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Diurnal and nocturnal ranging behaviour by moorland-breeding Short-eared Owls Asio flammeus in Scotland

Capsule The total area encapsulated by the hunting ranges of breeding Short-eared Owls Asio flammeus was broadly similar between day and night and also through the season, but activity was concentrated more on lower altitude graminoid-dominated areas during daylight and early in the breeding season. Aims To assess the representativeness of diurnal observations alone for determining ranging behaviour and habitat-use of breeding Short-eared Owls. Methods The diurnal and nocturnal ranging behaviours of four radiotagged Short-eared Owls were compared. Further diurnal field observations were used to assess the representativeness of the behaviour of the radiotagged birds. Results Short-eared Owls tended to hunt more over higher ground (where there was a greater ericaceous component within the moorland vegetation) at night and also later in the breeding season; the area of the total hunting range remained similar through the season (mean about 200 ha). Nocturnal activity rates closely matched those during peak diurnal activity periods. Conclusion Diurnal observations of breeding Short-eared Owls could provide representative data to inform breeding population densities and distribution but they may not be representative of overall habitat-use or ranging behaviour within home-ranges. Assessments of the risks to breeding Short-eared Owls of developments on moorland (e.g. forestry, wind turbine construction, grazing changes) based on diurnal observations alone may not reflect the true scale of potential impacts, which would also need to consider nocturnal behaviour.