Birds of prey on grouse moors

Male hen harrier ( illegal killing of several species of birds of prey, including golden eagle, peregrine and particularly hen harriers has repeatedly been shown to occur on grouse moors, especially those managed for driven shooting. The conflict is well researched but remains unresolved, and there are still gaps in our understanding of the social and ecological issues involved.

This document draws on the extensive body of research into hen harriers on grouse moors to explore the root of the problem and how we know it is happening. Importantly, it will discuss why and how grouse managers could and should accommodate raptors on grouse moors in the future alongside maintaining economic grouse shooting and its conservation benefits.

Photo credit: Laurie Campbell


The conflict

Q: Why is there a conflict between birds of prey and driven grouse shooting?
A: Hen harriers and some other larger birds of prey (raptors) eat grouse and their chicks1, which can reduce the number available to shoot2. While the harrier’s main prey species are voles and meadow pipits, grouse chicks are an important portion of their diet at times, particularly during the breeding season when they are feeding their young3-5.

Q: Why has this led to illegal killing?
A: When bird of prey numbers are high and grouse numbers are low, predation by some raptor species can reduce or suppress grouse numbers and prevent their population recovery. Many gamekeepers, grouse moor owners and managers believe that predation by birds of prey, particularly hen harriers, can reduce grouse shooting bags (the number shot) to a point where the shoot cannot be sustained. This means the loss of jobs and income both directly for moor owners and managers, keepers and in the wider community6. Fear of this situation has driven illegal killing.

This perception was confirmed by the Joint Raptor Study (JRS) at Langholm Moor7 (discussed in detail here). The JRS showed that in some situations, particularly when grouse numbers are already low, a high density of raptors can depress grouse populations further and then suppress their recovery enough that grouse shooting cannot continue1,8. In this study, hen harrier numbers in the area rose steeply from 2 to 20 breeding females and grouse numbers in late summer fell to such low levels that driven shooting was no longer economically viable and had to stop1,7,9,10.

Q: What is the history of this conflict?
A: Raptor killing by humans took place throughout the 18th and 19th centuries. Combined with the loss of lowland habitat, this led to the extinction of the hen harrier from mainland UK by the late 19th and early 20th centuries, leaving regular breeding only on the islands of Orkney and the Outer Hebrides11-13. There was a gradual recovery back into upland areas of the mainland from the 1930s and 40s onwards. Hen harriers received full legal protection in 1954 with the Protection of Birds Act. However, the recovery has plateaued in recent decades and there was a significant decline in breeding numbers between 2004 and 2016, particularly in areas of moorland and forestry13-15.

Q: Is there evidence that hen harriers are killed on grouse moors?
A: Yes, and the illegal killing of other birds of prey has also long been associated with grouse moors. Evidence published 25 years ago (1988-1995) showed that hen harriers had lower nesting success on grouse moors compared either to other areas of moorland, or to young forestry plantations across Scotland12. Recent evidence using data from satellite tagged hen harriers collected up until 2016 also strongly suggests that illegal killing was still widespread on British grouse moors at that time, with hen harriers being ten times more likely to die or disappear in areas managed for grouse than non-grouse managed moorland16. Other evidence based on distribution, breeding performance and recovery of dead birds strongly suggests that illegal killing of an array of raptor species still occurs on some grouse moors3,16,31.

Q: Why only ‘some’ grouse moors?
A: We know attitudes are changing toward illegal killing, with some moor owners protecting and valuing the birds of prey on their moors. However, the evidence shows that illegal killing still happens16, and it is vital to address the conflict and end this practice.

Q: How many hen harriers might we expect without illegal killing?
A: In 1998 another study estimated how many hen harriers could in theory be supported by the available habitat in the UK. The paper gathered evidence from across the world, and predicted that if all suitable UK habitat were occupied, numbers could reach 1,660 nesting females11. At the time there were thought to be around 600-700 breeding females. Some subsequent estimates of potential harrier numbers have been higher17, but many assumptions behind these figures have been challenged. The most recent national hen harrier survey (also using data from 2016) found 575 territorial pairs in the UK and Isle of Man13.

Q: Is all this habitat on grouse moors?
A: No. Potential hen harrier habitat in this study included heath/grass, open shrub heath, dense shrub heath or mire (or bog)11. Approximately half of English upland areas are thought to be managed as grouse moors18, so there are also large areas of potentially suitable harrier habitat in the uplands that are not grouse moors.

