Could translocation aid hen harrier conservation in the UK?

INTRODUCTION

Translocation, defined as the deliberate or mediatedmovement of wild individuals or populations from onepart of their range to another, is increasingly used in con-servation (Griffith et al., 1989; Cade & Temple, 1995;Wolf et al., 1996). Translocation is most frequently usedto re-establish extirpated populations or to augmentsmall populations of endangered species (Kleiman,Stanley Price & Beck, 1994; Hodder & Bullock, 1997;Millar et al., 1999). It has been successfully used torestore threatened populations of raptors such as theperegrine falcon (Falco peregrinus), the Mauritiuskestrel (Falco punctatus) and the red kite (Milvusmilvus) (see Cade, 2000, for a recent review).Translocation has also been used, with varying degreesof success, to remove individual ‘problem-animals’ fromareas of human–wildlife conflict (Linnell et al., 1997;Fischer & Lindenmayer, 2000). Intervention to removeindividual animals typically involves large carnivoresbut there have also been a number of cases in which rap-tors have been translocated in response to predation of

livestock or game (Matchett & O’Gara, 1987; Kenward,Marcstrom & Karlbom, 1991; Phillips, Cummings &Berry, 1991; Waite & Phillips, 1994). Two factors com-bine to focus attention on restoring raptor populationsthrough translocation. First, human–raptor conflicts haveresulted in widespread persecution and populationdecline; and second, techniques developed in falconryfor handling and release have been readily transferableto conservation (Cade, 2000).

One of the more contentious conservation issues inthe UK concerns the relationship between hen harriers(Circus cyaneus) and red grouse (Lagopus lagopus scoti-cus) (Thirgood et al., 2000a). Harriers are generalistpredators that can breed at high density on moorlandmanaged for grouse shooting (Redpath & Thirgood,1999). Predation by harriers can limit grouse populationsat low density and reduce shooting bags (Thirgood etal., 2000b,c). With predation reducing grouse densitybelow the level required for driven shooting, the incen-tive to maintain and improve heather moorland isreduced and the land is vulnerable to conversion toforestry or sheep farming, with the resultant loss of bio-diversity (Thirgood et al., 2000a). Currently, however,illegal persecution on grouse moors restricts hen harrierabundance and distribution such that the species is

Animal Conservation (2001) 4, 37–43 © 2001 The Zoological Society of London Printed in the United Kingdom

Mark Watson1 and Simon Thirgood2

1Game Conservancy Trust, Fordingbridge, Hampshire, SP6 1EF, UK2Game Conservancy Trust, ICAPB, University of Edinburgh, EH9 3JT, UK

(Received 28 February 2000; accepted 8 August 2000)

AbstractTranslocation is increasingly used in conservation to re-establish or augment populations of threat-ened species or to remove individual animals from areas of human–wildlife conflict. We assess thefeasibility and utility of translocating hen harriers (Circus cyaneus) in the UK to enhance their dis-tribution and abundance whilst simultaneously reducing the impact of harrier predation on red grouse(Lagopus lagopus scoticus) populations and shooting bags. Current knowledge of hen harrier feedingecology, dispersal, survival and recruitment suggests that they would be suitable subjects for translo-cation with the aim of increasing their distribution in the UK. Assessment of habitat and food avail-ability suggest that there are suitable recipient sites beyond the current range of the hen harrier in theUK. However, translocation would not be a sustainable method of reducing predation on grouse moorsbecause it would have to continue indefinitely as long as grouse moors attracted harriers. Translocationof harriers to grouse moors where they have been locally extirpated would not be appropriate untillevels of illegal control are reduced. Establishing new harrier populations through translocation awayfrom grouse moors may become desirable if initiatives to reduce human–raptor conflicts on grousemoors are unsuccessful, or as an interim measure to accelerate the recovery of hen harriers in theUK.

All correspondence to: S. Thirgood. E-mail: simon.thirgood@ed.ac.uk.

absent from much suitable habitat (Gibbons, Reid &Chapman, 1993; Etheridge, Summers & Green, 1997).

