Wind and Prey Nest Sites as Foraging Constraints on an Avian Predator, the Glaucous Gull

Abstract

Predators often face a trade-off between energy gain and risk of injury, and their foraging decisions should thus vary with conditions that influence this trade-off. We studied how wind speed affected the foraging decisions of Glaucous Gulls preying on Thick-billed Murres breeding on Coats Island, Northwest Territories, Canada in 1990–1992. At this colony, murres bred at varying densities on broad and narrow cliff ledges. Under calm wind conditions (<10 km/h), aerial search and attack activity by gulls was low. At wind speeds >15 km/h, gulls searched more actively and focused their aerial attacks on nest sites on narrow ledges. Aerial attacks on narrow ledges were less successful (in number of eggs or chicks taken per attack) than attacks made on foot on broad ledges. However, aerial attacks also had a lower incidence of contact with defending murres because: (1) gulls could approach ledges readily by gliding; (2) murres on narrow ledges could not turn to fend off attacking gulls without dislodging their eggs or chicks; and (3) dense groups of murres on broad ledges defended their eggs and chicks communally. Gulls foraged successfully on foot on broad ledges despite communal defense by murres, although they suffered a greater risk of being struck on the head by defending murres. Because gulls favored aerial attacks under windy conditions, we concluded that a trade-off between maximizing energy gain and minimizing risk of injury determines foraging decisions by gulls in this system, and that this trade-off was mediated by wind.