In today’s blog, Senior Conservation Scientist and lead author, Steven Ewing explains his latest paper on Hen Harrier mortality.
A new RSPB study published today in Biological Conservation shows that annual survival of Hen Harriers in Britain is unusually low, and that illegal killing is a dominant cause of death, accounting for up to 75% of annual mortality in some age classes of birds.
It provides convincing evidence that illegal killing is primarily associated with areas managed for Red Grouse shooting, indicating the widespread and ongoing role that persecution on some grouse moors continues to play in limiting modern-day raptor populations in Britain.
Hen Harrier is flight (c) Pete Morris RSPB
Largest tracking programme of Hen Harriers globally
Since 2014, RSPB and partners have fitted satellite transmitters to juvenile Hen Harriers at nests across Scotland, England, Wales, and the Isle of Man, deploying tags on over 140 individuals up until 2020. This is the single largest tracking programme for Hen Harriers globally. These tags, which provide real-time information on the location of tracked birds, have yielded fascinating insights into the ecology of this species, including the long-distance migrations of some birds as far afield as southern Spain.
However, the extent of the ecological insights has been constrained by the fact that many of these Hen Harriers have not lived for long, with a median lifespan after fledging of only 121 days. Through this study, we set out to try to better understand the factors contributing to these high rates of mortality.
Hen Harriers followed by RSPB’s satellite tracking project have been monitored intensively. Every day, data from the tags has been downloaded to ensure the birds are still mobile and that the tags are functioning optimally. Where there has been evidence that a harrier has been stationary for longer than expected, indicating it has died, we have dispatched members of RSPB’s Investigations team to the last known recorded location to recover the individual. A goniometer, which allows us to triangulate the location of actively transmitting tags in the field, has enabled a high percentage of recoveries. Retrieved harriers have been sent for veterinary post-mortem to try to diagnose the cause of death.
Hen Harrier deaths attributed to natural causes or illegal killing
Based on information from the post-mortems, data from the tags and information from the recovery sites, we have been able to allocate most Hen Harriers to a likely cause of death. At the end of the study (May 2021), 17 harriers were still alive, four tags had suffered technical failures, 38 had died of natural causes and 32 had died of unknown causes (but were likely natural deaths).
A further six individuals were confirmed, by veterinary post-mortem, to have been illegally killed and 43 were classified as Stop No Malfunctions. This latter category is where a previously well-performing tag disappears suddenly with nothing found despite rigorous follow-up, a strong indication that the individual has been illegally killed and the body and tag concealed. It is unlikely that these Stop No Malfunction cases represent tag failures, because there would need to have been a failure rate much greater than previously documented and highly implausible.
Flow chart indicating the allocation of satellite tagged Hen Harrier into the six fate categories used in this study, with a breakdown outlining the criteria used to assign individuals to different classes. To avoid potentially misclassifying GSM-tagged harriers to the fate SNM, when they may have died of natural causes in an area of poor signal reception, we also inspected the predicted strength of mobile phone coverage (https://checker.ofcom.org.uk/en-gb/mobile-coverage) at the final transmission location. Birds were only assigned to SNM if they disappeared in areas of good reception. 2Tag failure indicated by declining or low battery voltage, intermittent fixes, or fluctuating temperature readings (Whitfield & Fielding 2017).
Illegal killing is a key cause of low survival in British Hen Harriers
Intensive follow up of tracked harriers by RSPB Investigations staff has meant not only have we been able to estimate survival rates for different sexes and age classes, but we can estimate the relative contributions of natural causes of death and illegal killing to overall rates of mortality. Survival of juvenile Hen Harriers was low, especially in males, with only 14% of males and 30% of females surviving their first year. Older age classes survived at higher rates (50-57% for subadults and 80% for adult females), but still below that which might be anticipated for similar-sized raptors. Illegal killing accounted for between 27-41% of mortality in first-year birds and up to 75% in harriers aged between one and two years old. The contribution of illegal killing to deaths of adult birds could not be established due to the low numbers surviving over two years of age.
Strong links between illegal killing and upland areas managed from Red Grouse shooting
To establish whether there was a link between illegal killing of Hen Harriers and areas managed for Red Grouse shooting, we used a map that shows where grouse moor management in Britain takes place based on satellite imagery.
We found that the risk of dying due to illegal persecution during any given week increased by 43% with every 10% increase in the use of areas managed for grouse shooting by Hen Harriers. In contrast, we did not find the same higher risk of dying due to natural causes with increased use of grouse moor areas. We also found a strong overlap between illegal killing and grouse moor extent in 20km squares across Britain, which identified hotspots of illegal killing in northern England and the central and eastern Highlands of Scotland.
There were also strong differences in the seasonal timing of deaths due to natural causes and illegal killing. Peaks in natural mortality coincided with the time soon after fledging and leaving their natal area and winter, times when juvenile birds may be particularly susceptible to predation, starvation or the impacts of inclement weather.
The timing of mortality differed between birds dying of natural causes and illegal killing. Timing of mortality of illegally killed birds was either i) consistent throughout the year or ii) peaked around the grouse shooting season and during hen harrier breeding territory establishment (both patterns were equally well supported by the data).
Ongoing illegal persecution continues to limit the Hen Harrier population in Britain
Hen Harriers have been legally protected in Britain for almost 70 years under the Protection of Birds Act, 1954, but this study adds to the already overwhelming evidence base that illegal killing on grouse moors remains a key cause of this species’ low population size and its ongoing absence from large areas of the uplands, particularly grouse moors.
For more information on the country-specific policy and conservation implications, you can read the latest blog posts on Scottish Nature Notes and RSPB England.
Acknowledgements and funders
RSPB would like to acknowledge the organisations and individuals that have funded the Hen Harrier tracking project, particularly the European Commission’s LIFE programme through the Hen Harrier LIFE project, and thank all staff members, licensed raptor workers, taggers and land managers/owners that have played a role in this project.
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