Habitat composition as a determinant of reproductive success of Tengmalm's owls under fluctuating food conditions

Abstract

The effect of landscape composition on the breeding success of vole‐eating Tengmalm's owl (Aegolius funereus) was studied in western Finland at five different spatial scales (250–4000 m) around the nests during two consecutive three‐year population cycles of voles. Landscape composition had strongest effects on owl breeding in the decrease phase of vole cycles. Significant variation in owl breeding occurred along the productivity gradient from farmland predominated areas to barren hinterland. Owls tended to produce earlier clutches on territories predominated by agricultural areas in increasing vole years. A similar trend was observed in the decreasing phase of the vole cycle; owls breeding on barren hinterland seemed to delay breeding compared to owls breeding near agricultural areas. Surprisingly, nestling survival and fledgling production in the decreasing phase declined steeply with increasing proportion of farmland. Clutch size was not significantly related to landscape composition. The number of fledglings decreased with increases in clear‐cut and sapling areas in the decrease phase. During the declining years of vole abundance nestling survival increased from western farmland areas towards the eastern outlying district. These results indicate sudden summer decline of vole populations on farmland predominated habitats. This is probably due to that the number of vole‐eating predators, and hence their impact on vole populations is apparently higher in farmland areas than on forested hinterland. This finding gives support for the ‘spill‐over’ hypothesis, which states that predators and their exploitation tends to ‘spill over’ from luxuriant habitats to the barren habitats.