Bird Study (2008) 55, 86–93
Selection of nesting habitat by Montagu’s Harriers Circus pygargus and Hen Harriers Circus cyaneus in managed heaths JEAN-PAUL CORMIER1, JOELLE FUSTEC2*, JOSEPHINE PITHON2 and PATRICE CHOISY3 1Animal
Ecology Laboratory, UFR Sciences, 2 bd Lavoisier, 49045 Angers Cedex, France, 2Plant Ecophysiology and Agroecology Laboratory, ESA, 55 rue Rabelais, BP 30748, 49007 Angers Cedex 01, France and 3Aillé, 86130 St Georges des Baillargeaux, France
Capsule Montagu’s Harriers avoided tree plantations and selected plots with dense heath of intermediate height, while sympatric Hen Harriers preferred afforested or taller plots of heath. Aims To investigate relationships between vegetation structure and nest selection by Montagu’s and Hen Harriers. Methods We defined three vegetation strata: upper, intermediate and lower. Height and cover of plant species in each stratum were recorded in a 9-m2 area centred on nests or unused random points. Statistical analysis was performed to reveal predictors for classifying plots (i) with a Montagu’s Harrier nest, (ii) with a Hen Harrier nest and (iii) without a nest. Results Montagu’s Harriers selected places where tree cover was less than 25%, the maximum height of the upper stratum was 1.87 m, and the cover of the intermediate stratum was greater than 75%. Hen Harriers nested either in plots where the upper stratum was higher than 1.87 m, or where tree coverage was greater than 25%. Conclusions To encourage the breeding of Montagu’s Harriers, the planting of pine should be avoided and the heath should be rejuvenated before it reaches 1.80 m in height. For Hen Harriers, certain plots of heath higher than 2 m should be kept. In agricultural landscapes, patches of managed shrubland or heathland could help conservation of both species.
Montagu’s Harriers Circus pygargus L. and Hen Harriers C. cyaneus L. are ground-nesting birds of prey. Both species are protected by the Bonn Convention (1982) and the European Union Birds Directive (Annex 1). Typical natural and seminatural nesting habitats of Montagu’s and Hen Harriers are low plant communities such as shrubland, wet grassland, marsh or young tree plantations (Watson 1977, Schipper 1978, Simmons & Smith 1984, Cormier 1985, Redpath et al. 1998, Madders 2000). As a consequence of natural habitat decline, the two species are currently most often found breeding in crop fields where they are exposed to threats associated with harvesting (Corbacho et al. 1997, Pain et al. 1997, Arroyo et al. 2004, Limiñana et al. 2006). Management of remaining or restored natural habitat for bird breeding may be important for the conserva*Correspondence author. Email:
[email protected]
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tion of the species, as these may act as sources of recruits for other areas (Soutullo et al. 2006). However, the factors influencing nest selection by the two bird species remain unclear. Habitat selection for nesting by these wide-ranging species has mostly been studied at the landscape scale, often in relation to hunting or prey availability (Schipper 1978, Martinez et al. 1999, Madders 2000, Amar & Redpath 2005). Rather less work has been conducted on the finer-scale relationships between nest-site selection by harriers and vegetation structure, as vegetation characteristics may be of importance to ground-nesting birds when selecting a nest-site (Powell & Steidl 2000, Fondell & Ball 2004, Limiñana et al. 2006). Better knowledge of the vegetation structure of nest-sites chosen by each species of harrier would be valuable for informing management at this scale (Redpath et al. 1998, Cormier 1984). In spite of an overlap in breeding habitat between Hen and Montagu’s Harriers, there seem to be some differences in nest-site choice. Montagu’s
Harrier nests in heathlands
Harriers tend to choose tall grassland while Hen Harriers more often nest in moorland, bogs, swamp forest or in forest open space (Balfour 1962, Cramp & Simmons 1980, Hamerstrom & Kopeny 1981, Cormier 1984). Both species can occasionally be found nesting in the same areas (Cormier 1984, Millon et al. 2002, Garcia & Arroyo 2005). In the Poitou-Charentes region of southwest France, the area known as Le Pinail includes wet heathland in which both migratory Montagu’s Harriers and nonmigratory Hen Harriers can find suitable nesting habitat. We therefore compared vegetation characteristics around Montagu’s and Hen Harrier nests with that of sites without nests in order to investigate differences in nest plot selection between the two species. STUDY AREA
