Bird Study The status of the Hen Harrier, Circus

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The status of the Hen Harrier, Circus cyaneus, in the UK and Isle of Man in 2010 a

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b

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Daniel B. Hayhow , Mark A. Eaton , Stephen Bladwell , Brian Etheridge , Steven R. Ewing , ef

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Marc Ruddock , Richard Saunders , Chris Sharpe , Innes M.W. Sim & Andrew Stevenson a

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Conservation Science Department, RSPB, The Lodge, Sandy, Bedfordshire SG19 2DL, UK

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RSPB Wales, Uned 14, Llys Castan, Ffordd Y Parc, Parc Menai, Bangor, Gwynedd LL57 4FD, UK c

RSPB Scotland, Etive House, Beechwood Park, Inverness IV2 3BW, UK

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Conservation Science, RSPB Scotland, 2 Lochside View, Edinburgh Park, Edinburgh EH12 9DH, UK e

Northern Ireland Raptor Study Group (NIRSG), Cemetery Cottage, 12 Church Street, Greyabbey, County Down BT22 2NQ, UK f

Golden Eagle Trust, 22 Fitzwilliam Square, Dublin 2, Republic of Ireland

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Natural England, Dragonfly House, 2 Gilders Way, Norwich, NR3 1UB, UK

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Manx Birdlife, 35 New Road, Laxey, Isle of Man IM4 7BG, UK

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Scottish Natural Heritage, Kilmory, Lochgilphead, Argyll, PA31 8RR, UK Published online: 03 Oct 2013.

To cite this article: Daniel B. Hayhow, Mark A. Eaton, Stephen Bladwell, Brian Etheridge, Steven R. Ewing, Marc Ruddock, Richard Saunders, Chris Sharpe, Innes M.W. Sim & Andrew Stevenson (2013) The status of the Hen Harrier, Circus cyaneus, in the UK and Isle of Man in 2010, Bird Study, 60:4, 446-458, DOI: 10.1080/00063657.2013.839621 To link to this article: http://dx.doi.org/10.1080/00063657.2013.839621

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Bird Study (2013) 60, 446–458 http://dx.doi.org/10.1080/00063657.2013.839621

The status of the Hen Harrier, Circus cyaneus, in the UK and Isle of Man in 2010 DANIEL B. HAYHOW1*, MARK A. EATON1, STEPHEN BLADWELL2, BRIAN ETHERIDGE3, STEVEN R. EWING4, MARC RUDDOCK5,6, RICHARD SAUNDERS7, CHRIS SHARPE8†, INNES M.W. SIM4 AND ANDREW STEVENSON9* Conservation Science Department, RSPB, The Lodge, Sandy, Bedfordshire SG19 2DL, UK; 2RSPB Wales, Uned 14, Llys Castan, Ffordd Y Parc, Parc Menai, Bangor, Gwynedd LL57 4FD, UK; 3RSPB Scotland, Etive House, Beechwood Park, Inverness IV2 3BW, UK; 4Conservation Science, RSPB Scotland, 2 Lochside View, Edinburgh Park, Edinburgh EH12 9DH, UK; 5Northern Ireland Raptor Study Group (NIRSG), Cemetery Cottage, 12 Church Street, Greyabbey, County Down BT22 2NQ, UK; 6Golden Eagle Trust, 22 Fitzwilliam Square, Dublin 2, Republic of Ireland; 7Natural England, Dragonfly House, 2 Gilders Way, Norwich, NR3 1UB, UK; 8Manx Birdlife, 35 New Road, Laxey, Isle of Man IM4 7BG, UK; 9Scottish Natural Heritage, Kilmory, Lochgilphead, Argyll, PA31 8RR, UK

Downloaded by [RSPB] at 01:19 28 August 2014

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Capsule The fourth national survey of Hen Harrier showed that the population in the UK and the Isle of Man declined significantly between 2004 and 2010. Aim To estimate the size of the breeding Hen Harrier population (with associated 95% confidence intervals) in the UK and Isle of Man, constituent countries and Scottish regions, in 2010 and calculate population change since previous surveys in 1998 and 2004. Methods Complete surveys were made of 10-km squares likely to be occupied by breeding Hen Harriers in England, Wales, Northern Ireland and the Isle of Man, using standard methods developed for previous national surveys. In Scotland, a ‘census’ area was non-randomly selected for survey by volunteers, and randomly selected squares were surveyed in two strata covering the rest of the known range. Results The UK and Isle of Man Hen Harrier population was estimated at 662 territorial pairs (95% confidence interval (CI): 576–770), a significant decline of 18% since 2004. Scotland holds the bulk (76%) of the population (505 territorial pairs; 95% CI: 417–612), with smaller numbers in Northern Ireland (59 pairs), Wales (57 pairs), the Isle of Man (29 pairs) and England (12 pairs). Declines of 49% and 20% were observed in the Isle of Man and in Scotland, respectively, whereas the Welsh population increased by 33%. A significant decrease was recorded in numbers of pairs using young and mature plantation forest in Scotland. Conclusion The breeding population of Hen Harriers in the UK and Isle of Man declined between 2004 and 2010. Notable decreases in Scotland and the Isle of Man may be related to habitat change and illegal persecution. Illegal persecution continues to limit the population size of harriers in England to very low levels.

