Hen Harriers were once more widespread in Britain and Ireland than they are now, though it is impossible to know with any certainty what their population size or distribution were before the second half of the 20th century, when systematic surveys of the species started. We do know that Hen Harrier became extinct in mainland Britain during the 19th century, due largely to persecution and land use change (Watson 1977).
After the Second World War, Hen Harriers re-colonised much of their previous upland range from extant populations in Orkney and the Hebrides, benefitting from new legislation protecting wildlife, a reduction in game-keepering activity, and land use changes including the widescale availability of young forest plantations in many upland areas (Bibby & Etheridge 1993). However, the species has remained rare in the UK, with a breeding population of less than 1000 pairs. As such, it remains vulnerable to the effects of environmental change and human activities.
Bird Atlas 2007–11
The breeding distribution change map in the BTO/BirdWatch Ireland/SOC Bird Atlas 2007–11 shows a recent decline in Hen Harrier range in the southwest of Scotland, but also modest gains in other areas like the west of Scotland, north of England, Wales and Northern Ireland. Elsewhere, Hen Harrier distribution appears to have been broadly stable during the past 20 years; overall, breeding occupancy at the 10km scale has increased by 29% over the past 40 years. The breeding relative abundance change map largely shows a pattern of decline across most of Scotland and Northern England (though the latter changes are small) over the past 20 years, with increases in some areas of Wales and Northern Ireland.
The Bird Atlas 2007–11, although a fantastic source of information about spatial changes in distribution and abundance for many bird species over the past 40 years, does not tell the whole story for Hen Harrier populations. This is partly because scarce and elusive birds like Hen Harriers are not reliably detected during timed Atlas fieldwork that is used to derive measures of change in abundance. This means that apparent gains and declines in abundance may simply be caused by stochastic variation in the birds seen, between surveys. It should also be noted that occupancy and abundance of Hen Harriers had already declined in some parts of the UK before the first Atlas survey was carried out.
Hen Harrier cannot currently be monitored via the BTO/JNCC/RSPB Breeding Bird Survey because it is not encountered frequently enough during this survey to estimate reliable population trends. However, annual monitoring data are collected by hundreds of dedicated individuals around the UK, including Wales, the Isle of Man, the north of England, and Scotland. By far the largest annual consolidation of this data is carried out by the Scottish Raptor Monitoring Scheme (SRMS), which aims to gather distributional and productivity data on raptors from all over Scotland. The data are currently used to produce annual SRMS Reports, the most recent of which covers the 2015 breeding season (PDF). It is apparent from this report that there are large regional differences in the occupancy of known home ranges checked by surveyors (from 81% in the Uists and 77% in Argyll to 31% in Central Scotland and 44% in Orkney), and in the productivity of breeding pairs (from 2.2 and 1.9 fledged juveniles per pair in the Uists and Argyll, respectively, to 0.4 in Central Scotland and 0.9 in Orkney). The annual nature of this dataset, and the species-specific focus of many contributors, makes it better suited than Bird Atlas data to elucidating spatial and temporal variation in Hen Harrier populations, and to understanding the likely drivers of population. However, more detailed analyses to examine population variation over time and between different areas, must take account of differences in annual survey coverage, which can be considerable. These analyses, which are being undertaken by BTO in collaboration with partner organisations in the SRMS, are ongoing.
Five national surveys of Hen Harriers have been carried out, the first in 1988-1989 (Bibby & Etheridge 1993) and the most recent in 2016 (Wooton in press). Each of these has been aimed principally at enhancing and expanding on the annual monitoring of Hen Harriers that takes place around the UK to allow accurate assessments of the number of pairs of Hen Harriers breeding in different parts of the UK, thereby allowing population trends to be assessed for each inter-survey period. Over the past 25 years, there have been both increases and contractions at regional and national levels, but the national population has increased modestly.
However, the variability underlying this trend is sufficient that to characterise the national population as stable would be misleading. In Orkney, for instance, the number and productivity of breeding females dipped markedly towards the middle of this period (Sim et al. 2001), only to recover to its previous level within a relatively short space of time (Sim et al. 2007). A comparison of the three most recent national surveys (in 2004, 2010 and 2016) suggests a 27% decline and some even more severe declines at the regional level. The biggest decline over this 12 year period has been in England, where just four territorial pairs (from a maximum of 12 in 2010) were found in 2016. In some regions, there were substantial changes in the numbers of harriers nesting in plantation forests between the 2004 and 2010 surveys, probably associated with changes in the age structure of this habitat.
Hen Harrier conservation framework
In 2011, the Joint Nature Conservation Committee published a report outlining the conservation status of Hen Harrier in the UK (Fielding et al. 2011). The report built on and synthesised existing knowledge of Hen Harrier distribution, breeding numbers and productivity taken from the national surveys described above, as well as other survey work undertaken by SNH for the purposes of SPA designation, RSPB wing-tagging studies, and smaller datasets from England and Wales.
