Bird Study Nest site selection by Hen Harriers in ...

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Nest site selection by Hen Harriers in Scotland S. Redpath , M. Madders , E. Donnelly , B. Anderson , S. Thirgood , A. Martin & D. Mcleod Published online: 29 Mar 2010.

To cite this article: S. Redpath , M. Madders , E. Donnelly , B. Anderson , S. Thirgood , A. Martin & D. Mcleod (1998) Nest site selection by Hen Harriers in Scotland, Bird Study, 45:1, 51-61, DOI: 10.1080/00063659809461077 To link to this article: http://dx.doi.org/10.1080/00063659809461077

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Bird Study (1998) 45, 51–61

Nest site selection by Hen Harriers in Scotland

Downloaded by [218.57.136.200] at 17:29 21 March 2014

STEVE REDPATH,1* MIKE MADDERS,2 ERIC DONNELLY,1 BRUCE ANDERSON,3 SIMON THIRGOOD,4 ANN MARTIN1 and DAVID MCLEOD5 1Institute of Terrestrial Ecology, Hill of Brathens, Banchory, Kincardineshire, Scotland AB3 4BY, UK, 2RSPB, Carnduncan, Gruinart, Isle of Islay, Argyll, Scotland PA44 7PS, UK, 3RSPB, The Neuk, Coupar Angus, Perthshire, Scotland, UK, 4The Game Conservancy Trust, Crubenmore Lodge, Newtonmore, Inverness-shire, Scotland PH20 1BE, UK, 5Department of Geography, Edinburgh University, Edinburgh, Scotland, UK

The aim of this study was to examine nest site selection by Hen Harriers Circus cyaneus at two spatial scales in Scotland and to assess whether breeding success was influenced by choice of nest site. At the landscape scale, we compared availability and utilization of habitats in 610 km2 of Argyll, and at the local scale, we compared vegetation and topography at 52 harrier nests and random points within three areas of heather moorland covering a total area of 462 km2. At both scales, harriers showed a clear preference for nesting in heather. Within heather moorland, harriers nested in taller heather (average height 46cm) and nearer streams than expected by chance. More nests were on northwest-facing slopes than expected by chance. We found no evidence that breeding success was influenced by habitat or topography. Heather moorland is declining in the uplands due to overgrazing and afforestation. The association of harriers with heather suggests that their future may become increasingly dependent on moorland, where heather is maintained for grouse.

T

he distribution of nesting raptors is influenced by nest site and food availability.1 Provided suitable nest sites are available, raptors can be expected to settle in areas where food is abundant. Within areas, breeding success may be significantly improved by selecting sites which minimize the risks of predation,2–4 and optimize the thermal environment.3–5 In reality, the choice of nest site may represent a compromise between these factors.4 The Hen Harrier is a raptor of open country, which nests on the ground and selectively hunts the edges between habitats.6,7 In Britain, the principal prey species by number during the breeding season is the Meadow Pipit Anthus pratensis, although a variety of other species, including Red Grouse Lagopus lagopus *Correspondence author. Email: [email protected]

© 1998 British Trust for Ornithology

scoticus chicks are also taken.8–10 In continental Europe, North America and historically in Britain,8 harriers nest in a variety of habitats, although moorland dominated by Heather Calluna vulgaris currently appears to be the preferred breeding habitat of British Hen Harriers, with young forestry plantations used to a lesser extent.11 Of 922 nests examined by the RSPB (unpubl.), 76% were in heather moorland, of which 49% were located on moorland managed for Red Grouse. Despite the number of harrier nests located in Britain, nest site selection has yet to be quantified. Studies have indicated that Hen Harriers appear to prefer tall heather below an altitude of about 500 m and often choose nest sites close to streams in valley bottoms.8,12 In this paper we examine the distribution of Hen Harrier nests in relation to vegetation at two scales. First we consider nest selection at the landscape scale, using data collected from