The Hen Harrier Circus cyaneus was widespread and relatively common until the 19th century, but was subsequently eliminated from the mainland largely due to persecution and changes in land use (Watson 1977, Thom 1986). Following the Second World War, the population began to expand from remnant populations

*Correspondence author. Email: [email protected] † Present address: Greenbank, 33 Mines Road, Laxey, Isle of Man, IM4 7NH * The Old Stores, Bornish, South Uist, Western Isles HS8 5SA © 2013 British Trust for Ornithology

in Orkney and the Western Isles (Etheridge et al. 1997), benefitting from a reduction in gamekeeper activity during the war years, and an increase in the availability of young forestry with post-war afforestation in upland regions (Thom 1986). A survey of Hen Harriers in Scotland in 1988–89 estimated the population at between 408 and 594 breeding pairs (Bibby & Etheridge 1993). This was combined with records from Raptor Study Group members and other Hen Harrier enthusiasts from the remainder of the UK and the Isle of Man, generating a


2010 Hen Harrier survey

national population estimate of 478–669 pairs. A second survey in 1998 produced an estimate of 570 territorial pairs in the UK and Isle of Man, identifying apparent increases in the Northern Ireland population, but significant declines in numbers on Orkney. Subsequent studies suggest that reduced prey availability affecting productivity was principally responsible for this decline through a reduction in polygynous breeding (Amar & Redpath 2002, Amar et al. 2003, 2005). In 2004, a third national survey showed a statistically significant increase of 41% in Hen Harrier population since 1998 to an estimated 806 territorial pairs (95% confidence interval (CI): 732–889; Sim et al. 2007). Increases were noted in the Isle of Man, Wales, Northern Ireland, Orkney, and North and West Scotland, although populations in South and East Scotland and England declined. A substantial body of research suggests that illegal persecution is the principle factor limiting Hen Harrier population growth in parts of the UK, particularly in areas where heather moorland is managed for driven Red Grouse Lagopus lagopus scoticus shooting (Etheridge et al. 1997, Redpath & Thirgood 1997, Stott 1998, Redpath & Thirgood 1999, Holmes et al. 2000, 2003, Summers et al. 2003, Anderson et al. 2009, Fielding et al. 2011). Fielding et al. (2011) estimated that in the absence of illegal persecution the Hen Harrier population in the UK could be as high as 2514–2653 pairs, more than three times the 2004 level. Changes in habitat (Amar et al. 2008, Arroyo et al. 2009), prey abundance (Redpath & Thirgood 1997, Amar et al. 2003) and climate (Whitfield et al. 2008, Amar et al. 2011a) have also been demonstrated to influence Hen Harrier numbers and distribution. In Northern Ireland, distribution is considered to reflect habitat dissimilarities with other parts of the UK, particularly because moorland habitats in this region are in poor condition (Ruddock et al. 2008) and more recently because of the impact of moorland wildfires. Red Fox, Vulpes vulpes, presence is thought in some regions to negatively impact on local productivity, though it is thought unlikely that the effect would be sufficiently large to impact on distribution across a wider geographic area (Green & Etheridge 1999, Fielding et al. 2011). In the UK, the Hen Harrier is Red-listed as a Bird of Conservation Concern (Eaton et al. 2009). Regular monitoring of vulnerable species is essential to guide effective conservation effort, but is particularly important in species like the Hen Harrier involved in a sensitive human–wildlife conflict (Thirgood &

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Redpath 2008, Thompson et al. 2009). This paper presents the results of the fourth national survey, conducted in 2010, and compares the updated population estimates with those from two previous national surveys. METHODS Survey design

The 2010 Hen Harrier survey follows very closely the design of previous surveys in 1998 and 2004 (Bibby & Etheridge 1993, Sim et al. 2001, 2007). The species’ range in the UK and the Isle of Man was defined as all 10-km national grid squares with records of Hen Harriers in the breeding season (April to July) since 1968, using the Breeding Bird Atlases from 1968 to 1972 (Sharrock 1976) and from 1988 to 1991 (Gibbons et al. 1993). This was supplemented with records from 2008 and 2009 submitted to the 2007–11 British Trust for Ornithology/BirdWatch Ireland/ Scottish Ornithologists’ Club Bird Atlas, and from the Raptor Study Group and RSPB’s species surveys databases. Survey work was carried out by volunteers from Raptor Study Groups, Upland Bird Study Groups and paid field workers. The survey design for each country varied as described below (Fig. 1). Scotland

A complete survey of all 10-km squares recently occupied by breeding Hen Harriers was planned in Orkney. In the rest of Scotland, 135 squares from the occupied range were selected non-randomly for coverage by Scottish Raptor Study Group members, and the remaining 446 squares were allocated to one of two strata (Table 1). Stratum one comprised 104 10-km squares located in areas expected to hold relatively high densities of Hen Harriers (e.g. Special Protection Areas with Hen Harriers as a qualifying feature (Stroud et al. 2001); important non-designated areas, including Mull, Skye and the Uists). Forty-five (43%) of these squares were randomly selected for survey. Stratum two consisted of the remaining 342 squares within the known range, of which 40 (12%) were selected. Sampling intensities within the two strata were chosen based on the relative densities recorded in the 2004 survey (Sutherland 2000). Regional population estimates and trends in Scotland were calculated for the geographical regions used in the two previous surveys (Sim et al. 2007). © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

