Colony Attendance of Pigeon Guillemots as Related to Tide Height and Time of Day

The attendance of Pigeon Guillemots (Cepphus columba) at three colonies in Skidegate Inlet, Queen Charlotte Islands, British Columbia was investigated according to time of day and tide level during May - June 1990 and 1991. Guillemot attendance in all three colonies was correlated most strongly with tide height, but also with time after sunrise and tide direction. An analysis of colony attendance on Mandarte Island, British Columbia, however, found time after sunrise to be the only significant factor influencing attendance. The maximal tide range at Skidegate Inlet, 7.8 m, compared to only 3.8 m at Mandarte Island, was likely responsible for the patterns observed. The optimal time to determine nesting populations of Pigeon Guillemots is at high tide in the morning.