Science: Nomadic Short-eared owls make surprising movements
16 January 2024
By Alex James
By Alex James
Flying Short-eared Owl. Credit: Liz Cutting / BTO
The Short-eared Owl has fascinated birdwatchers and ecologists for decades, a bird whose appearance and disappearance from regions has baffled the best minds and a bird for which we lacked key information on movements and the degree of connectivity between populations.
A new study by BTO researchers, along with the University of Iceland and the Spanish National Research Council (CSIC), just published in the journal Ibis, fills this important knowledge gap and provides new evidence that will enable us to better protect and conserve this enigmatic species.
The Short-eared Owl has an extensive, but declining, 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 not available, their more common response is to move to other areas, resulting in irruptive or nomadic movements. Studying these movements has proved challenging, but advances in tracking technologies have enabled BTO Senior Research Ecologist John Calladine and colleagues to collect new information.
The study reveals an unexpectedly large degree of movement in the tracked individuals, with some of the birds tagged in Scotland and Spain travelling widely across Europe and North Africa, and one bird tagged in Iceland travelling to Britain. Nine individuals of these 47 were monitored while nesting over two seasons, and the distances between nest sites used by the same individuals in sequential years ranged from 41 to 4,216 km.
Of two females tagged while nesting just 2 km apart on the Isle of Arran (Scotland) in 2021, one went on to breed in northern Norway in 2022 and the other to the Pechora Delta in Arctic Russia. Incredibly, another female tagged at its nest in Scotland in 2017 - bred twice in 2018, once in Scotland and again in Noway. However, travelling so widely to find places where there is sufficient prey where large broods can be reared does have its consequences – less than half the adult birds survive from one year to the next.
So, rather than leading a hedonistic lifestyle, these birds are actually behaving selflessly. Their extensive travel to find enough voles to rear the next generation of owls has a knock-on effect on the condition of the individual and its chances of surviving for another year.
The types of movements recorded in this study, and the scale of these, implies 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).
John Calladine, BTO and lead author on the research paper, said: "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. Importantly, conservation action for the birds will need to be on a similarly wide scale."
John continued: "This study was only possible because of the generosity of those individuals and organisations who provided the funding necessary to purchase and deploy the tracking devices fitted to these amazing birds."
Neil Morrison, of the Tay Ringing Group and who has been key to the study right from its conception, said "I've been studying these owls for decades and thought I knew what they did. How wrong could I have been! To think that owls breeding in 'my glens' could then go on to breed in the Arctic or try and cross the Sahara –, who would have thought that! It's been a real privilege to be able to follow these birds in such detail. Now we have the knowledge to help ensure they continue to amaze us into the future."
A new study by BTO researchers, along with the University of Iceland and the Spanish National Research Council (CSIC), just published in the journal Ibis, fills this important knowledge gap and provides new evidence that will enable us to better protect and conserve this enigmatic species.
The Short-eared Owl has an extensive, but declining, 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 not available, their more common response is to move to other areas, resulting in irruptive or nomadic movements. Studying these movements has proved challenging, but advances in tracking technologies have enabled BTO Senior Research Ecologist John Calladine and colleagues to collect new information.
The study reveals an unexpectedly large degree of movement in the tracked individuals, with some of the birds tagged in Scotland and Spain travelling widely across Europe and North Africa, and one bird tagged in Iceland travelling to Britain. Nine individuals of these 47 were monitored while nesting over two seasons, and the distances between nest sites used by the same individuals in sequential years ranged from 41 to 4,216 km.
Of two females tagged while nesting just 2 km apart on the Isle of Arran (Scotland) in 2021, one went on to breed in northern Norway in 2022 and the other to the Pechora Delta in Arctic Russia. Incredibly, another female tagged at its nest in Scotland in 2017 - bred twice in 2018, once in Scotland and again in Noway. However, travelling so widely to find places where there is sufficient prey where large broods can be reared does have its consequences – less than half the adult birds survive from one year to the next.
So, rather than leading a hedonistic lifestyle, these birds are actually behaving selflessly. Their extensive travel to find enough voles to rear the next generation of owls has a knock-on effect on the condition of the individual and its chances of surviving for another year.
The types of movements recorded in this study, and the scale of these, implies 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).
John Calladine, BTO and lead author on the research paper, said: "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. Importantly, conservation action for the birds will need to be on a similarly wide scale."
John continued: "This study was only possible because of the generosity of those individuals and organisations who provided the funding necessary to purchase and deploy the tracking devices fitted to these amazing birds."
Neil Morrison, of the Tay Ringing Group and who has been key to the study right from its conception, said "I've been studying these owls for decades and thought I knew what they did. How wrong could I have been! To think that owls breeding in 'my glens' could then go on to breed in the Arctic or try and cross the Sahara –, who would have thought that! It's been a real privilege to be able to follow these birds in such detail. Now we have the knowledge to help ensure they continue to amaze us into the future."
Tagging a Short-eared Owl. Credit: John Calladine / BTO
OTHER POPULAR STORIES TO READ:
- Nature Reserves: The best places to visit in 2024
- Review: SWAROVSKI OPTIK ATC 17-40x56 Spotting Scope
- Review: Kowa TSN-66 PROMINAR Spotting Scope
- Review: Páramo Men's Halkon® Jacket
- Review: Viking Optical Swallow ED 20-60x80 Spotting Scope
- Rarities & Conservation: Declining British seabirds are on a cliff edge
- Rarities & Conservation: Golden Eagle numbers in southern Scotland reach highest levels for centuries
- Rarities & Conservation: Uncovering the secrets of one of Europe's rarest eagles
- Rarities & Conservation: Lambs are no longer a major food source for breeding White-tailed Eagles in Scotland
- Nature Reserves: Record-breaking breeding season for Pied Flycatchers in 2023 despite many complex challenges