Le Pinail (46°43′N, 0°31′E, 800 ha, France) is situated on the edge of Moulière Forest (4171 ha) in the ‘département’ of Vienne, a part of the PoitouCharentes region dominated by open fields with an altitude ranging from 138 to 190 m. Average annual precipitation is between 600 and 850 mm and the average temperature is 11.4°C. The site was once a quarry producing millstones. The extraction activities have created an unusual landscape consisting of a mosaic of ponds and heath. About 400 ha of the area are divided into rectangular patches planted with Maritime Pine Pinus pinaster Ait. (Fig. 1). Le Pinail includes a 385-ha wet heathland (EU
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Habitats Directive codes 4020–4030). A 135-ha part of this site belongs to the European network of biogenetic reserves established as a result of the 1979 Bern Convention. It comprises a mix of wet heathland and ponds. Le Pinail is managed by the Office National des Forêts (ONF), together with the Association de Gestion de la Réserve Naturelle du Pinail (GEREPI) for the biogenetic reserve. Le Pinail’s heathlands are typically dominated by Besom Heath Erica scoparia L., a tall plant reaching 3–4 m in height. They are also characterized by Bell Heather Erica cinerea L., Crossleaved Heath Erica tetralix L., Common Heather Calluna vulgaris (L.) Hull, Dwarf Gorse Ulex minor Roth and Purple Moor-grass Molinia coerulea (L.) Moench. Every ten years, sections of the heath are either cleared or burned to benefit biodiversity and more specifically to maintain harrier nesting habitat. Therefore, aside from those areas where ponds are abundant, the vegetation within a given field tends to be rather homogeneous as it has usually regenerated at the same time. METHODS Bird and nest survey
Breeding harriers have been closely monitored every year since 1983, from late April to fledging, by the ONF and the Ligue pour la Protection des Oiseaux (a non-government organization for bird protection). The nests of Hen and Montagu’s Harriers are located by watching adults land after a prey pass. Each year they are mapped and numbered and a piece of cloth impregnated with a repellent substance is hung near the nest to reduce mammal predation. At the end of the breeding period, Montagu’s Harriers are caught and ringed. Vegetation data collection
Figure 1. Location of sampling plots from 2001 to 2004 at Le Pinail. ●, Montagu’s Harrier nests; , Hen Harrier nests; ●, plots without harrier nests for at least the previous three years.
The vegetation study was conducted from 2001 to 2004, during the months of June and July. Structure and composition of the vegetation were recorded in plots of area 9 m2 (3 m × 3 m) centred on active nests of Montagu’s and Hen Harriers (hereafter MHN and HHN, respectively). Each year, data were also collected in 9-m2 plots selected randomly in areas in which no harrier nest had been recorded for at least three years (hereafter called NN; Fig. 1). NN plots did not include any trees higher than 12 m, as harriers have never been observed nesting in such habitat at Le Pinail. © 2008 British Trust for Ornithology, Bird Study,
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In each plot, we recorded the plant species present. The plant community was divided into three strata – upper (A), intermediate (B) and lower (C) – based on plant structure, height and composition. Stratum A was mainly composed of erect branches of Besom Heath (nanophanerophyte). In young patches of heath, stratum A was not present. Stratum B was composed of compact chamaephytes intermingled with M. coerulea. Stratum C consisted of low herbs. The maximum height of each stratum was measured. For strata A and B, the maximum heights of each species present were also recorded. Finally, we collected cover data based on visual estimates from the nest-site position (Elzinga et al. 2001). Cover estimation was a quick method to use, well-adapted to bushy heathland vegetation, and disturbance to birds was therefore minimized. Thus, canopy cover was estimated (i) for each stratum, and (ii) for each plant species within each stratum as a proportion of total plot area minus the area occupied by the nest cup. Values of canopy cover for individual species within a stratum were categorized using a scale ranging from 0 to 6: 0 = 0%, 1 = 1–5%, 2 = 6–25%, 3 = 26–50%, 4 = 51–75%, 5 = 76–95% and 6 = 96–100% (Mueller-Dombois & Ellenberg 1974). In fields planted with P. pinaster ( 0.05).
