|Habitat and Ecology:
Behaviour The Common Guillemot is a pursuit-diving marine bird which forages primarily during daylight. One parent remains at the colony with the chick whilst the other is on a foraging trip. Birds departing colonies usually splash-down to form large rafts close to the colony before departing to foraging areas. External radio tagging has been shown to adversely effect breeding (Wanless et al. 1988, Nevins 2004), whereas birds fitted with internal transmitters behaved as normal (Wanless et al. 1988). Diet During the breeding season, schooling pelagic fish species are the most important prey for adults, though benthic species can also be important.In Labrador, Canada, Shannies (Sticheaidae) were the main source of food, comprising 84% of the diet in 1996 and 52.9% in 1997 (Bryant and Jones 1999). Capelin (Mallotus villosus) were also important, forming 44.7% of the diet in 1997 (Bryant and Jones 1999). In the UK, the main prey taxa are sandeel (Ammodytes spp.) and clupeids. Small gadoids are also important at some colonies. Foraging range This species dives to maximum depths of 170-230m. During the breeding season, surveys recorded the highest densities of birds in the 51 - 100 m depth zone, although birds were still abundant in water less than 50 m and 101 - 200 m deep. Very few were seen in deeper areas (Wanless et al. 1990). The foraging range of this species appears variable across seasons and years. At the Isle of May, Scotland, during 1986 around 70% of foraging trips were over 7 km from the colony, whereas in the following year the birds tended to make shorter trips (Wanless et al. 1990). Foraging trips in eastern Canada are within 100 km (Cairns et al. 1987, Davoren et al. 2003), and in Pribilof Islands, Alaska, foraging occurred mostly within 60 km. In Witless Bay, Newfoundland, foraging aggregations formed over large Capelin schools within 5 km of breeding sites. Along the Newfoundland coastline, aggregations occurred within 15 km of the colonies, and at an offshore ridge about 80 km southeast of the colonies. Despite these large foraging radii, waters close to the colony were the most frequent destination of feeding birds. This was particularly true during chick rearing, when only one third of feeding trips could have exceeded 10 km from the colony (Cairns et al. 1987). In Pribilof Islands, Alaska, birds showed a consistent preference for shallower waters (Schneider and Hunt 1984). High densities of foraging birds have also been observed foraging over a submarine ridge (Coyle et al. 1992). In Kachemak Bay, Alaska, it appears that birds tend to feed over rocky substrates in water depths of about 18 to 55 m (Sanger G.A. 1987). They have also been observed to forage in riptides (Wanless et al. 1990), and in areas of sandy sediment suitable for sandeels (Wanless et al. 1998, BirdLife International 2000). Near Flamborough Head, UK, the Farallones, California, and Pribilof Islands, Alaska, birds forage at fronts between thermally distinct bodies of water. In the UK example, this occurs at a seasonally-occuring front between thermally-mixed and thermally-stratified water, whereas in the Farallones, the front was between cold and salty upwellings and estuarine outflow (Decker and Hunt 1996, BirdLife International 2000).