The conflict between harrier conservation and grousemanagement presents a dilemma for conservation agen-cies and wildlife managers that is unlikely to be resolvedthrough enforcement of wildlife legislation alone. Whilstmost conservation and game management interests agreeon the nature of the conflict, there is less agreement onits resolution. A number of potential solutions have beensuggested ranging from habitat management to reduceharrier carrying capacity to active intervention to locallyreduce harrier numbers (Thirgood et al., 2000a).Translocation of harriers from grouse moors where theyare abundant to other areas where they have been extir-pated has been put forward as a solution (Potts, 1998).Translocation is appealing because it could potentiallyenhance the conservation status of harriers by extendingtheir breeding range whilst simultaneously reducing theimpact of harrier predation on grouse moors. Under thesecircumstances, harrier translocation would be of widerinterest, as there are few examples of translocation pro-grammes that successfully combine reintroduction withreducing human–wildlife conflicts (Linnell et al., 1997).In this paper we examine the feasibility and utility oftranslocation of hen harriers in the UK. We assesswhether harriers could be translocated from high den-sity populations to areas where they formerly bred butare currently absent and, if so, whether translocationcould help to reduce harrier conflicts with grouse man-agement.

LEGAL CONSIDERATIONS FORTRANSLOCATION

Whilst the focus of this paper is biological, it is neces-sary briefly to consider the legal context. Hen harriersare rare birds in Britain with numbers in 1994 estimatedat 686 (± 150) breeding females (Stone et al., 1997).However, there are approximately 27 000 breedingfemales in the Western Palearctic (Snow & Perrins,1998). The European Community Birds Directive(79/402/EEC) places obligations on member states to‘establish a general system of protection of all speciesof birds’ (Article 5). For rare or vulnerable species,including hen harriers, it states that ‘they shall be thesubject of special conservation measures concerningtheir habitat in order to ensure their survival and repro-duction in their area of distribution’ (Article 4). TheBirds Directive has been implemented in the UK by theWildlife and Countryside Act 1981 and the Wildlife(Northern Ireland) Order 1985. Management of hen har-riers through translocation would deviate from orthodoxprotection under these laws and require special exemp-tion. Provision is made for this through derogation underlicence. Under Article 9 of the Birds Directive, MemberStates may derogate ‘where there is no other satisfac-tory solution . . . to prevent serious damage to crops,livestock, forests, fisheries and water . . . for the protec-tion of flora and fauna’. The species associated withgrouse moors, including red grouse, fall into the last cat-

egory, and heather moorlands receive further protectionthrough the European Community Habitats Directive(92/43/EEC). If management for grouse shooting is the best option for conserving heather moorland and theassociated flora and fauna, then there is a case for dero-gation to manage harrier predation for the protection of flora and fauna. Moreover, Article 9 further allowsderogation ‘for the purposes . . . of re-population . . . ofreintroduction and for the breeding necessary for thesepurposes’. Thus it appears that there are grounds forderogation on wider conservation issues as well as toenhance hen harrier status.

BIOLOGICAL CONSIDERATIONS FORTRANSLOCATION

Feeding ecology and population fluctuations

Hen harriers are highly mobile predators adapted to habi-tats with fluctuating resources.

A number of studies in Europe and North Americahave demonstrated that where cyclic populations ofmicrotine rodents are the main prey, harrier breedingdensities tend to fluctuate with the abundance of the microtines (Hagen, 1969; Hamerström, 1979;Korpimäki, 1985; Simmons et al., 1986; Salamolard etal., 2000). The inference is that if harrier densities nat-urally fluctuate it would be difficult to use translocationto establish new populations and also that managing har-rier numbers and predation on grouse would also beproblematic. In many areas of Europe, however, harri-ers have a diversified diet and generally stable numbers.Such is the case, for example, of both hen harriers andMontagu’s harriers (Circus pygargus) in central Spainand eastern France (Arroyo, 1997, 1998; Millon et al.,2000). A number of studies in Scotland have shown thatbirds such as the meadow pipit (Anthus pratensis) andred grouse tend to dominate the summer diet of hen har-riers with field voles (Microtus agrestis) forming only asmall proportion of the prey (Watson, 1977; Picozzi,1978, 1980; Redpath, 1991; Redpath & Thirgood, 1999).As a result, breeding densities of hen harriers tend tofluctuate with a lower amplitude than in areas wheremicrotines are the main prey (Watson, 1977; Picozzi,1978, 1984a; Redpath & Thirgood, 1999). The greaterstability in breeding density of hen harriers in the UKhas positive implications for the success of translocationin establishing new breeding populations. Furthermore,strong numerical responses to microtine fluctuations donot necessarily mean that harriers are nomadic, rather itimplies that a proportion of the population may not breedin certain years. In a conservation context this impliesthat translocation might only be conducted in certainyears according to food abundance.

