Farmland simplification linked to halving of Finnish Short-eared Owls
A 47-year study links the loss of grass-covered vole habitat and open field ditches with a decline of more than half in breeding Short-eared Owls.
A long-running study in western Finland has linked the loss of grassy farmland and open drainage ditches with a decline of more than 50% in a local breeding population of Short-eared Owls.
The research followed birds in the Alajoki agricultural area of Ostrobothnia between 1977 and 2023. The region was regarded as one of northern Europe's principal Short-eared Owl strongholds from at least the 1960s to the 1980s.
Across the 47-year study period, the number of breeding pairs fell at an average rate of 2.8% a year. At the same time, pasture, hay and set-aside fields were increasingly converted to cereals, potatoes and oilseed crops.
Those changes reduced grass-covered habitat suitable for voles by approximately 46%. The number of open field ditches, whose grassy margins give voles food and shelter through winter and early spring, also fell by 52%.
Short-eared Owls are mobile predators capable of travelling long distances to find concentrations of small mammals. In Alajoki, annual breeding density continued to rise and fall with the abundance of Microtus voles, showing that prey cycles remained a powerful influence on where birds settled.
The detailed nest records nevertheless produced an important distinction. Neither clutch size nor brood size showed a long-term decline among pairs that established territories. Reproductive output remained positively associated with vole abundance even in the later years of the study.
“Habitat degradation primarily reduced the number of breeding pairs rather than the reproductive success of pairs that successfully established territories.”
Study authors, Ibis
Agricultural operations and nest predation were the two leading identified causes of failure, each accounting for 44% of nests where a cause could be assigned. The cause remained unknown in more than half of all failed nests, so those percentages do not describe every failure.
The researchers argue that the disappearance of grass-covered patches and ditches reduced vole habitat during the period when owls were choosing territories. A more homogeneous landscape may consequently require a larger area to support each pair and could also intensify competition with Long-eared Owls and Eurasian Kestrels for the remaining prey.
Because the findings come from one intensively studied Finnish site, the exact rate of decline cannot be applied to Short-eared Owls everywhere. The work nevertheless provides rare long-term evidence that farmland simplification can reduce the number of breeding raptors even when the pairs that remain continue to produce similar numbers of young.
9 Sep 2026
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