The White-Crowned Sparrow: Reproductive Success (1975-1980)

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Abstract
We report the results of a 6-yr study of two banded populations of sedentary White-crowned Sparrows (Zonotrichia leucophrys nuttalli). All birds were banded and territory holders identified each year. We determined the outcome of 1,264 nests and located all nests of 186 pairs during a breeding season. Each pair produced an average of 2.5 fledglings in a season. Given the fact that a territorial bird has an expectancy of remaining on territory about 1.5 yr, the population seems to be at a stable level. The birds at one of the study locations, Twin Peaks, were less successful overall and more variable year to year than at the other, the Presidio. The differences between the two areas could be the result of severe weather changes that altered the characteristics of the vegetation. Only 0.34 of the total number of nests (978) were successful, predation being the greatest cause of nest failure. Of the total nests, 0.39 were lost to predation (0.13 to egg predation and 0.26 to nestling predation). Survival rates indicate that nest mortality is 0.0197 per day for the incubation stage, 0.0450 per day for the nestling stage, and 0.0366 per day for the total nesting period. Although it is possible that observer disturbance biased the results, the increased understanding of these birds that is derived from establishing natal sites, parentage, and kinship is of greater benefit than the probable cost of any such disturbance.

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