The foraging ecology of Glausous Gulls preying on the eggs and chicks of Thick-billed Murres
Creators: name: Gilchrist, H. Grant
Publisher: name: University of British Columbia
Year: 2009

description: The glaucous gull (Larus hyperboreus) is a generalist predator with a circumpolar distribution. It commonly depredates the eggs and chicks of birds nesting in the Arctic, and often breeds in association with colonial nesting waterfowl and seabirds. The aim of this thesis was to examine how environmental factors constrained the ability of glaucous gulls to depredate cliff-nesting thick-billed murres (Uria lomvia), and to determine whether these constraints vary through time, in space, or between individuals. I also wanted to determine how changes in gull foraging constraints could affect the impact of gulls on murre reproductive success and population dynamics. This study was conducted from 1990-1992 at a murre colony on Coats Island, Canada, and in 1993 at colonies in the Upernavik region of Greenland. Based upon results from egg placement experiments and field observations of predation, gull foraging success was constrained by high murre nesting densities, collective murre defence, and the accessibility of narrow cliff ledges. However, windy conditions enhanced the ability of gulls to overcome these constraints. Wind improved the aerial maneuverability of gulls, and enabled gulls to reach weakly-defended narrow ledges and avoid contact with murres during attack. Murre defence on narrow cliff ledges was less effective because murres had difficulty turning to face attacking gulls without dislodging their own eggs and chicks. Gull search activity, attack activity, and predation rates were strongly correlated with windy conditions. Consequently, the impact of gull predation on murre reproductive failure, which ranged from 0% to 21% depending on nest type, was determined largely by wind. Under calm wind conditions, adult gulls were often inactive and they rarely fed their chicks. This inactivity may reflect the reluctance of gulls to forage on foot, because although this was the most successful attack mode, it also incurred the greatest contact with defending murres. Alternatively, gulls could have been responding to the varying energetic demands determined by changing weather conditions, so that the danger of injury while foraging did not influence mode selection or periods of foraging activity. I explored these two alternatives using a dynamic simulation model which integrated field data and energetic estimates of gull foraging behaviour. The model suggested that the first explanation, which was based upon energy considerations alone, was not sufficient to explain gull foraging inactivity under calm wind conditions. However, the model supported the idea that gulls were sensitive to risk of injury, and that they should select low-danger foraging modes that provide low energetic gains. The model also revealed that gulls can afford to use foraging modes that yield low energetic gains relative to more productive and dangerous ones (e.g. scavenging vs. foraging on foot under calm conditions), because even poor foraging modes are sufficient to meet their energetic demands under most circumstances. I predicted that a decline in the density of nesting murres should enhance the ability of gulls to overcome the constraints of calm wind conditions, cliff ledge accessibility, and prey defence. Thus, murre colony declines should increase the impact of gull predation on murre reproduction. To examine the impact of gull predation at declining thick-billed murre colonies, I compared gull foraging mode selection, predation rates, and murre nest site selection at Coats Island, N.W.T., with that at declining murre colonies found near Upernavik Greenland. I found that gulls at declining colonies foraged on broad cliff ledges and were less constrained by calm wind conditions, apparently because population declines increased the availability of low nesting density ledges where gulls could maneuver on foot and attack murres with little risk of injury. Perhaps because of this, predation rates at declining murre colonies were consistently higher than at the stable Coats colony, particularly at low wind speeds.