Predation of Tengmalm's Owls: Numerical Responses, Functional Responses and Dampening Impact on Population Fluctuations of Microtines

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Abstract
The predation of Tengmalm's owls in the breeding season was studied during 1977-87 in western Finland. The yearly number of breeding pairs (range 1-26) and non-breeding males (0-10) in the study area (100 km2) was positively related to the number of available Microtus (M. agrestis and M. epiroticus) and Clethrionomys glareolus. The mean number of fledglings produced per pair was also positively correlated with the numbers of voles. The owls were able to track without time lags the population fluctuations of their microtine prey due to the high degree of mobility, vole-supply-dependent adult and juvenile survival, large reproductive potential and early maturity. The density of Microtus voles was the most important factor determining the diet composition of breeding owls. The functional response curve to the changing numbers of Microtus spp. was very close to linear and did not level off at the high vole densities. This indicated a constant predation rate without satiety when voles peaked. The predation impact on microtines was positively dependent on vole densities and suggests that the owls dampen microtine cycles. The following factors seemed to promote the dampening impact: rapid numerical and functional responses to changes in vole densities, owl populations only slightly limited by territoriality, and spatial heterogeneity of the study area.