Causes of change

Based on multiple field studies providing good evidence, the main driver of declines in Hen Harrier populations appears to have been illegal persecution, causing a reduction in nesting success, annual productivity and survival of breeding females.

Further information on causes of change

Demographic data presented here show that clutch size decreased between the late 1960s and the mid-1990s (although further investigation has shown that this trend was due to the increased proportions of records from Orkney, where clutch sizes tend to be smaller than on the mainland: Summers 1998, Crick 1998). Clutch size has increased again since the late 1990s, but is currently slightly lower than it was in the late 1960s.

There is good evidence showing that, although the Hen Harrier has been protected under UK law since 1961, many are still unlawfully killed or disturbed in efforts to protect the economic viability of driven shooting of Red Grouse (Redpath & Thirgood 1997, Thompson et al. 2009, 2016, Rebecca et al. 2016, Challis et al. 2018, Murgatroyd et al. 2019). A study combining Atlas data and a two-year field study provided good evidence that nesting success, annual productivity and survival of female Hen Harriers was lower on grouse moors than on other moorland or in young conifer forests, due to destruction by humans (Bibby & Etheridge 1993, Etheridge et al. 1997). Fielding et al. (2011) conclude that illegal killing is the biggest single factor affecting the species and that it is having a dramatic impact on the population in core areas of its range in northern England and Scotland. An expert knowledge assessment of threats across Europe also identified direct persecution as the main threat to breeding Hen Harriers in the UK (Fernandez-Bellon et al. 2020).

Management of moorland habitats, including keepering that remains within the law, however, can benefit harrier populations by increasing their prey and reducing their nest predators, especially crows and foxes, to which, as ground-nesters, they are sensitive (Baines & Richardson 2013, Ludwig et al. 2017, 2020a).

In areas where illegal persecution is minimal, food availability can restrict numbers. Habitat characteristics around harrier nest-sites (at a 1-km radius) can have a strong influence on breeding performance (Amar et al. 2002). Rough grass, a preferred habitat for field voles, is a critical foraging habitat for Orkney Hen Harriers (Amar & Redpath 2005, Amar et al. 2008a). On the Isle of Mull, nesting birds are associated with habitat mosaics which can include moorland, scrub and open forestry, and avoid grazed land (Geary et al. 2018) Research in Ireland suggests that breeding success is positively influenced by the extent of heath/shrub and bog habitat but negatively influenced by climatic instability and early season rainfall (Caravaggi et al. 2019a). Areas suitable for Hen Harriers in Ireland are strongly affected by anthropogenic activities including forestry, agriculture and recreational activities which can influence site occupation and breeding productivity (Caravaggi et al. 2019b).

Food shortage just before the laying period can result in low levels of polygyny and reduced nesting success among secondary females, resulting in reduced productivity (Amar & Redpath 2002, Amar et al. 2005). The area of rough grassland has decreased during the same period as sheep numbers have increased and this is thought have reduced food supplies (Amar et al. 2003, 2005, Amar & Redpath 2005), but there was no detectable effect of rough grass area on fledging success or fledged brood size (Amar et al. 2008). Further, these studies provide no evidence that the effects on breeding success have an impact on abundance.

However, Redpath et al. (2002a) present good evidence from a different field study in Scotland which also shows that food availability, notably numbers of field voles, can influence population change in Hen Harriers, where there is no persecution. Harrier densities were highest in areas and years where their small prey animals were most abundant. Clutch size was positively correlated with the number of field voles, although fledging success was not significantly correlated with the relative abundance of small prey (Redpath & Thirgood 1999, Redpath et al. 2002a). Madders (2000) also highlighted the importance of foraging habitat in Scotland, finding that the extent of young first-rotation forestry, the preferred foraging habitat in this area, is currently in decline and states that this has contributed to many of the reported changes in local Hen Harrier populations (although no specific research into demographic parameters were presented).

There is some evidence that climate also affects demography, although this is secondary to drivers outlined above and there is no evidence for effects on abundance. In Scotland, chick mortality increased in cold temperatures and annual values of harrier fledged brood size were positively related to summer temperature (Redpath et al. 2002b). Recovery of the Welsh harrier population until 2010, in contrast to those elsewhere in the UK, has been attributed to an increase in the breeding productivity, apparently due to a combination of cessation of human interference in recent years and warmer temperatures, leading to increased productivity (Whitfield et al. 2008). A 44% decline in Wales between 2010 and 2016 may have been caused by poor spring weather during that period, lack of prey availability and changes to upland management (Wotton et al. 2018).

A study in Ireland which investigated whether wind farms may have an effect on population trends found that the evidence for an impact was weak; the relationship between wind farm presence and population trends was negative but (marginally) non-significant and may not be causal (Wilson et al. 2017).

Information about conservation actions

The main driver which has caused declines and limited the recovery of the Hen Harrier in the UK is illegal persecution (Fielding et al. 2011), which affects both birds at their breeding sites and dispersing young birds. Therefore, stopping illegal killing is the most important action that can be taken to reverse declines. The confrontation between different stakeholders on moorland, in particular between grouse shooting interests and conservationists, remains a key area of conflict and movement towards a consensus will be important. Some observers have questioned the legitimacy of grouse shooting if it is only viable when birds of prey are killed (Thompson et al. 2009), whilst others contend that grouse shooting protects the conservation of heather moorland and populations of some declining birds such as waders that depend upon it (Sotherton et al. 2009). The final report of the Langholm Moor Demonstration project concluded that whilst some targets had been met, the project was unable to achieve compatibility for management between raptor and Red Grouse interests and grouse numbers remained insufficient to make driven shooting viable (LMDP Partners 2019).

A recent expert review of grouse moors commissioned by Scottish Government, suggested that, among other recommendations, grouse shooting in Scotland should be licensed if populations of Hen Harriers on and in the vicinity of grouse moors had not returned to favourable status within five years (Werritty et al. 2019).

In areas where illegal persecution does not occur, breeding productivity of Hen Harriers might be improved by habitat management: a mosaic including of moorland alongside foraging habitats such as rough grassland, heath/shrub, bogs or open forestry is preferred (Amar et al. 2008a; Geary et al. 2018; Caravaggi et al. 2019a), although habitat edges may represent an ecological trap (Sheridan et al. 2020). Increases in sheep grazing numbers may have led to decreases in prey (Amar et al. 2003) and hence reductions in grazing may also benefit Hen Harriers. Restricting forestry activity and avoiding recreational activity during the breeding season may reduce anthropogenic pressures (Caravaggi et al. 2019b). Keepering that remains within the law can also benefit harrier populations by increasing their prey and reducing levels of their nest predators, especially crows and foxes (e.g. Ludwig et al. 2017, 2020a).