52

S. Redpath et al. extracted from a geographic information system (ARCINFO) based on the interpretation of 1:24 000 black and white aerial photographs, by the Macauley Land Use Research Institute. From these 1988 photographs, the data were captured for 50 m × 50 m tiles (0.25 ha). Originally 1501 habitat classes were identified, although we excluded unsuitable habitat such as water and aggregated remaining classes into five broad groups, corresponding with NVC (National Vegetation Classification)13 categories: heaths (1); mires, swamps, tall herbs and upland grassland (2); woodland, divided into open canopy woodland (3) and closed canopy woodland (4); and remaining habitats (5). Within the four areas, we measured the amount of each of these five habitat categories and then examined the habitat at harrier nest sites. Although harrier nests were located from

aerial photographs, and secondly we look at finer scale selection within heather moorland areas. Finally we consider how breeding success is influenced by choice of nest site and discuss the implications for Hen Harrier populations in Britain. METHODS


Habitat at the landscape scale At a landscape level, we compared availability with utilization by nesting harriers, of various habitat types in four areas of Argyll (Fig. 1), covering a total of 610 km2 (Table 1). Within these four areas, the harriers were relatively free from persecution and had a wide range of habitats available to them. The areas were searched thoroughly for harrier nests from 1988 to 1996. Habitat data for each area were

C

Argyll

B

A

Fig. 1. Map of Scotland showing location of study areas. Within the Argyll box, the four study areas are indicated. Areas A, B and C refer to areas of heather moorland where nest site details were taken.

© 1998 British Trust for Ornithology, Bird Study,

45, 51–61

Hen Harrier nest sites

53

Table 1. Size of areas where study was conducted and number of nests on each. Random points indicate number of initial random quadrats and number of additional quadrats from rank heather stands (H). The area of habitat measured indicates the size of area measured from aerial photographs.


Area

No. of estates

Size of area

No. of nests

No. of random points

Area of habitat measured

Landscape scale Argyll

–

610 km2

34

–

610 km2

Local scale A B C

1 2 3

178 km2 120 km2 164 km2

11 19 22

30 +30H 60 +60H 35

164 km2 48 km2 21 km2

1988 to 1996, to ensure that nest data were independent, we only used data for the year when most nests were present in each of the four areas. For analysis the data from the four areas were combined. Habitat at the local scale At a finer scale, we compared habitat availability versus utilization on six upland estates in three areas (A, B & C) in Scotland between 1993 and 1995 (Fig. 1). Area A consisted of one large estate in Dumfriesshire, southwest Scotland; area B consisted of two neighbouring estates in Ayrshire, southwest Scotland; and area C comprised three neighbouring estates in Perthshire, central Scotland (Table 1). These estates were initially selected because harriers were allowed to breed freely on them in the absence of persecution, although some broods were destroyed on one of the areas in 1993. All areas were managed for Red Grouse and this meant that heather was the principal vegetation type. Active gamekeepers were employed on each area and consequently mammalian predators and crows Corvid spp. were controlled. Purple Moor Grass Molinia caerulea was abundant on area A, Bracken Pteridium aquilinum was common on areas A and B and rushes Juncus spp. were common in all three areas. To ensure that nest data were independent, we again used data from each area for the year when most nests were present. In addition, data were collected from a minimum of 30 randomly chosen points for each area. For the random points, a grid was placed over maps of the areas and coordinates

obtained using a random number generator. Any points that fell in unsuitable habitat types (e.g. water or bare rock) were ignored. In order to avoid inadvertent observer bias in habitat selection, random points were located in the field by walking to where the observer considered the point to be from the map, then taking a bearing due north and walking 100 m. At each of the nests and random points, data on vegetation and topographic features were obtained during the autumn and early winter. Vegetation data were collected within a 2-m quadrat, placed over the nest. Within quadrats, species presence and percentage cover were determined. Vegetation composition in nest and random sites was compared by classifying the vegetation into NVC categories using TABLEFIT.14 Vegetation height was measured at five 10-cm intervals on the diagonal in from each corner. This gave a total of 20 measures and avoided the actual nest cup. In addition to vegetation, the following measures were obtained: 1. Distance to the nearest stream; for consistency, only streams that were marked on 1:25 000 OS maps were considered. 2. Angle of slope; measured by eye on a scale of 1 to 5 where 1 = flat, 2 = < 20°, 3 = 20–40°, 4 = 40–60° and 5 = > 60°. 3. Direction of slope (0–360°); for statistical analysis, directions were grouped into four categories (1 = 1–90°, 2 = 91–180°, 3 = 181–270° and 4 = 271–360°). 4. Height above sea-level (m). For two of the nests in area B, the vegetation


