The effect of age at first breeding on Ural owl lifetime reproductive success and fitness under cyclic food conditions



1.  Individuals are expected to balance the costs and benefits underlying the trade-off between current and future reproduction. If starting to breed does not seriously lower future reproductive output, individuals that start breeding early in their life should have a higher fitness than individuals that postpone their breeding career. We studied how the fluctuations in food supply interacted with Ural owl’s age at first breeding, lifetime reproductive success (LRS) and fitness.

2.  During the period 1977–95, 126 Ural owl females started and ended their breeding career in a study area in southern Finland. Voles, the owls’ main food source, showeda 3-year cycle of low, increase and peak population numbers. We recorded when the females started to breed and how many fledglings they produced. For 57 females the age at first breeding was known.

3.  LRS of female Ural owls varied from 0 to 33 fledglings (mean 6.7±0.52 SE). The variance in LRS was explained by variation in the components: breeding lifespan (97%); nest success (23%); and average clutch size (15%).

4.  Survival of breeding females was low (62%) after a peak year, when the vole population crashed. In other phases the survival was 85–95%. Females that started breeding in a peak year had half the LRS of females that started in an increase year.

5.  There was a strong interaction between the vole cycle and age at first breeding. 1-year-olds started in a peak and 2-year-olds in an increase year.

6.  There was no effect of age at first breeding on LRS for females that started breeding in the same phase of the vole cycle.

7.  Females that started breeding at age 1–3 years had equal fitness, whereas females that started at age ≥4 had a lower fitness. Females that postponed first breeding as a two-year-old in an increase year had a lower fitness than females that did not do so. Females that postponed first breeding as a 1-year-old in a peak year had equal fitness to females that did not do so.

8.  Cyclic fluctuation in food supply clearly constrains the option as to during what phase and at what age to start breeding. In terms of fitness, the optimal age to start breeding depends on the phase of the vole cycle at hatching.