Q: Is 1,660 nesting harriers realistic?
A: The estimates in this paper don’t account for other variables like the availability of prey, changes in vegetation, predation on harriers and harrier nests, or the willingness of grouse moor managers to continue to produce good conditions for harriers in the event they can no longer shoot grouse. We know that harriers and their nests are predated by foxes10,19. Some of the habitat may not actually have been, or may soon have become, unsuitable. For example, when commercial forestry plantations mature, they become unsuitable for nesting hen harriers11. However, the UK could accommodate a much larger number of harriers in the absence of illegal killing.

Q: Would more harriers cause problems for grouse moors?
A: The study suggested that the estimated 1,660 harriers wouldn’t have too large an impact on grouse moors but critically this was if they were evenly spread across the suitable habitat. However, hen harriers often tend to roost and nest in a semi-colonial way so high densities can build up in particular areas11. The potential problems on grouse moors come from an uneven distribution of nesting harriers, breeding well because keepering reduces predation on their young, rather than from the overall number of breeding pairs.

Q: What sort of numbers can there be without causing a problem?
A: Based on our knowledge about grouse and hen harriers, we can predict the effect of harriers on grouse populations. A paper from 2014 did exactly that, predicting that at one nest per 4,000 ha (9,900 acres), hen harriers would reduce autumn grouse densities by less than 10%2. Given the area of moorland in England managed for grouse, this density would result in approximately 70 pairs of hen harriers on grouse moors in England and around 220 pairs on all heather moorland in Scotland2. In 2016 there were estimated to be four pairs of harriers on heather moorland in England and 460 harrier pairs on heather moorland in Scotland13, although the number of nests in England appears to have risen to 15 in 201920.

Hen harrier numbers

Q: How many hen harriers are there in the UK?
A: The most recent survey of breeding hen harriers was carried out in 2016 and reported 575 territorial pairs in the UK. 460 of these (80%) were found in Scotland, with 46 in Northern Ireland, 35 in Wales, 30 in the Isle of Man and four in England13.

Q: Are these numbers changing?
A: Overall, the UK population has declined by 13% since 2010. This change is not statistically significant across the whole of the UK but on both grouse moors and in maturing conifer plantations the number of harriers fell by around half between 2010 and 201613. However, 2019 was a successful year for breeding harriers with 15 nests in England and 47 chicks fledged20. 11 of these nests were on grouse moors. Continued partnership working such as at Langholm Moor, and the Scottish Heads up for Harriers project, may be beginning to help address the conflict in conjunction with new approaches such as brood management.

Q: Are the low numbers just because of illegal killing?
A: No, but illegal killing is thought to be the main factor limiting their recovery. Harrier numbers can also be affected by the amount of suitable nesting habitat21, the abundance of prey species such as the field vole4,5,10,22, and predation10. Data on harrier and merlin from Langholm suggests that cessation of keepering can reduce breeding success of these largely ground-nesting raptors due to increased predation by foxes, which may lead to decreases in breeding abundance23. These findings are supported by similar trends found on two other Special Protection Areas in Scotland where keepering ceased24. Others have noted that predation by birds of prey, windfarms, the weather, and human recreational disturbance may also be affecting raptor numbers17,25-27.

Q: Are there target numbers to aim for?
A: In 2011, the Joint Nature Conservation Committee (JNCC), which advises the government on conservation, published a report outlining a “Conservation Framework for Hen Harriers in the UK”. Within this report, the JNCC identified target numbers for the hen harrier to be considered in Favourable conservation status in the UK17.

Q: What is Favourable conservation status?
A: In order to achieve “Favourable” conservation status, the different influences acting on a species need to be in balance, so that it can survive and thrive. For example, when considering a particular species there are four aspects which are taken into account: their range, population size, the habitat (extent and condition), and their future prospects.

Q: What does this mean for hen harriers?
A: The JNCC have calculated that, for hen harriers in the UK to be classified as Favourable there should be:

  1. At least 44% of apparently suitable habitat occupied.
  2. 2.12 pairs of hen harriers per 100km2 of suitable habitat.
  3. A minimum of 1.2 young fledged per breeding attempt.

The JNCC report estimated the amount of suitable habitat there is per country, so we can calculate the number of breeding pairs that could render each country favourable for hen harriers. Breeding success would also need to be high enough, but with variation between different areas these numbers do not tell the whole story. They give a minimum, rather than an ideal, number of hen harriers in the UK but may be a useful guide to progress. Based on this simple approach, conservation status can be thought of as favourable for hen harriers in Scotland and Northern Ireland, but not England or Wales, or the UK as a whole.