cleared in 1996–97. Eight of them were selected by Montagu’s Harriers about eight years after regeneration (7.5 ± 0.6 years, n = 8). When Montagu’s Harriers started using them, the mean heights (± 1 se) of strata A and B of these plots were 1.52 ± 0.07 m and 0.83 ± 0.04 m, while the canopy cover values were 19.7 ± 5.5% and 51.7 ± 10.5%, respectively (data not shown). Although there were no differences between groups in the height of stratum B (P > 0.05), canopy cover of this stratum was higher in MHN (70.4%) than in NN plots (58.8%, U = 488, U’ = 880, P = 0.034; Table 1). In this stratum, as the different plant species intermingled and covered each other, the summed canopy covers of the constituent species were higher than the canopy cover of the whole stratum. E. scoparia was one of the main species in stratum B, together with E. cinerea, U. minor, E. scoparia, C. vulgaris and M. coerulea. U. minor had clearly greater coverage in MHN sites than in NN (H = 6.28, P = 0.043). E. cinerea
occurred in all MHN plots and in 92% of HHN plots, but in only 78% of NN plots. E. tetralix and S. humilis were also common in plots, while Blackberry Rubus fruticosus L., Bracken Pteridium aquilinum (L.) Kuhn and Carex sp. were occasionally found in MHN and NN plots. E. cinerea and E. tetralix were 10% and 22% taller in MHN plots than in NN plots (U = 362.5, U’ = 701.5, P = 0.028 and U = 118.5, U’ = 259.5, P = 0.048, respectively). Similarly, E. cinerea cover was 30% higher in MHN plots than in NN ones (U = 378, U’ = 686, P = 0.045). Such differences in E. cinerea or E. tetralix were not found between HHN and NN plots. Selection of nesting habitat by harriers
Predictors retained by the DFA procedure for discriminating HHN, MHN and NN plots were tree height, stratum B height and cover, E. scoparia height and © 2008 British Trust for Ornithology, Bird Study,
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stratum A cover, E. scoparia cover in stratum B, E. cinerea height in stratum B and U. minor cover in stratum B (F16,154 = 4.36, P < 0.00005; Table 2). The first canonical variable was tree height, which accounted for 76% of the dispersion of the groups. Although cover of strata A and B contributed little alone (low tolerance value), they together enhanced the separation of MHN from other plots driven by tree height. Factor 2 was driven by stratum B height. A total of 66% of the plots were classified correctly. Of the 38 MHN plots, five were misclassified as NN plots and four as HHN. Of the 13 HHN plots, four were incorrectly classified as NN plots and two as MHN. Of the 36 NN plots, 21 were correctly classified while seven were misclassified as MHN and the other eight as HHN. The first AID procedure was conducted in order to find the best predictors for MHN and HHN plots. It was performed without deleting misclassified MHN and HHN plots. Canopy cover of trees and height of stratum A were revealed as good predictors for discriminating both kinds of nest (PRE for the regression tree = 0.538; Fig. 2): MHN plots were characterized by low tree cover (25%, or stratum A higher than 1.87 m. The second AID, performed to distinguish HHN from NN plots, did not provide any significant result, even after removing misclassified plots. The third AID was run to search for
predictors for classifying MHN and NN plots. After deletion of misclassified patches, about 70% of tested MHN (60% of all MHN nests) were found in places where stratum A was less than 1.87 m high, tree cover was less than 30% and stratum B cover was more than 75% (PRE for the regression tree = 0.693; Fig. 3). Ten MHN plots were found in places where stratum B cover was less than 75%. After removing plots with trees from the data sets, the AID revealed the same predictors as before (PRE for the regression tree = 0.614; data not shown). DISCUSSION Vegetation and nest selection in Montagu’s Harriers
Montagu’s Harriers are known to nest in average density or high vegetation, for instance in tall grassland, heathland, shrublands or young plantations (Schipper 1978, Cramp & Simmons 1980, Cornulier & Bretagnolle 2006, Limiñana et al. 2006). During a 12year survey of afforested heaths, Cormier (1984) reported that Montagu’s Harriers were able to nest in 15-year or even 21-year plantations (5.5-m and 7-m high trees, respectively), when areas without trees became scarce. Consistent with such observations, Montagu’s Harriers first selected plots without trees for breeding at Le Pinail. Vegetation height is known to be of importance for nest selection. The height of nests recorded in other
Table 2. Canonical discriminant functions standardized by within variance (CDFSV), eigenvalues, canonical correlations and cumulative proportion of total dispersion (CPTD) obtained from discriminant function analysis procedure. Factor 1
Factor 2
F-to-remove
Tolerance
CDFSV Tree height Stratum B height Stratum B cover E. scoparia height in A E. scoparia cover in A E. scoparia cover in B E. cinerea height in B U. minor cover in B
0.867 0.109 –0.475 –0.392 0.809 0.813 –0.455 –0.314
–0.253 0.928 0.317 –0.606 0.270 –0.472 –0.602 –0.397
8.78 3.88 3.81 3.12 7.31 6.40 5.79 2.34
0.553 0.553 0.797 0.566 0.557 0.449 0.848 0.811
Eigenvalues Canonical correlations CPTD
0.716 0.646 0.757
0.229 0.432 1.000
Wilk’s λ F16,154 (approx. value) P
0.474 4.357 1.87 m). When Hen Harriers nested in young plantations at Le Pinail, mean tree height was around 6.5 m, in agreement with other studies in similar habitats (Watson 1977, Cormier 1984). This result supports Schipper (1978), who reported that Hen Harrier breeding habitat looked less open than that of Montagu’s Harriers, due to the presence of high shrubs. These differences between the two species may be explained by nest size. The body size of Hen Harriers is larger and thus nest diameter as well as clutch size are often larger in Hen than in Montagu’s Harriers (Cramp & Simmons 1980, Millon et al. 2002, Millon &
Figure 3. Automatic interaction detection revealing factors discriminating Montagu’s Harrier (MHN) and plots without nests (NN). PRE for the regression tree = 0.614. Imp, impurity; A, stratum A; B, stratum B; SCOP, E. scoparia.
