Field Vole Microtus agrestis abundance and Hen Harrier Circus cyaneus diet and breeding in Scotland

Ibis (2002), 144 (on-line), E33–E38

© 2002 British Ornithologists’ Union

Blackwell Science LtdField Vole Microtus agrestis abundance and Hen Harrier Circus cyaneus diet and breeding in Scotland

STEPHEN M. REDPATH1*, SIMON J. THIRGOOD2† & ROGER CLARKE3

1CEH Banchory, Hill of Brathens, Banchory, Aberdeenshire AB31 4BW, UK 2The Game Conservancy Trust, ICAPB, Edinburgh University, West Mains Road, Edinburgh EH9 3JT, UK

3New Hythe House, Reach, Cambridgeshire CB5 0JQ, UK

In many parts of the global range, voles form an important part of the diet of Hen HarriersCircus cyaneus, and breeding numbers are correlated with the abundance of these smallmammals. In Scotland, however, little information is available on harrier diet in the springand our understanding of causes of variation in harrier breeding density is complicated byhuman interference. In this paper we explore the relationship between Field Vole Microtusagrestis abundance and harrier spring diet, density and productivity in southern Scotland.Over three years, voles occurred on average in 67% of pellets, and 79% in years of high andintermediate vole abundance. From 1992, the number of breeding harriers increased fol-lowing protection from illegal persecution. After accounting for this trend, harrier numberscorrelated strongly with vole abundance. Harrier clutch size was also correlated with voleabundance. Although fledging success tended to be greater in years of vole increase than inyears of vole decline, fledging success was not significantly correlated with the relative abun-dance of voles, or with the abundance of Meadow Pipits or Red Grouse chicks.

What determines the breeding density and produc-tivity of Hen Harriers Circus cyaneus is an importantissue in British conservation (Etheridge et al. 1997,Potts 1998, Green & Etheridge 1999). Hen Harriersare scarce raptors in Britain, yet they are still illegallykilled by gamekeepers (Etheridge et al. 1997, Simet al. in press). The conflict between those who wishto manage Red Grouse Lagopus l. scoticus and thosewho wish to conserve raptors has been highlightedby recent research that has strongly suggested thatHen Harriers breeding at high densities could limitpopulations of grouse at low density and reduceshooting bags (Redpath & Thirgood 1997, 1999,Thirgood et al. 2000a, 2000b). An understanding ofwhat factors are important in determining highdensities of breeding harriers is therefore crucial inhelping resolve this conflict.

In many parts of their range, the diet, density andclutch size of Hen Harriers appears to be determinedby the abundance of voles (Hagen 1969, Hamerstrom1979, Korpimaki 1985, Simmons et al. 1986). Voles

form an important component of the diet in springwhen birds are settling on their breeding territoriesand in the summer when they are feeding chicks (e.g.Simmons et al. 1986, Barnard et al. 1987). In Scot-land, where the majority of British harriers breed(Sim et al. in press), little is known about their dietduring the spring, although in one study volesformed a minor component of winter diet (Marquiss1980). Studies in Scotland have mainly focused onthe nestling period when birds predominate in thediet (Watson 1977, Picozzi 1978, 1980, Redpath1991, Redpath & Thirgood 1999). Yet despite thefact that voles are relatively unimportant as preyduring the summer, we have found evidence thatyear-to-year changes in the breeding density of har-riers were correlated with small mammal abundance,suggesting that voles may be important prey inspring (Redpath & Thirgood 1999).

In this paper we explore in more detail the rela-tionship between harriers and small mammals inScotland and investigate whether Field VolesMicrotus agrestris are important in the diet when har-riers are establishing territories. Specifically, we focuson the relationship between vole abundance andharrier diet in spring, breeding density and breedingsuccess in one study area over eight years.

*Corresponding author. Email: [email protected] †Current address: Centre for Conservation Science, StirlingUniversity, Stirling, FK9 4LA, UK.