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to conservation bodies in confidence and monitored in an overt manner with landowner cooperation. Wales


A complete census of Hen Harriers was planned in Wales, with 35 10-km squares identified for survey. This was a change from the approach used in 2004, when surveying focused on 190 1-km squares occupied by breeding Hen Harriers since 1954. As the population increase documented by the 2004 survey may have been accompanied by an expansion out of the known range, the survey area was expanded to include the 10-km squares in which these 1-km squares (and a further 25 identified since 2004) lay. Four 10km squares were excluded on the basis of poor habitat suitability. Northern Ireland

Figure 1. Location of 10-km squares surveyed in the 2010 Hen Harrier survey. Scotland, Wales and Northern Ireland: Black census squares, dark grey - stratum one, light grey - stratum two, white – planned but unsurveyed squares. In England, Black – indicates areas within which surveys were carried out, a number of specific census squares have been omitted for confidentiality grounds. For further details refer to text.

A census approach was taken in Northern Ireland. The breeding range comprised 10-km squares reported as occupied during previous national surveys and recent monitoring (annual monitoring scheme initiated in 2006). Surveys in 2010 were carried out by NI Raptor Study Group volunteers, staff from Forest Service NI, Northern Ireland Environment Agency (NIEA) and by a contracted RSPB fieldworker. In total, 49 10-km squares were identified as having had confirmed or possible breeding records and were prioritized for survey. A further 44 squares containing potentially suitable habitat, but with no prior evidence of breeding (although birds may have been sighted) were identified, and additional effort was directed to covering as many of these as possible.

England

A complete census was carried out by Natural England and RSPB staff and members of Raptor Study Groups. All squares with records of recent breeding season occupation (during or since the 1988–92 Atlas (Gibbons et al. 1993)) were surveyed, with particularly intensive coverage of the main breeding area. In addition, an RSPB-run ‘Hen Harrier (telephone) Hotline’ was set-up and advertised in local and national media, to receive records from the general public for follow-up by RSPB staff. A protocol was developed in cooperation with the Moorland Association, (representing owners and managers of heather moorland), whereby any Hen Harrier breeding attempts recorded by their members could be passed on © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

Isle of Man

A complete census was planned by surveying all 10-km squares known to have held breeding Hen Harriers since and including the 1998 survey, incorporating the vast majority of suitable habitat on the Isle of Man. Fieldwork methods

Surveys were carried out in suitable weather conditions between April and July (inclusive). A minimum of two visits were made (separated by two to three weeks if only two visits) to each 10-km square. If breeding was not confirmed during the two visits, a third visit was


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Table 1. Survey design, detailing numbers of 10-km squares planned and surveyed across each country, in the 2010 National Hen Harrier surveyed. Planned 10-km squares Country


Hebrides North Highlands Southern Uplands East Highlands West Highlands Scotlanda Orkney England Isle of Man Northern Ireland Wales

Census

Stratum one

27 12 24 47 25 135 12 nac 7 49 35

7 20 4 4 10 45

(12) (20) (2) (2) (22) (58)

Surveyed 10-km squares Stratum two 0 12 13 5 10 40

(4) (110) (70) (45) (74) (303)

Census

Stratum one

Stratum two

25 11 25 46 18 125b 12 nac 7 56 29

7 (11) 16 (25) 4 (2) 4 (2) 9 (29) 40 (69)

0 (7) 11 (111) 11 (71) 5 (46) 9 (76) 36 (311)

Note: Unsurveyed squares are in brackets. a Total range (10-km squares) = 580. b Includes ten supplementary squares not included in the planned survey coverage. c Extensive surveys carried out across area indicated in Fig. 1.

recommended between late June and the end of July (some areas covered by Raptor Study Group members received more visits). Suitable habitat within each 10km square was surveyed, with suitable habitat defined as moorland, both heather, Calluna vulgaris (L.), and grass dominated, and upland young forest (including first rotation, re-stocks of conifer plantations and regenerating birch, Betula pubescens). In particular, observers were alerted to the possibility of birds nesting within closed mature coniferous forest, and to consider such areas as suitable habitat (Scott et al. 1993, Ruddock et al. 2008, Wilson et al. 2009). Surveys were conducted through a combination of searches and scanning suitable habitat from appropriate vantage points. Where Hen Harriers were observed, information was collected on the location (a six-figure grid reference); the number and sex of birds; the behaviour of the bird(s); the dominant habitat within 100 m of each sighting; and any evidence of nesting. As male Hen Harriers can be bigamous, observers were advised to continue searching in the vicinity of confirmed pairs for additional nests. On subsequent visits, all suitable habitat was searched again and attempts were made to confirm breeding at all locations where birds were recorded on the previous visit. Field workers worked under disturbance licences from the appropriate statutory agencies. Calculation of population estimates

Hen Harrier records were classified according to the criteria outlined in Table 2 (e.g. Sim et al. 2001,

