Do variations in scanning behavior affect tolerance to human disturbance?

Tolerance to human disturbance can be estimated as the distance at which animals become noticeably alert to humans. Although this alert distance has been studied before, no consideration has been devoted to the relationship between vigilance behavior and the probabilities of detecting pedestrians. Our goal was to test whether and how differences in scanning behavior of two model species (spot-winged pigeon and eared dove) would affect the predicted and observed probabilities of detection of pedestrian approaches in two conditions: individuals foraging alone and in flocks. Overall, we found differences between species and between foraging conditions, but no interaction effects. Spot-winged pigeons and individuals foraging alone spent more time scanning, with longer scanning bout duration, than eared doves and individuals foraging in groups, respectively. Spot-winged pigeons showed greater alert distances (observed detection probabilities) than eared doves, and so did solitary individuals as compared to individuals foraging in groups. Greater scanning time may have increased predicted and observed detection probabilities of spot-winged pigeons. Expected probabilities of detection of pedestrian approaches were higher for group than for solitary foraging conditions, contrary to observed alert distances, probably due to a lack of collective detection in flocks. Our results suggest that vigilance behavior can affect the distance at which birds detect human disturbance and the estimation of tolerance. This behavioral effect could have implications in management scenarios (e.g. determining buffer areas), as similar levels of human visitation to protected areas may exert different effects according to the probabilities of detecting visitors within the detection windows of different species. (C) 2003 Elsevier B.V. All rights reserved.