  Total km2 of
suitable habitat
Favourable number
of hen harriers
National hen harrier
survey data (2016)
Scotland 36,971 345 460
England 6,636 61 4
Wales 5,068 47 35
Northern Ireland 3,049 28 46

The Joint Raptor Study

Q: What was the Joint Raptor Study?
A: The Joint Raptor Study (JRS) was a five-year project with joint partners including GWCT, the Centre for Ecology and Hydrology (CEH), the RSPB and SNH (now NatureScot). It studied raptor predation on grouse, to work out the likely effect this would have on grouse numbers and hence the sustainability of shooting. The main study area was on Langholm Moor, a driven grouse moor in South West Scotland, with data from five other study moors in Scotland.

Q: Why was it done?
A: Until the JRS there was little scientific information to support or reject the belief some people firmly held that birds of prey could significantly reduce grouse numbers. The JRS therefore assessed the impact of raptors on the numbers and bags of grouse over a five-year period7.

Q: Which raptor species were studied?
A: The hen harrier and peregrine were of particular interest, as were their prey species. The diet of harriers is largely small mammals such as voles and songbirds like meadow pipits and skylark, whereas peregrines eat a variety of mainly larger birds including crows, pigeons and thrushes. Both predate red grouse, with harriers taking chicks and adults and peregrines mostly hunting fully-grown birds7.

Q: What was done in the JRS?
A: Despite legal protection since 1954, it was recognised that illegal raptor killing was still ongoing in the British uplands. During the study, an agreement was made to ensure that all raptors were fully protected at Langholm. Active grouse moor management was carried out, including rotational strip burning of heather and lethal control of generalist predators permitted by law, notably foxes, crows, stoats and weasels. Numbers of red grouse and breeding raptors were monitored each year. The researchers studied the breeding success and diet of hen harriers and peregrines, as well as grouse abundance, and mortality of both chicks and adults. The abundance of other harrier prey, meadow pipits and skylarks, together with field voles, was also recorded7.

Q: What did it find?
A: Hen harrier numbers rapidly increased from 2 to 20 pairs in five years7,28,29, which initially followed an increase in vole numbers. Peregrines numbers also increased from three pairs to five or six. Predation by harriers and peregrines appeared to keep grouse numbers down, holding the population at a low level and preventing it from recovering.

Q: Why did harrier numbers rise so high?
A: If conditions are right, many hen harrier nests can be found in a small area11. The JRS habitat was a mixture of grass and heather areas, which is good for hen harriers4,9. These conditions are ideal for their main prey species and during the JRS there were years with high vole numbers10. Plenty of prey, together with no illegal killing and low levels of predation on the harriers’ own nests thanks to legal predator control by gamekeepers, helped boost harrier numbers10.

Q: What did these raptors eat on Langholm Moor?
A: During the breeding season, meadow pipits were the most important prey species for hen harriers. Meadow pipits provided 45% of prey items, and grouse chicks made up 12%. For peregrines, pigeons made up 56% of summer prey items, and grouse 10%. Grouse also formed an important part of the diet for both hen harriers and peregrines in the winter, based on studies of pellets and prey remains. 77% of hen harrier pellets and 85% of peregrine pellets showed evidence of grouse having been eaten. Many more grouse were killed by hen harriers than by peregrines. Most harrier predation was on chicks, and most peregrine predation on adults7.

Q: What impact of raptors on grouse was found by the JRS?
A: Until the start of the JRS, grouse numbers tended to show regular fluctuations in relation to the abundance of their parasitic worms, with peaks every six years or so. Before medication was used to control these parasites (administered on grit), these ‘cycles’ were usual on most driven grouse moors. However, during the JRS, raptor predation removed on average 30% of the potential breeding stock of grouse each spring. In the summers of 1995 and 1996, predation by harriers accounted for more than a third of grouse chicks7,8. This was estimated to have reduced autumn grouse numbers by 50%, and numbers failed to recover from the low part of the population cycle7,29.

On two nearby moors, grouse numbers had previously cycled in parallel with Langholm, but on those moors harrier numbers had not increased and grouse peaked in 1996 as expected29. This increase and normal peak was not seen at Langholm, suggesting that predation by raptors caused grouse numbers to remain low, rather than parasites. Grouse numbers were subsequently too low at Langholm to support driven grouse shooting, which stopped in 1997.

Q: Would changes in habitat management have helped?
A: Almost half of the heather moor at Langholm had been converted to grass between 1948 and 1988, largely as a result of heavy grazing by sheep9. This sort of mixed landscape of grassy and heather areas is good for harriers4, so it was thought that increasing heather cover and reducing grass may help tip the balance and give grouse an advantage. Over the course of the study, there was no evidence that predation on adult grouse at Langholm was influenced by habitat, but harriers were more likely to find grouse broods in areas of grass/heather mix compared to pure grass or pure heather stands30.

