Declining thick-billed murre Uria lomvia colonies experience higher gull predation rates: an inter-colony comparison Original Research Article

Avian predators of colonial-nesting seabirds preferentially attack individuals nesting alone or on the edge of groups, apparently because they can forage without being struck by defending neighbours. If nesting densities of colonial seabirds declined, predators are predicted to have fewer foraging constraints and consequently a greater impact on seabird reproduction. Thus, avian predation could prevent the recovery of seabird populations once sources of adult mortality have been removed. To test these predictions, glaucous gull Larus hyperboreus predation rates, foraging mode selection and murre nest-site selection at an expanding thick-billed murre Uria lomvia colony (Coats Island, Canada) were compared with those at declining murre colonies in Northwest Greenland (Timmiak, Kingittoq). At Coats Island, most cliff ledges were densely occupied by breeding murres so that gulls could not forage extensively on foot. Gulls relied on wind when foraging (>10 km/h), which enhanced their aerial maneuverability and enabled them to reach nest sites on narrow ledges that were poorly defended. In contrast, glaucous gulls at Kingittoq foraged on foot regardless of wind conditions because broad ledges supported small breeding groups of murres. This resulted in higher attack and predation rates of murres at low wind speeds at Kingittoq. At Timmiak, which had experienced the greatest population declines, murres nested at high densities on narrow ledges or in crevices inaccessible to gulls. It was concluded that lower nesting densities at declining colonies reduce the risk of injury for gulls during attack and that this increases predation rates at low wind speeds. These findings suggest that seabird population declines that result from human-induced mortality (e.g. over-harvest) can increase the impact of avian predators on seabird reproduction and potentially, slow the recovery of colonies.