Interactive effects of parental age and environmental variation on the breeding performance of Tengmalm's owls

1. Two main mechanisms have been proposed to account for the changes in breeding performance of long-lived animals with age: progressive appearance or disappearance of different quality individuals within a cohort and within-individual improvements with age. 2. We studied age-related breeding performance of vole-eating Tengmalm's owls (Aegolius funereus, L.), whose prey populations fluctuate in a cyclic manner in western Finland. In years of low vote abundance, most yearling owls were not able to breed. In years of initially intermediate but increasing vote abundance, clutch size increased with female age and partners of old males initiated egg-laying earlier than those of young males. In years of initially high but decreasing vote abundance, the differences in laying dates were still detectable, whereas age-related differences in clutch size of females tended to disappear. 3. Within-cohort analyses did not indicate quality differences between surviving and dying males of the same age in years of increasing vote abundance, whereas in years of decreasing vote abundance early breeding males were more likely to survive than late-breeding males. This novel finding indicates that quality differences between individuals may induce observed age-related differences in only some years, and especially that differential mortality may occur in a different year when the difference in breeding performance is detectable. 4. Within-individual analyses showed that individual males advanced their nest initiation with age, and there was a trend that females laid larger clutches with increasing age. In nests of long-lived males, a significant decline in the clutch size from mid-age (24 years) to older age (6-10 years) was found. 5. We conclude that environmental variation may mask age-related differences in breeding performance at the population level, since the differences appeared to emerge more clearly in poor and intermediate food conditions. Some evidence for the differential mortality-hypothesis was found for males, which are mainly responsible for subsisting their families during the entire breeding season. Substantial improvements in competence or increased reproductive effort from 1 to 3 years of age, and deteriorated skills due to senescence at older ages may account for within-individual changes in breeding performance of males.