Q: What did we learn from the Joint Raptor Study?
A: In some circumstances, particularly when raptor numbers are high and grouse numbers are low, raptor predation can depress grouse numbers to the extent that driven grouse moors are not economically viable for sporting purposes8. Without illegal killing, hen harriers can thrive when moorland is managed for grouse.

Q: Is this applicable to other moors?
A: When raptors are not illegally killed, their breeding numbers naturally vary considerably between moors. They tend to be highest where meadow pipits and voles are most abundant, which is generally on moors with a mixture of grassy and heather areas4,7. Langholm is considered an average moor in terms of its heather cover and meadow pipit numbers. This being the case, in theory similar moors could host similar numbers of harriers, which could impact upon grouse9 but much of our knowledge on the grouse/harrier conflict is still only based on data collected at Langholm.

Q: What happened after the JRS?
A: Driven shooting stopped on Langholm in 1997, but a low level of keepering continued until 1999, when grouse moor management largely ceased and only the head keeper remained. The GWCT continued to monitor bird numbers annually, together with vole and fox indices, until the start of the Langholm Moor Demonstration Project in spring 2008, so we have continuous data for birds and voles on Langholm Moor since 1992.

Q: What happened to the number of raptors?
A: Hen harrier numbers at Langholm fell from 20 in 1997 to between two and five pairs over the next five years. Hen harrier breeding success, which had averaged 2.5 chicks fledged per female per year with 80% of breeding attempts successful, fell to 1.2 chicks per female and 39% of breeding attempts successful when the moor became unkeepered (2000-2007) and foxes and crows were not controlled28.

Q: What happened to the number of grouse?
A: Grouse numbers also fell. Average spring counts between 1992 and 1999 when the moor was keepered had been 28 birds per km2, which fell to 12 per km2 in the period 2000-2007 when keepering had stopped. Post-breeding counts in July fell from an average 59 birds per km2, to 14. Grouse breeding success also dropped, having been on average 1.7 chicks per adult from 1992 to 1999, but falling to an average of 0.9 for 2000-200728.

Easing the conflict

The Langholm Moor Demonstration Project

Q: What can be done to unlock this conflict?
A: The conflict itself has been the subject of many research papers, looking for and exploring ways to resolve it32–36. The Langholm Moor Demonstration Project (LMDP) built on the findings from the JRS, running from 2008-2017 to investigate potential means of addressing the conflict.

Q: Why was another study done at Langholm?
A: Having demonstrated that raptor predation could indeed put a grouse moor out of business, the LMDP investigated whether the grouse population could be recovered from these low numbers, to a level that would support commercial driven grouse shooting, in the presence of breeding raptors37. The GWCT worked again with its partners Buccleuch Estates, SNH (now NatureScot), the RSPB and Natural England. The specific aims were to:

  • Demonstrate how to resolve the conflict between moorland management for red grouse and raptors.
  • Maintain the hen harrier population, for which Langholm Moor is recognised as a Special Protection Area.
  • Improve and extend the heather moorland habitat, compared to its condition in 2002.
  • Improve grouse production to a level at which driven grouse shooting becomes economically viable to support sustained moorland management.

Q: What did the project do?
A: The project included several parallel approaches: habitat improvement, predator control, diversionary feeding of hen harriers and disease management37, with the following components:

  • Habitat improvement – heather burning, cutting, spraying and reseeding, bracken control and reduced sheep grazing to improve the condition of the heather moorland, and expand the area of the moor on which heather was dominant.
  • Predator control – fox, corvid (except ravens) and mustelid (stoats and weasels) control.
  • Diversionary feeding – alternative food was provided on posts near to hen harrier nests, to try and reduce the number of grouse chicks taken by hen harriers to feed their own chicks.Day-old poultry chicks and rats were placed on a feeding post near the nest for up to 60 days after hatching of harrier chicks. This provided up to half of the food harriers took to their chicks31. There are practical considerations to this approach. The food has to be replenished daily and so the nests need to be accessible for the gamekeeper, but the technique can reduce the proportion of grouse fed to harrier chicks38.
  • Disease control – medicated grit was placed in trays and made freely available to grouse, which eat grit to help breakdown and digest heather. The grit contains the drug flubendazole which kills parasitic worms living in the grouse’s intestines.

The ten-year demonstration project was planned to run from 2008 to 2017, but land management ended in February 2016 when the board felt there was no likely prospect of achieving the grouse shooting objective.

