LIFETIME REPRODUCTIVE SUCCESS IN BARN OWLS NEAR THE LIMIT OF THE SPECIES RANGE

I Studied 357 nesting attempts by a minimum of 473 Barn Owls (Tyto alb a) in northern Utah from 1977 to 1995, and documented lifetime reprodu ctive success for 262 owls. Mean age of first breeding was 1.06 years (range <1 to 3), mean number of years breeding was 1.30 (range 1 to 7) , and mean number of years breeding successfully was 1.03 (range 0 to 6). Eleven percent of the pairs produced two broods in one year. Mean number of eggs produced in a lifetime was 9.76 (range 1 to 66), and me an number of young fledged was 5.58 (range 0 to 50). Eight percent of the females laid 25% of the population's eggs, and 55% laid 75%. Of th e females that laid eggs, 22% produced no fledglings. Twelve percent o f the females left breeding descendants in the population with up to f our generations traced; the number of direct descendants from these fe males ranged from 3 to 69. Longer-lived owls produced more eggs and fl edglings in their lifetimes, but age that breeding began did not stron gly affect lifetime reproductive success. Breeding age had a weak but nonsignificant effect on clutch size and the number of fledglings prod uced in a breeding season. Habitat variability did not affect LRS, but sites with higher usage were correlated with higher nesting success. Variability in the severity of winter weather had a strong influence o n LRS through mortality of adults, reduction in clutch size and in the likelihood of producing two broods in one season. Severe winters, tho ugh, had little effect on the number of fledglings in a brood in the f ollowing breeding season. Age and sex of Barn Owls had very little inf luence on individual LRS.