21st November 2023
Survival rates for hen harriers reared and released through an innovative scheme to help the recovery of the species are better than for wild birds, it has been reported, as the trial enters its final year.
Data from satellite-tagged hen harriers given a boost in the Brood Management Scheme trial shows the survival rate from fledging through the difficult winter months to the following May is 44 per cent, compared with 24 per cent for wild birds.
Fledging success of the trial to date has been nearly 100% – higher than observed in similar captive rearing programmes and in the wild.
The reasons for the improved survival rate are not fully understood, but a full formal investigation into the effects of the Brood Management Trial is underway.
Hen Harriers suffer high mortality rates, particularly in their first year of life. The hen harrier population in England is now at a 200-year high, with 141 chicks fledging successfully in 2023.
Since the first broods were managed in 2019, 58 chicks have been taken, safely reared and released back into the wild population.
The Moorland Association is a partner in the Brood Management Scheme trial and has helped satellite tag birds in each of the five years the trial has been operating. Thanks to this tracking technology, the researchers know that some of the birds from the cohort tagged in 2020, for example, are nesting and breeding successfully, adding further chicks to the increasing population.
Wet, cold and cloudy winter weather is not only a challenge for the young birds but also for their tracking devices which rely on solar powered batteries.
This year nine birds were tagged of which four are still transmitting regularly. It is hoped that the signals may return with some better weather, but the lack of signal from some is being treated as suspicious and is currently under police investigation.
Among the birds that epitomise the importance of the brood management trial is a male tagged in 2020 by the Moorland Association, who has fathered chicks three years in succession to date, favouring the grouse moors of the North Pennines, thus boosting biodiversity targets.
The most celebrated of the satellite-tagged wild birds in England is probably Frank, who was fitted with a tag by Natural England in August 2018 and who has gone on to father at least 21 chicks. Frank was not originally a brood-managed bird but some of his chicks have been taken into the scheme. Frank continues to be seen in the skies above Nidderdale and Coverdale in North Yorkshire and making use of the winter roost at Swinton, sometimes joined by up to 12 other adult and juvenile hen harriers, which can be seen from the estate bird hide.
Amanda Anderson, director of the Moorland Association, said: “Despite the challenges faced by hen harriers in England, we are on the right track.
“The figures show a complete turnaround in the English population. There were no nests or chicks fledged in 2013 compared to 141 chicks fledged this year. Since the first licence for brood management was available in 2018, an impressive improvement in the overall population has been recorded, with 485 chicks successfully taking to the wing in England, nine times the number in the six-year period before the trial.
“There is simply no way that this remarkable improvement could have been achieved without brood management.
“I am incredibly grateful to all those estates, keepers and partners such as the field teams from Natural England who have worked so hard on the conservation of this species. The trial has shown to date that grouse moor operators have stepped up to the plate and brought harriers back in significant numbers.”
Estates in Yorkshire, Cumbria, Northumberland, Durham and Lancashire have actively participated in the trial with many Moorland Association members hosting wild nests, further boosting the population.
The International Centre for Birds of Prey offers a bespoke facility for conservation breeding of rare birds of prey. Its involvement in the hen harrier brood management trial has been invaluable.
The dedication of those involved in the care and handing of the chicks reared in the scheme is also to be highly commended.
In 2018, 50 per cent of hen harrier nesting attempts were on land managed for grouse shooting, rising to over 80 per cent in 2022. Areas of England specially designated for hen harriers are thought now to have sufficient numbers to be classed as ‘in favourable condition’, although this has not been formally assessed.
The hen harrier faces huge challenges, from illegal killing and lack of original habitat, to predation of its eggs and chicks by foxes, stoats, weasels, crows and gulls. Once fledged the birds are also vulnerable to severe weather, disease, attack by other birds of prey and starvation.
Satellite-tag data shows that the birds continue to make use of their now preferred upland habitat, as expected, with the majority of their time spent on grouse moors in the uplands. Data also shows that when satellite tags have stopped transmitting the birds’ last known positions are often around these natural habitats.
Once the core habitat is re-colonised, it is expected that the increased number of birds would move further out and investigate previously suitable habitat. In just five breeding seasons, birds are being seen investigating the Lake District and have been tracked as far south as Somerset.