2007). In Orkney, the count unit was the number of breeding females due to the extent of polygyny in that population (Balfour & Cadbury 1979, Picozzi 1984). Elsewhere, polygyny occurs to an unknown, but lesser extent (Cramp 1985) and in these areas the survey count unit used is the number of pairs (i.e. a breeding male and female). Three population estimates are given for 2010: the number of proven breeding pairs, the number of territorial pairs (proven plus probable), and the number of territorial pairs plus apparently single birds. Population estimates for England, Wales, Northern Ireland and the Isle of Man were calculated as the sum of the counts made in census squares. For Scotland and Scottish regions, estimates were derived by adding the count from census squares to estimates calculated by extrapolating from counts made in randomly selected squares to the area of squares unsurveyed. Extrapolated estimates were calculated as [(n1/p1) + (n2/p2)], where ni is the total count from surveyed squares within stratum i and pi is the proportion of squares in stratum i that were surveyed. Estimates for Scotland and Scottish regions were made using only those squares relevant to them. CIs for extrapolated estimates were derived by bootstrapping (Efron 1982, Greenwood 1991), with squares randomly selected with replacement, up to the number actually counted. This was repeated 999 times for each population estimate, and the CI taken as the 2.5 and 97.5 percentiles of the replicates. Average regional densities per 10-km survey square were calculated from bootstrapped estimates and CIs taken as above. To test for significant changes in population estimates between © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

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Table 2. Classification criteria for Hen Harrier survey records, based on Sharrock (1976), adapted for Hen Harriers as used in both the 1998 and the 2004 surveys.

2004 and 2010, a randomization test was used. The observed difference between the estimates from each year is referred to as D. Using bootstrap estimates of population size from the 2004 and 2010 surveys (as above), 999 differences, d1 … d999, between paired estimates were calculated. Each of these differences was subtracted from D, to give d*1 … d*999. The proportion of d* values with an absolute value greater than D was taken as an estimate of the chance occurrence (P-value) of the observed difference, D. Habitat selection

Each Hen Harrier record was assigned to one of five land management classes based on a visual assessment by fieldworkers of the dominant habitat and land management within 100 m of the sighting: grouse moor (heather-dominated moorland managed using rotational strip burning for Red Grouse shooting); heather moor (heather-dominated moorland showing no visible form of intensive management for Red Grouse shooting); young conifer plantation (pre-canopy closure); mature conifer plantation (post-canopy closure); scrub/brash (dense understory growth); and grass moor (grass-dominated moorland). Land management class details were not recorded for 7% of records, and 11% of records on heather moor habitats were not classified with regard to grouse management in 2010. In order to allow the estimation of the total © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

number of territorial and single birds associated with each land management class, the proportions obtained for those records where land management class details were recorded were applied to those unclassified records. Analysis of land management class data used all records of territorial and apparently single birds to allow comparability with the previous three surveys. Country estimates of the number of territorial pairs plus apparently single birds in each land management class were made in the same way as for population estimates described above. Extrapolation of the recorded density from each habitat in randomly selected squares across each stratum is justifiable as these squares are assumed to represent the distribution of habitat types across the range

RESULTS 2010 survey coverage

Complete census coverage was achieved in the Isle of Man, Orkney and England (Table 1). The additional survey coverage conducted in England produced no records and no additional records were received through other means (via the RSPB ‘Hen Harrier (telephone) Hotline’ or the Moorland Association protocol): all Hen Harriers found in 2010 were at previously used sites. In Northern Ireland all of the 49 10-km squares with previous records of at least possible



breeding were surveyed, as were 7/44 (16%) of squares thought to have potential breeding habitat. In Wales, 29 of the 35 (82%) 10-km squares intended for coverage were fully surveyed. In Scotland, 115 of the 134 (84%) 10-km squares identified for census coverage were surveyed fully, with the remaining 21 only being partially surveyed. Partially surveyed squares were treated as unsurveyed (and any records omitted from analyses) and added to the relevant strata from which estimates were extrapolated (10 to stratum one and 11 to stratum two). Ten squares were surveyed by volunteers with no prior indication of intent to do so, and these squares were added to the areas of census coverage and removed from the relevant sampling strata. Of 457 remaining squares available for sampling, 76 were surveyed; 40 of 112 (36%) in stratum one and 36 of 345 (10%) in stratum two. Access to survey was denied in one 10-km square. Population estimates for 1998, 2004 and 2010

In the UK and Isle of Man, the number of territorial pairs (proven and probable) was estimated at 662 (95% CI: 576–770), of which 472 (95% CI: 393– 575) were classified as proven pairs (Table 3). The inclusion of single birds produced a larger estimate of 842 pairs (95% CI: 748–969). The number of territorial pairs was significantly lower (P < 0.05) than in 2004, declining by 18% from 806 pairs (Sim et al. 2007), and was not significantly different to the numbers of territorial pairs in 1998 (Sim et al. 2001). On the Isle of Man, 29 territorial pairs were recorded

