Birds may exhibit different movement patterns, from the dispersal that takes them away from the nest in which they were raised, to annual migrations between breeding and wintering areas located thousands of miles apart. Some of these movements are more difficult to study than others, and this is particularly true for those species that are nomadic in their behaviour, which leads to poor knowledge of their life history which has repercussions for their conservation.
One such species is the Short-eared Owl, the subject of a long-term tracking study by BTO Senior Research Ecologist John Calladine, working with collaborators in Scotland, Iceland and Spain. The species has an extensive distribution, across which it is predominantly a specialist predator of small mammals.
The abundance of its favoured vole prey can vary markedly, both in time and space, making these rodents an unpredictable food resource. While Short-eared Owls can switch to other prey when voles are unavailable, their more general response is to move to other areas, resulting in irruptive or nomadic movements.
As such, the research focused on the movements of 47 Short-eared Owls from multiple European locations (Iceland, Scotland, Spain), tracked with GPS devices. The birds tracked were predominantly female, because males are smaller than females and, therefore, were often below the weight limit for carrying a tracking device.
The study reveals an unexpectedly large degree of movement in the tracked individuals, with some birds tagged in Scotland and Spain travelling widely across Europe and North Africa. One bird, a female tagged at its nest in Scotland in 2017, bred twice in 2018, once in Scotland 41 km from its previous site and again in Norway, a further 926 km distant. Birds tagged in Iceland tended to spend the least time making long-distance movements (2%), and those tagged in Spain the most (19%), with those tagged in Scotland being intermediate (8%).
Thus, birds tagged in the most geographically isolated location (Iceland) spent proportionately more time undertaking short-distance movements. Whereas, long-distance movements included crossings over both land and sea. Most movements were made at night, with about 80% of the movements outside of a home range.
The types of movements recorded in this study imply that there are not distinctly separate populations of Short-eared Owls across much of Europe. Rather, there is a single potentially integrated population across most of the range, albeit with some comparatively more, but not completely, isolated populations (such as that in Iceland).
In Ireland, the species is a rare and sporadic visitor, but may be seen during winter in areas such as the Wicklow coast – and a recent influx was noted across the east of the country, with many being recorded by our Ornithology team on our field surveys.
In summary, this new knowledge has significant implications for attempts to protect and conserve this species and underlines a need to collect information on Short-eared Owl populations from sufficiently wide geographic scales and/or over the long term.
Reference: Remote tracking unveils intercontinental movements of nomadic Short-eared Owls Asio flammeus with implications for resource tracking by irruptive specialist predators. Calladine, J. et al. 2024. Ibis. No. (DOI): 10.1111/ibi.13304