Survival of male Tengmalm's owls under temporally varying food conditions.

We analysed whether annual survival of male Tengmalm's owls in western Finland varies according to changes in the abundance of their main prey, Microtus voles. Our analyses were based on capture-recapture data over five 3-year population cycles of voles from 1981to 1995, each cycle consisting of consecutive years of low, increasing and decreasing vole abundance. Survival estimates of males in the increasing vole years (ca. 55%) were higher than in the decreasing vole years. In the latter case, males faced a drastic crash in Microtus vole numbers, and only about one-third of males survived over this crash. After the crash of vole populations, male survival increased rapidly (up to 76%) according to the recovery of Microtus vole populations. These results show that temporal variation in the abundance of their main prey modifies the survival of male Tengmalm's owls. In addition to survival, recapture rates also varied, largely due to the fact that in poor vole years a majority of males skipped breeding. The large among-cycle phase variation in survival (25-76%) probably creates selection for phenotypic plasticity in life-history traits related to survival and reproduction.