies may change if the bias due to the load-size effect is taken into consideration.