Foraging behaviour of thick-billed murres breeding in different sectors of the North Water polynya: an inter-colony comparison

The North Water polynya is an area of open water that occurs year round between northwest Greenland and Canada. Oceanographic conditions differ between the western ('latent heat' polynya type) and extreme eastern ('sensible heat' polynya type) sectors of the polynya, and the effects of this variation on prey availability and foraging conditions for high trophic-level predators are unknown. Thick-billed murres Uria lomvia breed on both sides of the polynya, and we conducted inter-colony comparisons of their foraging ecology. We measured time allocation and foraging efforts of chick-rearing adults using electronic data-loggers which recorded dive profiles and flight activity. Murres on the western side of the North Water polynya foraged at relatively shallow depths and rarely (4.4% of dives) explored depths beyond 70 m. In contrast, murres on the eastern side searched for prey at >70 m in 23% of all dives, and spent a greater share of each trip actively diving. The Canadian birds made foraging trips of longer duration than the Greenland murres, but they also spent more time 'resting' at the sea surface. Rest time at sea was apparently the only time buffer available for increasing foraging effort. The Greenland birds had little room for increasing foraging effort, while the Canadian birds had spare capacity for additional work. Maximum potential foraging ranges were equal at the 2 colonies (75% within 50 km), and the murres had access to approximately equally-sized areas of open sea. We estimate that the density of foraging birds at sea within the 50 km of colonies would be 6.5 times higher on the Canadian side due to the large breeding colony at Coburg Island. Intra-specific competition for food should increase with increasing colony size because the predators may reduce food resources within their foraging range. Nevertheless, the Canadian birds worked less than Greenland murres, indicating a relatively good food availability in the western part of the polynya. However, the higher foraging effort by Greenland murres paid off in a higher chick growth rate, so it remains unclear why the Canadian birds did not also make use of their spare capacity to increase foraging effort. Since high-level zooplankton/fish stock interactions are seldom part of oceanographic studies in the Arctic, seabird foraging behaviour and breeding ecology serve as two of the few indicators of possible local variation within the polynya ecosystem.