Population dynamics and reproductive success of Short-eared Owls in relation to widely fluctuating food abundance and farmland habitat degradation

Abstract

Populations of the Short-eared Owl Asio flammeus have declined across Europe and North America, probably in response to habitat degradation driven by agricultural intensification. However, long-term evidence linking farmland habitat change to variation in breeding density and reproductive success remains limited. We investigated these relationships in an agricultural landscape in western Finland over a 30-year period (1977–2006), focusing on owl breeding parameters and temporal changes in grassy habitats supporting their primary prey (voles). Annual breeding density ranged from 0.0 to 1.5 (mean 0.35) nests km−2 and increased with spring vole abundance. Mean clutch size varied between 2.5 and 8.2 eggs, and mean brood size between 0.7 and 4.4 nestlings per nest, with significantly higher values observed during years of high vole abundance. Nest failure was primarily attributed to agricultural activities and predation. To assess long-term population trends, monitoring was extended to a broader 1000-km2 area through 2023. Over the 47-year study period, the local breeding population declined by more than 50%, corresponding to an average annual decrease of 2.8%. This decline coincided with substantial agricultural intensification, including the conversion of pasture and hay fields to cereal, potato and oilseed cultivation, as well as the reduction in open drainage systems. These changes resulted in a 46% reduction in grassy habitats suitable for voles and, consequently, for owl foraging. Our findings provide long-term empirical support for the hypothesis that farmland structural simplification associated with agricultural intensification is a primary driver of population declines in Short-eared Owls occupying grassland and farmland habitats.