Dispersal and recruitment

Most breeding bird populations contain individuals thatwere recruited from within the population as well asindividuals that were recruited from elsewhere. Research

38 M. WATSON & S. THIRGOOD

on an individually marked population of hen harriers insouthern Scotland during 1992–1996 demonstrated that25% of males and 36% of females that attempted tobreed were local recruits (Redpath & Thirgood, 1997).These data suggest that enough birds of both sexes arelikely to return to the natal area to establish a permanentbreeding population. Conversely, the data also imply thatan established breeding group would attract recruits fromsources other than the released birds.

Research in mainland Scotland during 1988–1995demonstrated considerable variation in the natal disper-sal of male and female harriers based on the land classof their natal origin (Etheridge et al., 1997). Male andfemale harriers that hatched on grouse moors tended tobreed on grouse moors whilst harriers of both sex thathatched on unmanaged moors or young forestry planta-tions tended to disperse to breed on grouse moors. Thisresulted in a net movement of harriers to grouse moors,compensating locally for the level of illegal control inthis land class. Studies of habitat selection by Etheridgeet al. (1997) pointed to differences in abundance ofpasserines, lagomorphs and grouse between grousemoors and the other land classes as an important factorinfluencing the net immigration of birds to grouse moors.Harrier clutch size and mean size of successful broodswere both larger on grouse moors than on other moorsand in conifer forests, providing further circumstantialevidence of differences in habitat quality.

An important conclusion of Etheridge et al. (1997)was that surviving female harriers usually nested in thesame area in successive years with the median distancemoved between breeding sites being less than 1 km.Etheridge et al. (1997) found no difference in distancemoved in successive years between successful femalebreeders and unsuccessful females. In contrast, earlierwork by Picozzi (1984a,b) demonstrated that femaleharriers that moved to a new territory moved further fol-lowing failure than after successful breeding. Theseobservations suggest that, undisturbed by illegal control,breeding hen harriers would be relatively sedentary, butthat translocation over long distances would be mostappropriate to establish new populations, since short dis-tances would be within the relatively localized move-ments between existing populations.

Response to breeding failure and timing ofintervention

Hen harriers are likely to respond to the removal of either eggs or broods for translocation as they would toa breeding failure. Early failures, at the start of incuba-tion after minimal parental investment, are more likelyto result in the laying of repeat clutches than later failures(Simmons, 1984; Etheridge et al., 1997). These studieson harriers are consistent with extensive research onclutch replacement in other raptors (Morrison & Walton,1980). A short period of captive rearing is a desirableprecursor to release during translocation becauseimproved body condition of juvenile raptors results inincreased survival (Meyers & Millar, 1992). This factor

argues for taking harrier chicks at about the mid-pointof the nestling stage. Optimum timing of intervention inany translocation programme is largely determined bythe aims of the programme (Cade & Temple, 1995).Translocation of hen harriers in the current context isrelatively novel in that it aims to reduce their predationimpact whilst simultaneously enhancing their range andstatus. If the aims of harrier translocation were restrictedto enhancing range and status, gathering reintroductionstock early during incubation could stimulate the layingof a second clutch and have little effect on local popu-lation size. Such ‘double clutching’ has been used inbreeding programmes for peregrine falcon, prairie fal-con (Falco mexicanus), Mauritius kestrel and bald eagle(Halioetus leucocephalus) (Cade & Temple, 1995).

Size, number and composition of release cohorts

Griffith et al. (1989) and Wolf et al. (1996) analysedavian and mammalian reintroduction success based onsize and number of release cohorts. They found thatreleases of more than 80–120 individuals did little toincrease the success of reintroduction, and that a typicaltranslocation consisted of releases over 3 years, but thatlonger programmes were more successful. Picozzi(1984b) estimated hen harrier survival (0–2 years) as14% for males and 29% for females in his Orkney study.If this difference in survival between the sexes holds forreleased birds during translocation, it suggests that thecomposition of the release cohorts should be biasedtowards males, although the polygynous breeding ofsome hen harrier populations suggests the opposite. It isnot known if harriers would be polygynous in a new site,but assuming it to be high-quality habitat, it is plausiblethat this would be the case. A programme releasingcohorts of 20–30 birds over 4–5 years is likely to fulfilthese criteria and produce a viable population. It wouldbe useful to see the results of demographic simulationson the relative success of different release programmes.A similar approach has, for example, been used to assessthe viability of reintroduced populations of bearded vul-tures (Gypaetus barbatus) and white-tailed eagles(Haleaeetus albicilla) (Bustamente, 1996; Green,Pienkowski & Love, 1996).

