| Summary: | 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. === Science, Faculty of === Zoology, Department of === Graduate
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