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which are known to be aggressive during the pre-laying period (Garcia & Arroyo 2002). Habitat management and species conservation
In areas where Montagu’s Harriers and Hen Harriers cohabit, management of shrublands and heathlands may be tailored to meet the breeding requirements of one or both species. In order to encourage breeding Montagu’s Harriers, afforestation should be avoided as planted trees may prevent the development of a stratum B suitable for nesting. For instance at Le Pinail, for Montagu’s Harrier, heaths should ideally be cleared before E. scoparia reaches around 1.80 m. In heaths as in shrublands, methods that favour resprouting of a greater number of buds, such as rotovating, may produce better nesting habitat, because they will lead to a dense B stratum (Tucker 2003). In the wet heathland of Le Pinail, eight years of growth are required to produce suitable nesting habitat, therefore heath regeneration should follow a 12-year rotation providing patches of different age-class for Montagu’s Harriers. These patches should also be grouped: as Montagu’s Harriers are semi-colonial birds, nest locations of conspecifics are also of importance in nest selection (Cramp & Simmons 1980, Cornulier & Bretagnolle 2006, Soutullo et al. 2006). Although the factors determining nest selection by Hen Harriers remain unclear, afforestation does not seem to directly benefit this species. It is still not clear which type of vegetation structure is preferred by Hen Harriers, only that HHN are usually found in plots where E. scoparia is taller than 1.87 m. Therefore, it would seem reasonable to maintain some plots with heather taller than 2 m (i.e. older than 12 years). Habitat quality also depends on food resources. Although no studies of prey populations have been carried out at Le Pinail, regular bird surveys and observations have shown that harriers mainly hunt in surrounding agricultural fields. Garcia & Arroyo (2005) studied sympatric Hen and Montagu’s Harriers and found that Hen Harriers spend much more time hunting around the nest than Montagu’s Harriers. This difference in behaviour suggests that the presence of Montagu’s Harriers may affect Hen Harrier breeding by creating competition for food resources in the surrounding heath. In our study, the seven Hen Harrier nests in which breeding failed were located at 232 ± 41 m from a Montagu’s Harrier nest, the mean distance separating two neighbouring nests of sympatrics being © 2008 British Trust for Ornithology, Bird Study,
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323 ± 23 m. Conversely, Montagu’s Harriers are less affected by this type of competition as they hunt mainly at long distances from the nest (Schipper 1978, Salamolar 1997). In our study, we did not find any possible influence of Hen Harriers on Montagu’s Harriers breeding. From this point of view, habitats with suitable vegetation but where hunting areas are distant may be less suitable for Hen Harriers than for Montagu’ Harriers, in particular in years in which prey are scarce. As a consequence, in habitats managed for both species, patches kept for Hen Harriers should not be mixed with those for Montagu’s Harriers and should be located as close as possible to food resources. In landscapes dominated by open fields, the dense vegetation of a natural area such as Le Pinail provides concealment and shelter for nests away from human disturbance and afforestation, especially for Montagu’s Harriers, which are threatened by intensive agriculture (Millon et al. 2004, Cornulier & Bretagnolle 2006). In agricultural areas, restoration or management of patches of natural heath or shrublands may help to protect this endangered species. ACKNOWLEDGEMENTS We are grateful to Dominique Antigny and his colleagues at ONF for giving us useful information and permission to work at Le Pinail. We are also grateful to Beatriz Arroyo and Phil Whitfield whose comments improved an early draft of the manuscript. We thank the Museum National d’Histoire Naturelle (Paris, France), who provided bird rings.
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(MS received 18 October 2006; revised MS accepted 2 July 2007)
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