Environmental variation and the demography and diet of thick-billed murres
Conditions in arctic marine environments are changing rapidly, and understanding the link between environmental and demographic parameters could help to predict the conse- quences of future change for arctic seabirds. Over 20 yr (1988 to 2007), we studied colony atten- dance, adult survival and reproductive success of thick-billed murres, as well as the departure masses and diets of their chicks at Coats Island, Nunavut, Canada (62.95° N, 82.00° W). We eval- uated how each parameter responded to climatic conditions near the colony during the breeding season, and in the winter range during the non-breeding period (delineated using geolocation). We used the Arctic Oscillation (AO) and North Atlantic Oscillation indices, as well as local vari- ables to describe ice, oceanographic and weather conditions. We demonstrate that adult survival varied little among years but was higher after winters with lower AO indices, more ice in the south-western part of the winter range in spring, and cooler sea surface temperatures (SST). By comparison, interannual variation in breeding parameters (breeding success, chick mass and diet) was pronounced and responded to SST and ice conditions near the colony. Counts of birds attend- ing the colony, influenced heavily by pre-breeders, were most strongly related to the conditions that influenced adult survival; counts were positively related to ice concentration in the south- west of the winter range. Relationships between climatic conditions and demographic parameters were often lagged, suggesting effects mediated through the food web. The trend towards higher SST and less ice in the vicinity of the colony has not yet reduced reproductive success. However, a significant, ongoing decline in the rate of energy delivery to nestlings suggests that a critical threshold may eventually be crossed.