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(24 proven breeders, 5 single birds) representing a decline of 49% since 2004. The UK total (excluding the Isle of Man) was estimated at 633 territorial pairs (95% CI: 547–741), but was not significantly different from the 749 pairs (95% CI: 675–832) estimated in 2004. Scotland held 76% of the total UK and Isle of Man Hen Harrier population. The Scottish population estimate was 505 territorial pairs (95% CI: 417–612) in 2010, 20% lower than in 2004 (633, 95% CI: 563–717), although this population change was not statistically significant. The population is now not significantly different from the estimate of 436 territorial pairs (95% CI: 365–506) in 1998. In England, 12 territorial pairs were located in the census squares, one more than in 2004, but this is still a substantial decrease from the 19 pairs in 1998. In Northern Ireland, the 59 territorial pairs recorded nationally was little changed from the 63 territorial pairs (95% CI: 58–68) estimated in 2004 despite expanding the survey area greatly in 2010. This apparent stability follows a 66% increase between 1998 and 2004. However, within the subset of 20 10-km squares surveyed in both recent national surveys, numbers declined from 62 territorial pairs in 2004 to 45 in 2010. In contrast, the Welsh population increased by 33% since 2004, continuing the trend reported between 1998 and 2004. A total of 381 territorial pairs were located in the UK and Isle of Man in 2010, 58% of the total population estimate. As in previous surveys, the West Highlands held the largest proportion of the Scottish population (36%;

Table 3. Numbers of proven, territorial, and territorial pairs plus single birds of Hen Harriers in the UK and Isle of Man in 2010, and changes in the number of territorial pairs between 2004 and 2010. Number of pairs in 2010a Country

No. of territorial pairs in 2004

Proven

Territorial

Territorial pairs + single birds

Change 2004–2010 (%)b,c

England Isle of Man Northern Ireland Scotland Wales UK total UK and Isle of Man total

11 57 63 (58–68) 633 (563–717) 43 749 (675–832) 806 (732–889)

12 24 30 366 (287–469) 40 448 (369–551) 472 (393–575)

12 29 59 505 (417–612) 57 633 (547–741) 662 (576–770)

12 34 78 642 (548–769) 76 808 (714–935) 842 (748–969)

9.1 −49.1 −6.3 −20.2 32.6 −15.5 −17.9*

Note: Classification of proven pairs, territorial pairs (proven + probable) and territorial pairs plus single birds is based on criteria defined in the text. Numbers in parentheses are 95% CI, estimate given without CI are from areas where complete coverage was achieved. b Change figures are based on numbers of territorial pairs. c Significance of change: *P < 0.05. a

© 2013 British Trust for Ornithology, Bird Study, 60, 446–458

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Table 4. Estimated number of territorial pairs of Hen Harriers by region in Scotland in 2004 and 2010, percentage change between squares surveyed in both 2004 and 2010, average density of territorial pairs per 10-km survey square (with 95% CI) and percentage of the 2010 populations by region. Number of territorial pairsa


Region Orkneye Hebrides North Highlands Southern Uplands East Highlands West Highlands Scotland

2004

2010

Change 2004–2010 (%)b

74 48 (45–54) 102 (72–137) 73 65 (54–76) 234 (191–285) 633 (563–717)

74 48 (44–56) 59 (38–87) 57 (49–74) 115 (65–183) 194 (110–281) 505 (418–612)

0 0 −42.2 * −21.9 76.9 −17.1 −20.2

% change 2004–2010 in squares surveyed in both surveysc 1.4 (9) −4.9 (20) −38.5 (9) −35.0 (27) −23.8 (36) −27.7 (21) −16.0 (122)

d

Territorial pairs per 100 km2 2010

% Scottish population 2010

7.40 (3.9–11.5) 2.75 (1.9–3.6) 1.56 (1.3–1.9) 2.53 (1.9–3.1) 2.39 (1.9–3) 2.89 (1.8–4.3) 2.92 (2.1–3.3)

13.5 8.8 10.8 10.4 21.0 35.5

The sum of the regional totals differs slightly from the estimated national total due to the differing sampling intensities and small sample sizes in different regions. a Numbers in parentheses are 95% CI. b Significance of change: *P < 0.05. c Sample size given in parentheses. d Density calculated from the number of pairs per surveyed, and occupied 10-km square. e Orkney estimate was a complete census.

Table 4). A significant decline (−42%) was estimated in the number of territorial pairs in the North Highlands, decreasing from 102 (95% CI: 72–137) in 2004 to 59 (95% CI: 38–87) in 2010 (bootstrap randomization test, P = 0.029). This followed an increase from 30 (95% CI: 13–53) in 1998. No other Scottish region showed a significant change based on extrapolated population estimates. However, considering only the subset of 10-km squares surveyed in both 2004 and 2010, we found evidence of decreases of territorial pairs (ranging from −24% to −39%; Table 4) in the North, West and East Highlands and the Southern Uplands. The highest density of territorial pairs was in Orkney, with 7.4 (95% CI: 3.9–11.5) pairs per 100 km2; elsewhere in Scotland, the average density of harriers was 2.9 (95% CI: 2.1–3.3) pairs per 100 km2. In Scotland (excluding Orkney), there was no significant change in estimates of Hen Harriers on heather moor or grouse moor between 2004 and 2010 (Table 5). However, there were significant declines in the number of Hen Harriers recorded in both mature and young plantation, having decreased by 55% (randomization test, P-value = 0.004) and 74%, respectively, since 2004 (P-value = 0.01). The pattern of habitat use in Scotland in 2010 now resembles that recorded in 1998 when the majority of records (84%) were from heather moor, with none in mature plantation or scrub (Sim et al. 2001). In England (100%), Wales (92%), Orkney (99%) and the Isle of Man (57%), all or the majority of territorial pairs © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

and singles were found on heather moorland (Table 5). Only one pair was recorded on heather moorland managed for grouse shooting in England. On the Isle of Man, numbers on other types of heather moor dropped substantially from 23 in 2004 to 4 in 2010. In Northern Ireland, the majority of the records were from young plantation, a substantial increase from eight pairs (less than 10%) in 2004 to 43 pairs (56%) in 2010. One area of heather moorland being managed for grouse shooting held seven pairs of Hen Harriers in 2010, while numbers recorded on other heather moor remained apparently stable despite increases in survey effort. Use of mature forest declined by 97% since 2004 with only one male recorded hunting over this habitat in Northern Ireland in 2010.

