Stability, Regulation, and the Determination of Abundance in an Insular Song Sparrow Population

The population dynamics of the Song Sparrow, Melospiza melodia, were studied for 15 yr on Mandarte Island, in southwestern British Columbia, Canada. This population exhibited a high median density (7.8 females/ha) and fluctuated strongly (10—fold or more) from year to year. The population received few successful immigrants, even at low densities, and its dynamics were thus driven by local events. Two strong density—dependent regulating mechanisms were detected. First, reproductive output was strongly depressed at high densities because of an increased rate of nest failure and a decline in mean clutch size. Greater nest failure at high density was due to increased predation on eggs and nestlings. Nest failure increased with the rate of nest parasitism by Brown—headed Cowbirds (Molothrus ater) and was lowest when cowbirds were absent from Mandarte, suggesting that cowbirds either cause or facilitate nest failure. Second, the rate of juvenile recruitment was inversely related to the density of adults, because of the interference competition for breeding territories between adults and yearlings. The survival rate of adults was independent of population density. Survival rates of adults and juveniles were negatively affected by weather in February 1989, when an annually cold spell coincided with the loss of over 90% of the population. A second severe population crash occurred during the winter of 1979—1980, but the cause of this crash is not known. On two other occasions, periods of severe winter weather occurred at the same time as a substantial fraction of the population disappeared. However, not all periods of poor weather were accompanied by marked disappearances of birds. Adult females survived better than adult males during the breeding season but equally well outside it. We found no correlations between the survival rates of juveniles and adult males and females in years without a population crash suggesting that there is no common cause underlying the observed variation in survival among age and sex classes in most years. Simple models of the population indicate that either density—dependent reproductive success or density—dependent recruitment of locally hatched juveniles can regulate population size. These two regulatory factors operate in a sequential and independent manner. Despite the presence of strong regulating factors, population crashes caused marked instability in population size, nearly leading to extinction in 1989. The rate of juvenile recruitment was the only factor closely related to the net rate of population growth in noncrash years and in all years combined, indicating that juvenile recruitment is the central factor affecting the abundance of Song Sparrows on Mandarte Island.