Prey handling in raptors in relation to their morphology and feeding niches

Abstract

Most raptors take large prey for their size compared to other birds, but tear the prey apart into small morsels before swallowing. Little is known about how the efficiency of this prey handling varies among raptors, and how it relates to their feeding niches, diets and gape dimensions. We offered 202 mammalian and 224 avian prey items to 37 wild raptors kept temporarily in captivity, representing ten species and three orders. Feeding efficiency was measured as the proportion of a prey item that was ingested. The proportion of a prey item ingested was larger for mammalian than for avian prey, declined with prey size, increased with raptor size, and was larger for typical vole feeders (with shorter and wider bills) than typical bird feeders (with longer and narrower bills). The proportion of a prey item ingested was not related to raptor sex when controlling for focal raptor body mass. The probability that the head of a prey was ingested was higher if the prey was a mammal than if it was a bird, whereas the opposite was the case for the entrails of a prey. The results suggest that the traditional use of prey remains to estimate raptor diets may lead to severe biases, the magnitude of which would depend on prey type and size, raptor sex, species and order, and raptor feeding niche. Failure to correct for uneaten remains of a prey would lead to overestimating the profitability of large prey, and in particular of large avian prey. The results are consistent with the idea that vole feeders can afford to have a well‐developed digestive tract, and thus swallow prey in large pieces, because they pounce on prey from above. Bird feeders, however, must ingest food in smaller pieces because they have smaller guts as a result of selection to keep the body mass low to capture agile prey.