PREY DETECTION BY COMMON NIGHTHAWKS - DOES VISION IMPOSE A CONSTRAINT

Echolocating insectivorous bats under natural conditions, do not make the fine-detailed discriminations based on target shape and texture th at laboratory data suggest they are capable of. The purpose of this st udy was to test the discrimination ability of a visually-orienting pre dator, the common nighthawk (Chordeiles minor), to allow for a compari son with ecologically similar bats. We hypothesized that vision is bet ter for discrimination than echolocation and predicted that nighthawks would be better than bats at discriminating between targets. We teste d the ability of nighthawks by presenting edible and non-edible target s to free-flying birds in a cafeteria-style experiment. Like bats, bir ds responded to all targets presented, apparently not discriminating, at least initially, between targets based on size or shape. However, u nlike bats, nighthawks rejected the targets in the final phase leading to attack and rarely made contact, We conclude that visually orientin g nighthawks are capable of more fine-grained target discrimination th an echolocating bats. However, because the birds almost never attacked targets, we cannot say what cues such discrimination is based on.