45, 51–61

54

S. Redpath et al.

was burnt before measurements were taken, so for these only topographic measures were taken.


Potentially suitable sites As harriers invariably nested in tall heather (see Results), we increased the number of random points in equivalent habitat in order to compare nest sites with randomly chosen suitable habitat. On two of the estates (A & B), we selected a further 30 tall heather stands (greater than or equal to the minimum height of nest site vegetation) from random points. Points were selected as before, although this time observers visited the stand of tall heather nearest to the random point and sampled the approximate centre. On one estate (A), the distribution of rank or degenerate heather (i.e. tall heather with the canopy opening up) was mapped from 1:24 000 black and white aerial photographs taken in 1988. Using a stereoscope, a 1-ha grid was laid over the photographs and the vegetation type and percentage cover within each hectare estimated. Validation in the field (groundtruthing) revealed that it was relatively straightforward to determine the presence of rank heather. The distribution of rank heather was measured as the number of hectares containing that habitat in each km2. Prey abundance Meadow Pipit abundance was estimated in 18 1-km2 sites chosen at random over estate A. Random sites chosen in enclosed pasture or woodland were excluded. Two parallel 1-km transects, set 500 m apart, were walked within each site. Surveying was conducted in early summer, between 0500 and 0900 hours on calm days with good visibility.15 For each transect, abundance was simply measured as the number of pipits counted within a 200-m strip of the transect. Abundance within each km2 was taken as the mean of the two transects. Abundance was compared to the amount of heather and rank heather within each count area. Breeding success To see whether or not there was any association


45, 51–61

between nesting habitat and breeding success, we compared the number of young fledged (measured as the number of young over 25 days old at nests) with the dominant vegetation at the nest site. Again we repeated this analysis at two scales: first we utilized data collected from a total of 395 harrier nests over nine years (1988–1996) from Argyll. These data were divided into those from the island of Islay, where there are no foxes, and those from mainland Argyll, where foxes are present and only lightly controlled. Secondly, we compared breeding success with habitat and environmental measures at nests on the heather moorland study areas. RESULTS Landscape scale At the two scales in which we considered site selection, Hen Harriers showed strong preference for heath vegetation. In Argyll, there was a highly significant association between nest location and habitat, with more nests located in heaths and open canopy woodland than expected by chance (Table 2) (χ2 = 39.7, 4 df, P < 0.001). In Argyll the open canopy woodland referred to young conifer plantations and, within this habitat, 14 (87%) of the nests were located in heather-dominant vegetation. In the closed canopy woodland, the nest occurred in a heather ride within the wood.

Table 2. Nest site location in four habitat types in 610 km2 of Argyll. Number of expected nests is derived from the relative area of that habitat. Number of nests Habitat

Area (ha) Observed

Expected

Heaths Mires/swamps/ upland grassland Open canopy woodland Closed canopy woodland Rest

10 244 17 272

15 2

5.7 9.6

13 711

16

7.6

11 353

1

6.3

8 386

0

4.8

Total area

60 965


55

Moor A

Proportion

1 0.8 0.6 0.4 0.2 U20

U16

U5

U4

U2

SD17

M25

M23

M15

H21

H18

H11

H10

H1

0

NVC category Moor B

Proportion


1 0.8 0.6 0.4 0.2 U20

U16

U5

S11 M25

U2

M25 M23

M23

M17

M16

M15

M6

H21

H18

H13

H10

H9

H1

0

NVC category Moor C

Proportion

1 0.8 0.6 0.4 0.2 U6

U5

S11

M20

M15

H21

H18

H13

H11

H10

H9

H1

0

NVC category Fig. 2. The proportion of (■) nest and (❏) random sites in various NVC (National Vegetation Classification) categories for areas A, B and C.