Hacking and captive rearing

The falconers’ technique of hacking has been widelyemployed to release wild-caught and captive-reared rap-tors in translocation programmes (Cade & Temple,1995). Hacking refers to the technique of releasing birdsto the wild by gradually reducing the amount of food provided until they are sufficiently independent tolook after themselves. Studies of Montagu’s harrier in Spain (Pomarol, 1994) and France (Amar, Arroyo &Bretagnolle, 2000; Arroyo & Bretagnolle, 2000) haveshown how hacking can be applied to harrier conflictswith agriculture. Whilst published data refer only toresults for Montagu’s harrier, the programme run by theCentre d’Etudes Biologiques de Chizé in western France

39Hen harrier translocation

also includes the captive rearing and release of hen har-riers (B. Arroyo, pers. comm.). In the Chizé study, post-release monitoring demonstrated that captive-rearedMontagu’s harriers behaved normally compared to wildfledglings observed simultaneously (Amar et al., 2000).

Survival and philopatry in captive reared and wildharriers

Amar et al. (2000) compared re-sighting rates of cap-tive-reared and wild-reared Montagu’s harriers over 2 years in Chizé in western France. Re-sighting ratesaround the release site or natal nest were 14–19% forcaptive birds and 9–10% for wild birds. This differencewas related to the better body condition of the captive-reared birds suggesting either that they had better post-fledging survival or that they were more stronglyphilopatric. These data compare with re-sighting rates of7% for wild populations in Spain (M. Pomarol, pers.comm.). Estimates of yearling survival and philopatry inhen harriers vary but appear to be higher than inMontagu’s harriers. Female survival to 1 year in main-land Scotland was estimated at 36% (Etheridge et al.,1997) and survival to 2 years on Orkney was estimatedat 29% (Picozzi, 1984b). Wing-tagging studies in south-ern Scotland indicated that the return rates of femaleswere as high as 50%, with four out of eight femalenestlings returning to breed in 1 year, although this mayhave been exceptional as this was a rapidly expandingpopulation (Redpath & Thirgood, 1997). Hen harriersthus appear more philopatric than Montagu’s harriers,suggesting that the protocols from the Chizé projectcould be employed with higher probabilities of successfor hen harriers. It is important to emphasize that theChizé project is not a reintroduction. The purpose oftaking harriers into captivity is to protect them from har-vesting activities and birds are released in the middle oftheir natal area close to communal roosts and otherbreeding adults (Arroyo & Bretagnolle, 2000). TheChizé project has shown, however, that both Montagu’sharriers and hen harriers can be captive-reared andhacked back to the wild with a proportion of releasedbirds returning to the vicinity of the hack site in subse-quent years.

SELECTION OF RECIPIENT SITES

IUCN guidelines for translocation state that recipientsites for translocated individuals must be within thespecies’ historical range, must contain sufficient extentof suitable habitat and have assured long-term protec-tion (IUCN, 1995). Evaluation of potential sites for har-riers should therefore include assessment of the extentof habitat change since extirpation, as well as the impli-cations of any legal, political or cultural change. An ini-tial assessment of potential sites for harrier translocationindicates that there are three categories of potential sites:(1) managed grouse moors where there are currently noharriers such as the North Pennine hills in northernEngland; (2) unmanaged moors where there are currently

no harriers such as Dartmoor and Exmoor in south-westEngland; (3) lowland heaths such as Breckland andSalisbury Plain in southern England. In each of thesecases there is good historical evidence of breeding byhen harriers (Holloway, 1996). Most grouse moors innorthern England are within 200 km of current henharrier breeding populations in Scotland whereas moor-lands and heathlands in southern England are more than500 km from Scotland, although small harrier popula-tions do occur in northern England and Wales.

