Fluctuating Food Abundance Determines the Lifetime Reproductive Success of Male Tengmalm's Owls

Abstract

1. I examined the lifetime reproductive success (LRS) of male Tengmalm's owls in western Finland where vole abundance fluctuates cyclically among years. Males were site-tenacious after their first breeding attempt. 2. The productivity of 141 males, all of which started to breed during 1979-87, was studied up to 1990. They spent 1-7 years (mean 1.5 years) as breeders and raised 0-26 fledglings (mean 5.4) during their lifetime. The lifetime number of fledglings produced by males was highly significantly correlated with their lifetime contribution of known recruits to the breeding population. 3. Most male fledglings (78%) died before the first breeding attempt. Of the young fledged in one year, 5% produced 50% of fledglings in the next generation, and 21% of males reared 50% of all fledglings produced by the population. 4. Offspring survival from egg to fledgling was the most important component of LRS, followed by the life span and clutch size. The age of first breeding did not affect LRS, but polygynous males produced twice as many fledglings as monogamous ones. 5. Among environmental factors, vole abundance was the most important determinant of LRS: males entering the breeding population in the low and increase phases of the vole cycle gained larger LRS than males recruiting in the peak and decrease phases. 6. As the breeding life span of polygynous males was longer than that of monogamous ones and as there was no negative relationship between the brood size and the duration of breeding life, no evidence of reproductive costs could be obtained. Superior males may easily compensate for the large parental investment by increasing their food intake without risking their future survival.