Q: What was found?
A: Findings have been published in a series of scientific papers, summaries of which are available on the LMDP and GWCT websites. The key findings were:

  • Hen harrier numbers remained low, at one to three breeding females, from 2008 until 2014, when they increased to 1228.
  • Red grouse densities quickly rose from low levels, to be two to three times higher within three years of management resuming in 200828, but did not rise high enough to support economically sustainable driven grouse shooting.
  • This was because the survival of grouse chicks and adults were both too low39.
  • Diversionary feeding reduced the number of grouse chicks that were taken by hen harriers, compared to what would have been expected40. However, diversionary feeding alongside grouse moor management did not increase grouse numbers sufficiently to allow sustainable driven shooting.
  • Average yearly increases were found for three species of wader: curlew 10%, golden plover 16% and snipe 21%. However, lapwing numbers remained low41.
  • The extent of heather was improved. Total heather cover increased by 10%, and the area over which heather dominated the vegetation mix increased by 30%42.
  • There was a high number of breeding buzzards feeding on the study area (12-14 pairs), together with an estimated 47 non-breeders43. Buzzards are opportunistic feeders, using a variety of food sources, depending on what is available. Depending on the method used to study it, red grouse were estimated to make up 1-6% of prey items taken to buzzard chicks44. Although grouse are a minor component of  buzzard diet in both summer and winter45, the presence of many buzzards at Langholm meant that overall they may have been impacting upon the ability of grouse to recover, assuming that that all grouse eaten by buzzards were killed by buzzards and not merely scavenged43.

A final report summarising the science and the partnership’s findings was published in 201946.

The Hen Harrier Action Plan

Q: How do we move forwards for hen harriers?
A: In 2016, the UK Government published its Joint Action Plan to increase the English hen harrier population47. It brought together several conservation approaches to try to safeguard the future of the hen harrier in England. It is a collaborative effort, supported by both conservation and field sports organisations, to protect both the hen harriers and the grouse moors.

Q: What is the plan based on?
A: There are six elements to the plan, which are:

  1. Law enforcement, prevention and intelligence: to reduce illegal killing.
  2. Ongoing monitoring of breeding sites and winter roost sites: to gather more information about the hen harriers we have and help with law enforcement.
  3. Further research of the movement of hen harriers using satellite tracking: to monitor hen harriers and their chicks.
  4. Diversionary feeding of hen harriers: to reduce predation on grouse chicks.
  5. Engagement study about their possible reintroduction across suitable habitat in England: to investigate whether moving hen harriers from a donor country such as France to suitable habitat in UK lowlands could be viable.
  6. Brood management: Trialling the temporary movement of hen harrier chicks to aviaries: where two nests are in close proximity, remove the chicks from the second nest, rear them in captivity and release them to suitable habitat further away.

Brood management

Brood management is the most controversial aspect of the Action Plan, but also that which might hold the key to success, as it addresses the underlying drivers of the conflict.

Q: What is brood management?
A: Brood management is a form of wildlife management. If a second hen harrier nest is established within 10km of another, the eggs or chicks from one of the nests can be collected and reared in captivity. When fledged, they will be released onto suitable moorland. If they are collected from a Special Protection Area (SPA) for harriers, they must be released back into this same SPA.

Q: Why will this help?
A: This is a trial of whether such management can allay the fear that many harriers will build up in a small area, and therefore eliminate the motivation to remove them. An even distribution of harrier nests could allow for increased harrier numbers, with a lower impact on grouse numbers.

Q: Why is brood management controversial?
A: Some people disagree with the principle of disturbing the nest and rearing chicks in captivity, even for release back to the wild. They suggest that more criminal enforcement should be the main focus of harrier recovery; they also feel that, as numbers are very low in England, a minimum population size should be established before brood management is instigated, and grouse shooting should be licensed or banned if illegal killing continues. Others feel that all techniques should be tried to improve the outlook for hen harriers, and that this aspect of the plan could be instrumental in giving grouse moor owners and managers the confidence that their business or livelihood is not at risk.

Q: Has this technique been used before?
A: Not for hen harriers, but it has been used successfully for the Montagu’s harriers in Spain48 and France49,50, to relocate them away from agricultural areas, where harvesting would otherwise destroy the nests.

Q: What are the overall aims of the plan? What can we hope for?
A: It is hoped that the trial of the brood management scheme, as part of the Defra Hen Harrier Action Plan, will contribute to increased numbers of hen harriers in northern England. It is hoped that such novel non-lethal management techniques can change the social attitudes of those involved in upland land management to accept the presence of hen harriers on grouse moors. It is also hoped that an improvement in the conservation status of hen harriers will help those who have sceptical views about grouse moors recognise that moors can and do deliver a net gain in biodiversity. The first hen harrier chicks to be brood managed were in 2019. Numbers for the 2020 breeding season are expected in August 2021.


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