Local scale Within heather moorland, a comparison of NVC categories (Table 3) used for nesting and those selected at random in the three areas showed that nests fell in eight of the 22 categories selected from the random quadrats (Fig. 2). Of the 50 nests where habitat could be measured, 94% were in Calluna-dominant vegetation, with the remaining 6% in rushes Juncus spp. (all in area B). On all areas, the

most widely used NVC category was H1 Calluna–Festuca ovina heath. For statistical comparison, NVC categories were grouped into four broad types: H, heaths; M, mires; S, swamps and tall-herb fens; U, upland grassland. There were significant differences in each of the three areas, with more nests located in heaths than expected by chance, and fewer nests in the other three categories (Table 4). For a comparison of vegetation height, only those quadrats which fell in NVC categories


45, 51–61

56

S. Redpath et al.

Table 3. Definitions of NVC categories determined from 2 × 2 m quadrats using the TABLEFIT program.14


NVC Category

Habitat definition

H1 H9 H10 H11 H13 H18 H21 M6 M15 M16 M17 M20 M23 M25 S11 SD17 U2 U4 U5 U6 U16 U20

Calluna vulgaris–Festuca ovina heath Calluna vulgaris–Deschampsia flexuosa heath Calluna vulgaris–Erica cinerea heath Calluna vulgaris–Carex arenaria heath Calluna vulgaris–Cladonia arbuscula heath Vaccinium myrtillus–Deschampsia flexuosa heath Calluna vulgaris–Vaccinium myrtillus – Sphagnum capillifolum heath Carex echinata–Sphagnum recurvum / auriculatum Scirpus caespitosus–Erica tetralix Erica tetralix–Sphagnum compactum Scirpus caespitosus–Eriophorum vaginatum Eriophorum vaginatum blanket/raised bog Juncus effusus/acutiflorus–Galium palustre Molinia caerulea–Potentilla erecta mire Carex vesicaria swamp Potentilla anserina–Carex nigra Deschampsia flexuosa Festuca ovina–Agrostis capillaris–Galium saxatile Nardus strict –Galium saxatile Juncus squarrosus–Festuca ovina Luzula sylvatica–Vaccinium myrtillus Pteridium aquilinum–Galium saxatile

which were utilized for nesting were used. Combined data from the three areas indicated that nests were located in significantly taller Calluna vegetation than the random points (Fig. 3; nests: 46.0 ± 1.3 cm, random: 27.9 ± 2.0 cm; t-test: t = 7.6, df = 80, P < 0.001). When comparing topographic and other features between nests and random sites, only

potentially suitable vegetation types were selected. Therefore only random points (including the extra randomly located heather stands) which consisted of the NVC categories which actually held nests and were > 25 cm tall were used. This left a total of 78 random points between the three areas. Random points did not differ between the three areas in terms of

Table 4. Distribution of observed frequency of harrier nests and random points in four NVC categories. Two nest sites were excluded from area B because the vegetation had been burnt between nesting and vegetation surveying. Differences were significant for each area. A: G = 19.1, P < 0.001, B: G = 25.7, P < 0.001, C: G = 8.7, P < 0.05. Area A NVC category

B

C

Nest

Random

Nest

Random

Nest

Random

Heaths Mires Swamps + tall-herbs Upland grassland

10 1

9 13

12 4

13 27

21 1

26 4

0

1

1

1

0

1

0

7

0

19

0

4

Totals

11

30

17

60

22

35


45, 51–61


57

50 45 40

30 25 20 15 10 5

90–100

80–90

70–80

60–70

50–60

40–50

30–40

20–30

10–20

0