Factors Influencing Colony Attendance by Pigeon Guillemots on Southeast Farallon Island, California

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Abstract
I studied colony attendance in a marked sample of Pigeon Guillemots (Cepphus columba) in 1979 and 1980 on Southeast Farallon Island, California. These observations were supplemented with sightings of known-age birds in 1977, 1981, adn 1982. In 1979 and 1980 I compared the attendance of males and females from the time of colony occupation through the incubation and chick-rearing periods. Guillemots returned in early March in both years, but occupied territories earlier in 1979 than in 1980. Older, experienced birds arrived on land before young, nonbreeders. Males arrived earlier than females each year and spent more time on their territory each morning before and after egg laying. In 1979, colony attendance was higher for both sexes, egg laying began earlier, larger clutches were laid, and young chicks were fed at a higher rate. I suggest that the annual differences in attendance and egg laying dates may been due to the greater availability of food in 1979, as measured by higher chick-feeding rates. Sex differences in attendance probably result from the increased energetic requirements associated with egg formation in females.