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 that laboratory data suggest they are capable of. The purpose of this study was to test the discrimination ability of a visually-orienting predator, the common nighthawk (Chordeiles minor), to allow for a comparison with ecologically similar bats. We hypothesized that vision is better for discrimination than echolocation and predicted that nighthawks would be better than bats at discriminating between targets. We tested the ability of nighthawks by presenting edible and non-edible targets to free-flying birds in a cafeteria-style experiment. Like bats, birds responded to all targets presented, apparently not discriminating, at least initially, between targets based on size or shape. However, unlike bats, nighthawks rejected the targets in the final phase leading to attack and rarely made contact, We conclude that visually orienting nighthawks are capable of more fine-grained target discrimination than echolocating bats. However, because the birds almost never attacked targets, we cannot say what cues such discrimination is based on.