E34 S. M. Redpath, S. J. Thirgood & R. Clarke

© 2002 British Ornithologists’ Union, Ibis, 144 (on-line), E33–E38

METHODS

We collected data between 1992 and 1999 on Lang-holm moor in south-west Scotland (see Redpath &Thirgood 1997, 1999 for details of study area andmethods). For the purposes of this paper we onlyinclude data from the core moorland block where allthe harriers bred. We located harrier nests duringspring by watching harriers display during the pre-breeding season and then watching males deliverfood to their females during incubation. We found85 nests at the egg stage and three during the nestlingperiod.

From 1997 male and female harriers started usinga roost during early spring (February onwards) in themiddle of the study area. From sightings of taggedbirds we knew that individuals using this roost sub-sequently bred on our study area. We collected pel-lets from this roost from February to April, 1997–99.These were washed, dried and teased apart undera microscope. Birds were identified mostly fromplumage but bill parts, claws and bones were alsoused when present and diagnostic. Mammals wereidentified from hair, teeth and bones (Clarke et al.1993, Clarke 1999). We present the data as the per-centage of pellets containing a given prey type and asthe number of individual Field Voles (identified fromjaws and teeth) per pellet. Pellets overestimated theimportance of small mammals in the nestling dietof harriers, but percentage frequency of smallmammals in the pellets was correlated strongly withfrequency in prey seen during direct observations(Redpath et al. 2001).

We trapped small mammals each March and Sep-tember on 10 lines of 50 unbaited snap-traps set fortwo nights. We checked traps each morning to recordthe numbers of Field Voles, Wood Mice Apodemussylvaticus, Common Shrews Sorex araneus andPygmy Shrews S. minutus. This gave an index ofsmall mammal abundance on Langholm moor ineach season, expressed as the number caught per100 trap nights (Redpath et al. 1995). We comparedthe numbers and clutch size of breeding harrierswith the relative abundance of small mammals usingthe index based on spring trapping. We compared thefledging success of harriers with the small mammalabundance index in both spring and autumn.

During the nestling period, Meadow Pipits Anthuspratensis and Red Grouse chicks together with smallmammals formed the majority of the prey, so we alsoexamined harrier fledging success in relation to therelative abundance of these two prey types. We

counted Meadow Pipits in 15, 1-km2 moorlandsquares. In each square we walked two parallel,1-km transects between 06:00 and 09:00 h in June(Thirgood et al. 1995). We counted the number ofindividuals within 200 m either side of the transectand expressed this as the number counted perkilometre. Red Grouse broods were located withdogs in early June and we estimated densities (grousechicks per km2) using mean brood size and hen den-sity in July (see Thirgood et al. 2000b).

In 1998 and 1999, we provided some harriers withfood as part of an experiment to reduce predation ongrouse chicks (Redpath et al. in press). We excludeddata on breeding success from the nests with supple-mentary food. We use four breeding parameters:average clutch size, number of young fledged permale, number of young fledged per female, andfledging success measured as percentage of eggs pro-ducing fledged young. In 1992, two harrier nestsfailed as a result of suspected persecution and weretherefore excluded from analyses of breeding success.Statistical analyses were done using Minitab (v11).

RESULTS

Small mammal abundance

There were two peaks in the numbers of Field Volestrapped, one in autumn 1993 and one in autumn1996/97 (Fig. 1). From spring 1992 to autumn1999, we trapped 448 small mammals at Langholm,of which 69% were Field Voles, 18% were CommonShrews, 10% were Wood Mice and 3% were PygmyShrews. Numbers of Field Voles caught (per 100 trapnights) were correlated strongly with the numbersof all small mammals caught (spring: r = 0.93, n = 7,P = 0.001; autumn: r = 0.97, n = 7, P < 0.001).Because of this and because Field Voles were thedominant species caught and identified in pellets(below), we used Field Vole abundance in subse-quent analyses.