The report had two main aims: to assess whether Hen Harrier populations in different parts of the UK had favourable conservation status, and to identify the constraints likely to be influencing Hen Harrier populations regionally and nationally. Regional population estimates from the national surveys and estimates of suitable habitat availability (derived from land cover maps based on satellite imagery) were used to calculate breeding densities of Hen Harriers. The conclusion of the report was that Hen Harrier populations in England and several Natural Heritage Zones in Scotland were in unfavourable conservation status, either because of low harrier densities in suitable habitat, or because productivity in these areas was too low to sustain the breeding population. The main constraint identified in most of these areas was illegal persecution associated with grouse moor management, though food supply was found to be important in at least one NHZ (Orkney). Because it was written before the results of the 2010 national survey were published, it did not include findings from that study. A revised version of the report, taking account of more recent survey data, is in preparation.
Constraints on Hen Harrier populations
Several scientific studies (Etheridge et al. 1997; Potts 1998; Sim et al. 2007; Fielding et al.2011; Hayhow et al. 2013) have found that breeding Hen Harrier numbers in the UK, particularly in northern England and southern and eastern Scotland, are currently, or have been, constrained by illegal population control associated with management of grouse moors. Two broad approaches have been taken by such studies, both approaches having been undertaken at within-landscape and between region scales. The first approach relates breeding densities of Hen Harriers to traditional management of grouse moor, either by looking at changes in Hen Harrier numbers in response to changes in management at a particular site (Redpath & Thirgood 1997) or by comparing breeding harrier densities between areas dominated by grouse moor and other breeding habitats (Potts 1998; Sim et al. 2007; Hayhow et al. 2013). The second approach relates data on breeding success to patterns of management within landscapes (Etheridge et al. 1997) or between regions (Fielding et al.2011). These studies show that breeding densities and nesting success of Hen Harriers are lower in areas with a high proportion of grouse moor than in other areas.
Illegal killing is by no means the only factor that can impact on Hen Harrier populations in Britain. Because Hen Harriers are ground-nesting, they are vulnerable to predation of their eggs and nestlings by mammals such as Red Fox (Baines & Richardson 2013; MacMillan 2014). Like other raptors, Hen Harriers are also susceptible to variations in their food supply (Newton 1979). The effect of food availability on breeding success and population trends of this species is particularly well studied in Orkney, where availability of small mammal prey is related to both mating system and breeding success (Amar et al. 2003; Amar et al. 2005). There is abundant evidence that the effects of both of predation and food availability on Hen Harrier populations are moderated by land use and habitat management (Redpath et al. 2002; Amar & Redpath 2005; Amar et al. 2008; Wilson et al. 2012; Baines & Richardson 2013).
Given the potential for predation and food availability to affect the fortunes of Hen Harriers, it is clear that some aspects of grouse-moor management have the potential to benefit Hen Harriers. Through control of generalist predators, management for grouse can increase prey availability for harriers by decreasing competing predation on Hen Harrier prey (Redpath et al. 2002), and increase nest survival by reducing predation of Hen Harrier nest contents (Baines & Richardson 2013). In addition, management for grouse involves the creation and maintenance of landscapes dominated by heather Calluna vulgaris, which is strongly preferred as a breeding habitat by Hen Harriers in most of their UK range. Studies have shown that, when their breeding attempts succeed, Hen Harriers breeding in managed moorland may fledge more offspring than those in other habitats such as young plantations and unmanaged moorland (Etheridge et al. 1997). The relevance of these positive influences for overall productivity of harrier populations has been questioned, however, in the context of the levels of persecution associated with grouse moor (Green & Etheridge 1999). In the absence of persecution, there is evidence that control of generalist predators can have a strongly positive effect on Hen Harrier breeding numbers (Baines & Richardson 2013).
Stopping management for grouse has been suggested as a means of improving the fortunes of Hen Harriers (Thompson 2009). However, although this would remove the main proximal constraint on populations in some areas, it might not translate straightforwardly into increases in Hen Harrier populations. In areas currently dominated by grouse-moor, a shift to alternative land uses such as forestry, high-density stocking with sheep or deer or wind energy development could diminish the value of the land for harriers by decreasing food availability or nesting success (e.g. Wilson et al. 2012, Wilson et al. 2017). Efforts are still ongoing by scientists and practitioners on both sides of this conflict to find a way to manage for grouse without illegally controlling raptors (Amar 2014). If such a solution can be found, it has the potential to benefit both the grouse shooting industry and Hen Harrier conservation more than alternative scenarios in which the existence of one precludes the other.
Amar, A. 2014. www.bou.org.uk/about-the-bou/hen-harriers-going-going/
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