Distribution and Foraging Behaviour of Wintering Western Grebes

The Western Grebe, Aechmophorus occidentalis, is a species which breeds mainly in the prairie regions of Canada and the United States and winters on the Pacific Coast. Little is known of its distribution and behaviour during this period on coastal marine waters. I studied the seasonal distribution, the diurnal distribution, and the foraging behaviour of Western Grebes off southern Vancouver Island. I found that Western Grebes arrive in Saanich Inlet in late August and remain until mid May. Excluding migratory transients, coastal populations reach peak numbers in mid winter. Seasonal changes in flock numbers appear to be the result of both transitional migrants as well as birds that disperse to adjacent foraging areas. This research is the first to show that wintering Western Grebes are nocturnal predators. Roosting flocks disperse at dusk to take advantage of vertically migrating prey that is available in surface waters at night. In daytime Western Grebes form large roosting flocks as an anti-predator defense while resting. While nocturnal foraging is not an unusual strategy among birds, it is most unusual for a bird that specializes on pelagic fish as prey. How can birds find such cryptic elusive prey in darkness? I suggest that Western Grebes may use ambient marine bioluminescence to locate and capture prey. This mechanism of prey capture has not previously been suggested for birds and may explain how grebes can take advantage of the increased abundance of waters at night.