Estimates of instantaneous growth rates (lambda) of spotted owl (Strix occidentalis) populations have been based on demographic data that uniformly assumed an equal sex ratio among fledglings. In this study, sex ratios of subadults, banded as juveniles, and fledgling California spotted owls (S. o. occidentalis) were observed and compared to an assumed 1:1 ratio. The sensitivity of lambda to variations in fledgling sex ratio was then examined. The sexes of 73 fledgling spotted owls from 1993 and 1994 were 42:31 (males:females) in the Sierra National Forest (SNF) and Sequoia and Kings Canyon National Parks (SNP). Sex ratios of fledglings ranged from 2.5:1 (SNF 1993) to essentially 1:1 (SNF 1994, SNP 1993 and 1994) but did not differ significantly from 1:1 in any sample. The sex ratio of 25 subadults in both study areas was significantly different from 1:1 and strongly biased towards males (19:6). This more likely results from differential dispersal differences between juvenile male and female owls, with the females likely dispersing beyond study area boundaries more often than males. Lambda was only slightly sensitive to variation in fledgling sex ratio, and was influenced more by yearly variation in fecundity than in observed vs. assumed sex ratios within years. Variations in fledgling sex ratio observed in this study are probably not biologically significant when considered at the larger scale of a regional population, as opposed to the small subpopulations under study in SNF and SNP, or over longer time scales than the two years reported here.
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