Survivorship of Common Murres on Southeast Farallon Island, California

Abstract

Annual survival of color-banded Common Murres Uria aalge breeding in two colonies on Southeast Farallon Island, California was studied from 1986 to 1991. Survival of adult breeding birds averaged 93.9% in Colony I (-2500 pairs, stable) and 77.2% in Colony II (-50 pairs, growing, recently established). Variation in annual survival between colonies was due to unusually high Peregrine Falcon Falco peregrinus predation on Colony II from 1989 to 1990. Predation occurred when murres attended their breeding sites in winter. Excluding the 1989 to 1990 data, survival rates at Colony II were comparable with those in Colony I, averaging 92.6%. Survival of males at Colony I averaged 100% and of females 94.4%. Survival of males at Colony II averaged 81.7% and females 83.7%. Males experienced greater survival than females (99.0% versus 93.3%) combining the data from both subcolonies and excluding the year of anomalously high mortality at Colony II. Falcon predation appeared to influence survival of males more than females, possibly due to sex-specific differences in reproductive effort or winter attendance behavior. The role of predation in population regulation and habitat selection is discussed.