DISCUSSION Population trends

The 2010 Hen Harrier survey revealed a statistically significant decline in the UK and Isle of Man breeding population since the previous survey in 2004, with the number of territorial pairs falling by 18% from 806 (95% CI: 732–889) to 662 (95% CI: 576–770). Prior to 2004, the population had undergone a substantial increase since the second national survey in 1998, and the estimate in 2010 is still above that from 1998 (570 pairs, 95% CI: 499–640 (Sim et al. 2001)). Within the


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Table 5. Numbers of Hen Harriers (territorial pairs and single birds) estimated in each land management class in the UK and Isle of Man in 2004 and 2010. 2004


Habitat Orkney Grouse Moor Heather moor Young plantation Grass moor Mature plantation Scrub/brash Rest of Scotland Grouse Moor Heather moor Young plantation Grass moor Mature plantation Scrub/brash England Grouse Moor Heather moor Young plantation Grass moor Mature plantation Scrub/brash Wales Grouse Moor Heather moor Young plantation Grass moor Mature plantation Scrub/brash Northern Ireland Grouse Moor Heather moor Young plantation Grass moor Mature plantation Scrub/brash Isle of Man Grouse Moor Heather moor Young plantation Grass moor Mature plantation Scrub/brash

2010

Number

% of national/regional total

Number

% of national/regional total

% change in numbers between 2004 and 2010b

0 79 0 0 0 0

0.0 100.0 0.0 0.0 0.0 0.0

0 80 0 0 0 1

0.0 98.7 0.0 0.0 0.0 1.2

– 1.3 – – – Increase

185 (222–366) 204 (154–267) 140 (107–182) 67 (39–104) 50 (30–78) 16 (7–33)

28.0 30.8 21.2 10.1 7.6 2.4

194 (105–253) 222 (199–337) 63 (34–69) 55 (38–74) 13 (8–23) 13 (11–17)

34.6 39.6 11.3 9.8 2.3 2.3

4.9 8.8 −55.0* −17.9 −74.0* −18.8

2 9 0 0 0 0

18.1 81.8 0.0 0.0 0.0 0.0

1 11 0 0 0 0

8.3 91.7 0.0 0.0 0.0 0.0

−50.0 22.2 – – – –

6 39 0 0 0 0

13.3 86.7 0.0 0.0 0.0 0.0

0 70 0 6 0 0

0 92.1 0.0 7.9 0.0 0.0

−100 94.9 – Increase – –

0 27 (22–33) 8 11 (7–16) 30 (28–34) 6

0.0 32.9 9.8 13.4 36.6 7.3

7 26 43 0 1 0

9.1 33.8 55.8 0.0 1.3 0.0

Increase −3.7 437.5 −100 −96.7 −100

10 23 7 2 5 12

17.0 39.0 11.9 3.4 8.5 20.3

12 4 3 5 4 0

42.9 14.3 10.7 17.9 14.3 0.0

20.0 −82.6 −57.5 150 −20 −100

a

a

a

Numbers in parentheses are 95% CI. Significance of change: *P < 0.05.

b

overall decline, the 2010 survey results demonstrate substantial regional variation in population trends since 2004; notable declines were recorded in Scotland and the Isle of Man, increases were documented in Wales, and there has been little apparent change in Northern Ireland. The Hen Harrier population in England continues to remain at very low levels. Despite work describing a wide range of habitats broadly classified as woodland as very important for

Hen Harriers in Scotland (Haworth & Fielding 2009), significant declines were recorded in the number of Hen Harriers breeding within forest plantations (young and mature) in Scotland. Our results illustrate the difficulties of using national surveys to monitor population change at small spatial scales. Only one Scottish region, the North Highlands, showed a significant population decline between 2004 and 2010, but apparently substantial decreases © 2013 British Trust for Ornithology, Bird Study, 60, 446–458


