The diving behaviour of Pigeon Guillemots (Cepphus columba) off southern Vancouver Island

Abstract

The foraging behaviour of Pigeon Guillemots (Cepphus columba) was observed off southern Vancouver Island, British Columbia. Diving bouts comprised 1–24 dives. Birds returned to the surface with prey in 22 of 248 (9%) dives, and mean handling time for prey was 34.7 s. Dives averaged 87 s (ranging from 37 s in water 14 m deep to 144 s at 34 m) and the mean postdive pause lasted 98 s (range 24–232 s). Birds foraged in water depths from 6 to 45 m. The duration of both dives and pauses increased with water depth. Our model of Pigeon Guillemot diving behaviour predicts foraging time at the bottom to be maximized during dives to depths of 22–24 m, while foraging efficiency, (foraging time)/(dive + recovery time), is maximized at 10 m. Calculated work to resist buoyancy and drag during descent and foraging phases of the dive cycle suggest that energetic savings from reduced buoyancy at depth may not explain how birds increase dive duration with increasing depth. Pigeon Guillemots appear to maximize time spent in the foraging patch. In 82% of transects, the most frequently chosen foraging depth was 15–20 m (mode). Model predictions were supported by observations that 43.6% of Pigeon Guillemots preferred water depths of 15–20 m, while 19% preferred water depths of 10–15 m.