Hawk models, hawk mimics, and antipredator behavior of prey

Prey typically respond to potential predators by taking flight. This results in an optimal flight initiation distance (FID) at which the risk of remaining and the cost of flight are equal. Thus, FID is strongly negatively correlated with susceptibility to predation by the sparrowhawk Accipiter nisus across species of small European birds. However, not everything that looks like a hawk is in fact a hawk. Aggressive mimicry arises from the resemblance between a dangerous model such as a predatory hawk and an innocuous mimic such as a cuckoo that makes errors made by individuals that encounter models and mimics potentially dangerous and life threatening. A prime example of such aggressive mimicry is the hawk-like appearance of common cuckoos Cuculus canorus and other cuckoos. If mimicry is efficient, we should expect that species of small birds that are prey of hawks and hosts of cuckoos react just as strongly to a cuckoo as to the presence of a dangerous model like a sparrowhawk. We used FID of small birds as a measure of the reaction of prey to models and mimics, predicting that FID would be negatively related to susceptibility to predation and positively to risk of cuckoo parasitism. Both susceptibility to sparrowhawk predation and rate of brood parasitism by the common cuckoo independently explained variation in FID, consistent with the expectation that both hawks and cuckoos have imposed significant selection pressures on FID and that species of small birds perceive cuckoos as true mimics of hawks.