Within these divisions there are regional considera-tions. Some managed grouse moors in northern Englandare almost certainly affected by illegal control of rap-tors. A study of 128 harrier breeding attempts in north-ern England from 1994 to 1997 (Stott, 1998) showedsimilar differences in breeding success between man-aged and unmanaged moorland as in Scotland (Etheridgeet al., 1997) Therefore, it is likely that there is muchmoorland in this region that is presently unoccupied, butwhich constitutes suitable hen harrier breeding habitat.There may be protected areas or other sites free fromillegal control within this region that could provide goodrecipient sites. The following assessment therefore con-centrates on recipient sites where habitat suitability isuncertain.

Habitat availability

Good comparative data are not available to determinehow much habitat remains suitable for breeding hen har-riers in Britain. However, there is no doubt that harrierhabitat has declined in quality and extent particularly inthe south of England. For example, 75% of heathland inthe county of Dorset has disappeared since 1900(Woodrow et al., 1996). Moreover, socio-economicchanges have resulted in the increased recreational useof lowland heaths, suggesting that even if habitat weresuitable, these areas may be too heavily disturbed forground-nesting raptors. For example, Montagu’s harri-ers breed successfully in the New Forest in southernEngland only with permanent site protection (M. Noble,pers. comm.).

Nest site selection

Harrier nest site selection is believed to be a compro-mise between concealment from predators, optimizationof the thermal environment and proximity to foraginghabitat (Simmons & Smith, 1985; Redpath et al., 1998).Harriers selectively hunt along habitat edges in opencountry, thus it is important that the structure of vege-tation is suitable for hunting as well as nesting (Schipper,1977; Redpath, 1992). In areas of lowland heath wheresuccession may have resulted in habitat becomingunsuitable for harriers, appropriate grazing may reversethis process. Conversely, where overgrazing hasdestroyed areas of rank heather, exclusion of grazinganimals may allow enough growth to ensure that nest-ing habitat is available. Bullock & Pakeman (1997)noted that the purpose of lowland heath management in

40 M. WATSON & S. THIRGOOD

the UK is almost always nature conservation. Therefore,translocation of harriers could be linked to ongoingheathland projects and result in the integration of a vari-ety of biodiversity initiatives.

Food availability

We adopted a quantitative approach to assess food avail-ability in potential recipient sites. Ideally this would bedone for both passerines and microtines but data wereonly available at a suitable scale for passerines from theBritish Trust for Ornithology’s Breeding Bird Survey(BBS). Potential recipient sites were chosen on the basisof vegetation characteristics, degree of protection andlevel of human disturbance. Meadow pipit and skylark(Alauda arvensis) data were extracted from the BBS, asthese are the main passerine species in hen harrier diet and BBS abundance estimates are comparable tothose available from published research at Langholm insouthern Scotland and ‘Moor C’ in north-east Scotland(Redpath & Thirgood, 1997). Langholm supported ahigh density of harriers whilst ‘Moor C’ supported a lowdensity of harriers during the years of study (1992–1997). The BBS covered ten sites representative of thehabitat under assessment. As multiple counts were avail-able for Langholm and ‘Moor C’ for 1992–1997, butonly single counts were available for BBS sites for mostof these years, the least squares means were derived froman ANOVA model to give a comparison of sites con-trolling for year effects (Fig. 1). This suggests that thereare many sites across the UK that hold densities ofpasserines sufficient to support breeding hen harriers.

Human disturbance

Many lowland heaths in Britain suffer from recreationalpressure and it would be important to select recipientsites that were protected from this in some way. A studyof marsh harriers (Circus aeruginosus) in Spain demon-strated that human disturbance may lower provisioningrates, leading to reduced fledgling survival in additionto causing outright breeding failures (Fernandez &Azkona, 1993). Private land, nature reserves or militarytraining areas offer restricted access that could benefitground nesting birds.

Conflicts with other species

IUCN (1995) guidelines state that studies should evalu-ate the effect that translocations may have on recipientecosystems. In the case of generalist predators such ashen harriers, assessment of these effects should focus on potential prey species of both conservation andeconomic significance. Skylarks would be importantprey on lowland heaths and this species has declined by60% since 1970 (Chamberlain & Crick, 1999). Clarke,Combridge & Combridge, (1997) demonstrated thatDartford warblers (Sylvia undata), a Red Data Bookspecies with a range restricted to lowland heaths, con-stituted up to 29% of prey items in the diet of hen har-riers wintering in parts of southern England. If henharrier translocations were to be conducted, predationimpact on threatened species would need to be moni-tored. Translocation of harriers to lowland heaths mightalso bring conflicts with game shooting interests onnearby farmland, although the extent and severity of con-flict remain unknown.