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elsewhere were not statistically significant. The small number of random sample squares at the regional scale gave rise to estimates with poor precision, which are also sensitive to the inclusion of ‘outlier’ squares. For example, in the East Highlands, the inclusion of one square with high Hen Harrier numbers in a random sample of just five squares gave rise to a high population estimate, implying a substantial increase since 2004 (77%). This increase was not, however, statistically significant and was at odds with a decrease (−24%) in the 36 squares surveyed in both 2004 and 2010. Further comparison of results from squares covered in both surveys suggests that the decline in Scotland may have been widespread, with four of six regions showing declines of greater than 20%, although this latter comparison should be regarded with some caution due to the non-random selection of these squares (i.e. they are dominated by volunteer selected census squares). In Orkney, the number of territorial pairs has remained stable, following the substantial recovery in numbers detected between 1998 and 2004 (Sim et al. 2007). This recovery followed a major decline from an average of 69 breeding females in the 1970s to 25 in the late 1990s (Meek et al. 1998). The recovery has since continued; the Orkney population reached a 20-year high in 2011 with 120 occupied home ranges (Etheridge et al. 2013). The recovery has been attributed to an increase in rough grassland, and a recovery in the numbers of the Orkney vole Microtus arvalis orcadensis, precipitated by a 20% decline in sheep stocking levels (Amar et al. 2011a). The Welsh population has increased by at least 33% since 2004, and has doubled since 1998. Due to the fact that six survey squares were not covered in Wales, the country estimate should be considered a minimum, however, only two 1-km squares within these six 10-km squares have been occupied since 2004. None of the survey squares holding Hen Harriers were classified as moorland managed for grouse shooting reflecting the decline in grouse shoots in Wales since the late 1990s (Whitfield et al. 2008). The expansion of the Hen Harrier population in Wales has been well documented and is attributed to an increase in breeding productivity brought about as a combined result of warmer temperatures during the breeding season and an apparent cessation of human interference (Whitfield et al. 2008). More recently, however, productivity has reportedly been poor in a number of areas (S. Bladwell pers. comm.). Fielding et al. (2011) suggested there is suitable © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

habitat within Wales to support 246–260 pairs of Hen Harriers, so there appears to be potential for continued increase, although it remains to be seen whether this expansion can continue outside of protected areas. The severe decline on the Isle of Man is cause for concern and is notable given the increases recorded by surveys in 1998 and 2004. The expansion of the Hen Harrier population on the Isle of Man since the first records in the 1970s (Cullen 1991) was thought to demonstrate the scope for population increase in the absence of illegal persecution (Sim et al. 2001). Although further research is clearly required, there are no obvious drivers for this most recent decline. It is believed that persecution is absent (or at extremely low levels) and there have been no known changes in habitat or prey availability. That said, it has been shown that relatively subtle environmental changes (e.g. in grazing pressure) can bring about population change of Hen Harriers (Amar & Redpath 2002, Amar et al. 2003, 2011a). After the rapid expansion in Northern Ireland between 1998 and 2004, the population appears to have marginally declined despite extensive additional survey effort. While the national decline was less than 10% since 2004, there were more substantial regional declines including in the two SPAs for which Hen Harrier are a designating feature. In Northern Ireland there has been an apparent shift in the major habitat used by Hen Harriers. In 2004, the majority of territorial and single birds were found equally on heather moor (33%) and mature conifer plantation (37%), whereas in 2010, while 33% were still found on heather moor, 55% were found on young plantation sites, and with only one record in mature plantation. Nesting in the crowns of deformed Sitka Spruce Picea sitchensis trees was first reported in Northern Ireland in 1991 (Scott et al. 1993). However, during 2010, there were no confirmed records of treenesting and in recent years traditional tree-nesting territories appear to have relocated to heather moorland (M. Ruddock pers. obs.). Continued maturation of forest plantations in Northern Ireland alongside harvesting and restructuring of previously used mature plantation sites are likely drivers of this pattern of change in habitat use. However, these drivers are complex and vary locally (Ruddock et al. 2008). This survey reports a reduction in the number of Hen Harriers using mature forest plantations sites in Scotland and in Northern Ireland but opposing trends between the two countries in numbers found in young plantation. The substantial increase in use of young plantation forest



in Northern Ireland, composed mainly of second rotation restocks, contrasts with a decline in use of this habitat in Scotland. The area of pre-thicket restocks was predicted to exceed first rotation young forest in Scotland around 2008 (Haworth & Fielding 2009) and while this habitat is widely reported to be used by Hen Harriers in Ireland (Wilson et al. 2010, O’Donoghue et al. 2011), it has only rarely been reported in Scotland (Petty & Anderson 1986). Restocks are thought to be less attractive to harriers as they tend to be relatively small areas of clearfell surrounded by forest not moorland and therefore isolated from suitable foraging habitat. In addition the brash from the felled crop means that prey populations are limited and less accessible to harriers (Petty & Anderson 1986, Madders 2000, Wilson et al. 2010). Hen Harriers require substantial open areas in close proximity to the nest if they are to persist in a largely afforested habitat (Ruddock et al. 2008, Wilson et al. 2010), hence the age of the plantation, surrounding habitat availability, presence of open areas and rides as well as site-specific features such as soil and drainage must all be considered in ongoing research into the use of nonmoorland habitats by Hen Harriers (Madders 2000, Cormier et al. 2008, Wilson et al. 2009, 2012). Further research into the differences in forest composition and patterns of restructuring between Northern Ireland and Scotland would help to shed light on these opposing trends. The dynamic nature of forestry means that assessing the future potential of forests across the UK for nesting Hen Harriers is complex and will need continued monitoring (Fielding et al. 2011, Wilson et al. 2012). In England, Fielding et al. (2011) calculated that there was sufficient habitat to support between 323 and 340 pairs of Hen Harriers. The 12 breeding pairs recorded in 2010 represent only c. 4% of the potential population size in England, implying that harriers are absent from most of their suitable range (Anderson et al. 2009). Annual population monitoring of harriers in England suggests that numbers ranged from 12 and 24 territorial pairs between 2004 and 2010, with the majority of these pairs located in the Forest of Bowland Area of Outstanding Natural Beauty. However, since the 2010 survey, numbers have declined yet further, and only one pair is known to have bred in England in 2012, with none in their previous stronghold of Bowland, meaning that there is a real possibility that Hen Harriers could soon become extinct as a breeding bird in England. The 2004 Hen Harrier survey reported declines in those regions with the highest intensity of managed grouse moor, a result consistent with the hypothesis

