Summary1. 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 vole abundance, most yearling owls were not able to breed. In years of initially intermediate but increasing vole 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 vole 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 vole abundance, whereas in years of decreasing vole 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 (2-4 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.

We investigated how physiological stress in an area-sensitive old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), is associated with forest fragmentation and forest structure. We found evidence that the concentrations of plasma corticosterone in chicks were higher under poor food supply in dense, young forests than in sparse, old forests. In addition, nestlings in large forest patches had lower corticosterone levels and a better body condition than in small forest patches. In general, corticosterone levels were negatively related to body condition and survival. We also found a decrease in corticosterone levels within the breeding season, which may have been a result of an increase in food supply from the first to the second broods. Our results suggest that forest fragmentation may decrease the fitness of free-living individual treecreepers.

We manipulated the primary brood size of Eurasian treecreepers (Certhia familiaris) breeding in different sized forest patches (0.5-12.8 ha) in moderately fragmented landscapes. We examined the effects of brood size manipulation (reduced, control, enlarged) and forest patch size on physiological stress (heterophillymphocyte ratios; H/L), body condition and cell-mediated immunocompetence (phytohaemagglutinin test). Nestlings' H/L ratios were negatively related to forest patch area in control and enlarged broods, whereas no effects were found in reduced broods. The effects of forest patch area were strongest in enlarged broods, which had, in general, twofold higher H/L ratios than control and reduced broods. The elevated H/L ratios were positively related to nestling mortality and negatively correlated with bodycondition indices suggesting that the origin of stress in nestlings was mainly nutritional. Cell-mediated immunity of nestlings was not related to brood manipulation or to forest patch size. Also, the H/L ratios of adults were not related to brood manipulation or forest patch size. In addition, parental H/L ratios and body condition were not related to nestling H/L ratios. Our results suggest that during the breeding period the deleterious effects of habitat loss are seen explicitly in growing young.

We examined the effects of predatory and competitive interactions among three owl species on reproductive success and population composition of these raptors both experimentally and observationally. Eagle Owls (Bubo bubo, body mas $\approx$ 2700 g), Ural Owls (Strix uralensis, 900 g), and Tengmalm's Owls (Aegolius funereus, 130 g) coexist in Fennoscandia and feed mainly on small rodents. Predation may play an important role in interactions among these owl species, because Eagle Owls and Ural Owls can kill Tengmalm's Owls but cannot enter the small entrance hole of Tengmalm's Owl nest boxes. We asked (i) whether predation risk and interspecific competition due to Eagle Owls and Ural Owls reduced breeding density and fitness of Tengmalm's Owls, and (ii) whether these interactions increased intraspecific competition for safe nesting sites among Tengmalm's Owls. We manipulated breeding densities of potentially competing owls by erecting nest boxes, the control being boxes in areas where breeding attempts of competing owl species were absent. Control nest boxes in areas with no Eagle Owl and Ural Owl territories, and nest boxes within Eagle Owl territories, were used by breeding Tengmalm's Owls more than nest boxes within Ural Owl territories. Most breeding attempts of Tengmalm's Owls near Ural Owls failed during the courtsip period. The observational data revealed that breeding frequency of Tengmalm's Owls was reduced and the mean start of egg laying was delayed by 11 d within 2 km of Ural Owl nest. In addition, male Tengmalm's Owl at these nests were younger and paired more often with short—winged (i.e., young and generally subdominant) females than when farther away from Ura Owl nests. Our results suggest that inexperienced male Tengmalm's Owls are forced to establish their territories in the vicinity of Ura Owls where they often paired with subdominant females. The areas near Ura Owl nests are suboptimal habitats for Tengmalm's Owls, whereas those near Eagle Owls are not. We conclude that predatory and competitive interactions from Ural Owls decrease the breeding population size of Tengmalm's Owls by reducing the suitable habitats. This is the first experimental demonstration that such interactions may decrease fitness of raptors.

In western Finland, yearly median laying dates of Tengmalm's owls varied from 14 March to 27 April during 1973-1989 and were negatively correlated with the winter densities of voles. Yearly mean clutch sizes varied from 4.0 to 6.7 and were more closely related to the spring than to the winter densities of voles. The yearly mean clutch size decreased with yearly median laying date. The 3-year vole population cycle is typical of the study area. The start of egg-laying was earliest in the peak phase of the cycle (median laying date 22 March), when vole numbers are high during egg-laying, but decline rapidly to low numbers in the next autumn or winter. In the increase phase (1 April) vole abundances are moderate at the time of laying, but increase to a peak in the next autumn or winter. In the low phase (15 April) voles are scarce in spring and in the preceding winter, starting to increase in late summer. Clutch size and female body mass were independent of laying date in the low phase, decreased slowly but significantly in the increase phase, and declined abruptly in the peak phase. These trends also held when the effects of territory quality, female age and male age were ruled out. When comparing the same laying periods, clutch sizes were significantly larger in the increase than in other phases of the cycle, but there was no difference between the peak and low phases. Supplementary feeding prior to and during egg-laying increased clutch size independent of laying date. These results agreed with the "income" model (the rate of energy supply during laying determines clutch size). Tengmalm's owls invest most in a clutch in the increase phase, as the reproductive value of eggs is largest because of high survival of yearlings. A high reproductive effort may be adaptive during this phase, because the availability of voles is predictable during the laying period.