HUMAN–RAPTOR CONFLICTS ANDTRANSLOCATION

Current knowledge of the feeding ecology, dispersal,survival and recruitment of hen harriers suggests thatthey would be suitable subjects for translocation toincrease their breeding range in the UK. Breeding andforaging habitat and the other factors necessary for suc-cessful translocation are likely to be fulfilled at manysites, both in the lowlands and in the uplands beyondthe present breeding range. Prey abundance and theabsence of illegal control are the most important factorsinfluencing recipient site suitability.

Whilst it appears that translocation could be used toenhance the status and range of hen harriers in Britain,could it also reduce conflicts between harriers and grousemanagement? Translocation would be an imprecise andtemporary method of reducing harrier numbers and pre-dation on grouse moors because of the continual flow ofpotential recruits to grouse moors. Translocation wouldhave to continue indefinitely as long as grouse moorsattracted immigrant harriers. This is clearly logisticallyimpractical quite apart from there being a finite number of recipient sites. It thus appears unlikely thattranslocation could play a major role in reducing

41Hen harrier translocation

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Fig. 1. Comparison of indices of abundance of meadow pip-its and skylarks from 12 moorland and heathland sites in theUK. Data presented as least squares means plus one standarderror after adjusting for year effects. Data for Langholm andMoor C from Redpath & Thirgood (1997). Data for remain-ing sites from British Trust for Ornithology Breeding BirdSurvey.

human–raptor conflicts on grouse moors within thecurrent range of hen harriers in the UK. However,translocation could have an important positive effect inimproving the relationship between conservation andgrouse management interests and demonstrating the ben-efits of co-operation between stakeholders (Decker &Chase, 1997).

Could translocation be used to restore hen harriers tosuitable habitat in northern England where they havebeen locally extirpated by illegal control? Internationalguidelines for translocation state that there should be aclear understanding of why a species was lost to an areaand that the factors causing local extirpation have beenrectified (IUCN, 1995). In the current context it wouldbe necessary to first reduce the levels of illegal controlbefore attempting translocation of harriers to northernEngland. If illegal control of harriers ceased on grousemoors the goal of translocation in increasing harrierdistribution would almost certainly be achieved unaided through natural dispersal in large parts of theirpotential range. In this context, therefore, translocationwould be an unnecessary and inappropriate conservationtool.

Translocation of hen harriers to areas of suitable habi-tat remote from existing harrier populations, and remotefrom grouse moors where they would be illegally per-secuted, could however, enhance the security of thespecies in the UK. Such areas are concentrated in southand south-western England and there are historicalrecords of harriers breeding in these areas. Translocationof harriers in this context may become desirable in thefuture if no solution can be found to existing human–raptor conflicts on grouse moors, if the methods appliedon grouse moors are not effective enough to allow pop-ulation recovery and extensive natural re-colonization,or as an interim measure to accelerate the recovery ofhen harriers in the UK. Translocation of harriers to theseareas would have the additional benefit of improvingrelationships between stakeholders involved in the con-flict on grouse moors, as noted above. Balanced againstthese positive aspects, however, are the considerablefinancial costs and logistical difficulties that such atranslocation programme would entail.

CONCLUSIONS

In conclusion, whilst current knowledge indicates thatan appropriate translocation programme might result inthe establishment of a viable hen harrier population, itseems unlikely that translocation could play a major rolein reducing conflicts between harrier conservation andgrouse management. The resolution of theharrier–grouse conflict probably lies in the integration ofa number of different approaches, including habitat man-agement, diversionary feeding and direct intervention tolocally limit harrier numbers, although this latterapproach would require a change in the philosophy ofconservationists (Thirgood et al., 2000a). The hen har-rier case study should be seen as a cautionary demon-stration that translocation may not be a universal panacea

to increase numbers of a threatened species or to reducehuman–wildlife conflicts.

Acknowledgements

We thank N. J. Aebischer, I. Newton, G. R. Potts, S. M.Redpath and two anonymous referees for helpful com-ments and B. E. Arroyo and V. Bretagnolle for adviceand hospitality in Chize.

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