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that the persecution of breeding Hen Harriers is the main factor limiting population levels (Sim et al. 2007, Fielding et al. 2011). This study reports no significant change in the size of the population using managed grouse moor across Scotland and England. It is hoped that the data from this survey will enable previous models (e.g. Fielding et al. 2011) to be updated to analyse national and regional changes in Hen Harrier abundance up to 2010 at a finer scale in relation to grouse moor management and muirburn intensity. Such models have used the extent and distribution of muirburn as a surrogate for grouse moor distribution (Anderson et al. 2009, Fielding et al. 2011, Amar et al. 2011b, 2012). Anderson et al. (2009) found the distribution of muirburn explained the discrepancy between recent Hen Harrier distribution (Gibbons et al. 1993) and the distribution as would be predicted by climate and habitat suitability (Anderson et al. 2009). The same study also found that fledging success was lowest for Hen Harriers breeding in sites with the highest degree of muirburn, however, after removing all completely failed nests (assumed to be a result of illegal persecution), from the analysis, a positive relationship was found between fledged brood size and extent of muirburn. The intensity of moorland management varies depending on the type of grouse shooting being practised, ‘driven’ grouse moors are managed most intensively to maximize grouse numbers. As Hen Harriers are perceived to reduce grouse harvests, the risk of illegal killing is likely to be related to variation in management intensity. The absence of breeding Hen Harriers from many ‘driven’ grouse moors (Redpath 2010) supports this and it is likely that Hen Harriers recorded on grouse moors in this survey were on lower intensity ‘walked up’ grouse moors. Anderson (2009) and recently Baines & Richardson (2013) suggest that in the absence of persecution, moorland management itself can be beneficial to Hen Harriers through availability of suitable nesting sites and gamekeeper activity (of legally controlled predators). Indeed previous studies have found that clutch size and the average number of young fledged from successful hen harrier nests on managed grouse moorland are significantly greater than in other landuse categories (Etheridge et al. 1997, Green & Etheridge 1999) and there is evidence that this is related to improved foraging success on managed moorland (Redpath et al. 2002). More than 80% of nest failures on grouse moors were the result of persecution, while on other moorland areas and land © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

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classes; this figure was less than 30% (Natural England 2008). The same report highlighted that more than 60% of breeding attempts on grouse moors failed as a result of an adult disappearing, an occurrence recorded in less than 10% of breeding attempts in other habitat types. In addition, recent radio tracking work indicates that overwinter survival of first-year birds appears to be very low in the uplands of northern England, the area where grouse moor management is most predominant (Natural England 2008, Fielding et al. 2011).


Survey limitation and recommendations

One of the aims of this study was to calculate regional population estimates and change since previous surveys. As with all sample-based surveys, our ability to detect change is influenced by sample size and this is particularly true if the data are interrogated at smaller spatial scales (i.e. regionally). This can result in conflicting trends when comparing squares surveyed in both years with the extrapolated trends across the whole region (i.e. in the case of the East Highlands). We suggest that if future national surveys intend to analyse regional trends that sufficient resources be made available to increase survey effort. In this way, the study could produce robust regional estimates and detect change at smaller scales without compromising on national coverage. One way this could be achieved would be to increase the number of randomly sampled squares. The UK and Isle of Man population remains only around a third of the potential national population estimates of c.2500 (Fielding et al. 2011). It is important that the UK and Isle of Man Hen Harrier population continues to be monitored regularly, and we suggest that the current survey cycle of six years is maintained. The close links between wintering and breeding populations in the UK reported by Dobson et al. (2012) emphasize the need for further research to be carried out to investigate causes of the most recent declines, not just on breeding sites, but also at winter locations and roost sites.

ACKNOWLEDGEMENTS

The 2010 Hen Harrier survey was conducted under the Statutory Conservation Agencies/RSPB Annual Breeding Bird Scheme programme. We thank the BTO for provision of 2007–2011 Atlas data. We thank the paid field staff who worked on the survey, but in particular © 2013 British Trust for Ornithology, Bird Study, 60, 446–458

must pay tribute to the large amount of fieldwork undertaken by volunteers, especially members of Raptor Study Groups and Upland Bird Study Groups. William George and David Fouracre in the RSPB Conservation Data Management Unit and Dr George Henderson at NIEA who provided extensive support with the GIS manipulation for survey design and analysis. We thank the many landowners and managers who granted access permission. Comments made by Professor Jeremy Wilson, Dr George Henderson and Dr Aly McCluskie improved an earlier draft of this paper. FUNDING

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(MS received 2 November 2012; revised MS accepted 24 August 2013)