Harrier diet in spring

We collected 279 pellets in early spring 1997–99.Field Voles occurred in 67% of these and Shrews in3%. When voles were relatively abundant in 1997they occurred in 79% of harrier pellets, with an aver-age of 1.8 voles per pellet (Table 1). In 1998 whenvole captures were almost a third of those in 1997,they still occurred in 79% of pellets. However, in1999 when few voles were trapped, only 14% of


Field Voles and Hen Harriers E35

pellets contained voles, with an average of 0.2 perpellet. Clearly, voles were an important componentof the Hen Harrier diet during peak and intermedi-ate years of the vole cycle.

Harrier breeding numbers

Harrier breeding numbers have increased since thestart of the study (Fig. 2), possibly due to protection

against persecution suspected prior to the study(Redpath & Thirgood 1997). There was a significantlinear relationship between the numbers of breed-ing harriers and year (females: r = 0.79, n = 8,P = 0.013; males: r = 0.89, n = 8, P = 0.004). Wetherefore incorporated year into a multiple regres-sion model with vole abundance. After controllingfor the effects of year, numbers of breeding harrierswere strongly related to numbers of Field Volestrapped for both sexes of harrier (Fig. 3; females:F1,7 = 11.96, P = 0.018; males: F1,7 = 41.8, P = 0.002).

Harrier breeding success and prey abundance

Harrier clutch size correlated positively with thenumbers of Field Voles trapped in spring (Fig. 4;r = 0.86, n = 7, P = 0.01). However, there were nosignificant relationships between harrier fledgingsuccess and voles trapped in either spring or autumn(fledged young per male: r < 0.29, P > 0.53, fledgedyoung per female: r < 0.35, P > 0.5%, fledging suc-cess: r < 0.44, P > 0.32). There were also no signifi-cant relationships between harrier fledging successand our index of Meadow Pipit abundance (r < 0.49,

0

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s

92

a s

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a s

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a s

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a s

97

a s

98

a s

99

a

Fiel

d v

ole

s p

er 1

00 t

rap

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Figure 1. Numbers of Field Voles caught (± 1 se) at Langholm in spring (h) and autumn (j) from 1992 to 1999.

Table 1. Estimates of importance of Field Voles in Hen Harrier diet, based on the analysis of pellets collected during late winter/earlyspring from 1997 to 1999.

1997 1998 1999

Field Voles caught per 100 trap nights in spring 3.17 1.15 0.20No. pellets collected from communal roost 181 47 49% of pellets containing Field Voles 79 79 14Mean voles per pellet 1.8 1.3 0.2

0

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1992 1993 1994 1995 1996 1997 1998 1999

Bre

edin

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arri

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Figure 2. Numbers of harriers attempting to breed (i.e. lay eggs)on Langholm moor from 1992 to 1999.



n = 7, P > 0.27) or between harrier fledging successand the abundance of grouse chicks (r < 0.62, n = 6,P > 0.19). Over the eight years of the study, therewere two years (1994 & 98) when numbers of volestrapped decreased from spring to summer. In theseyears, fledging success was low (27% & 42%) com-pared to years when voles increased (57.6 ± 7.0%,range 34–75%).

DISCUSSION

Our data confirm the importance of food supply inthe pre-nesting period in influencing the breeding

density and productivity in a range of raptors (e.g.Newton 1979, Korpimaki 1985, Rohner 1996,Steenhof et al. 1997, Salamolard et al. 2000). Thequestion here was whether voles were importantprey for Hen Harriers during the spring and impor-tant drivers of population change in Scotland, asthey are elsewhere. The answer appears to be yes;voles formed a major component of the spring dietduring peak and intermediate years of the vole cycleand vole numbers were correlated strongly withharrier breeding density and clutch size.

The importance of voles as prey during springcontrasts with the summer diet of Hen Harriers. AtLangholm voles formed on average 6% of nestlingprey from 1993 to 1996 and a maximum of 22%of items identified at any one nest (Redpath &Thirgood 1999, Redpath et al. 2001). Similarly, datafrom 11 harrier nests from five studies around Scot-land indicated that small mammals formed only 7%of prey items on average (summarized in Table 4 inRedpath 1991). In continental Europe and NorthAmerica voles are a key element of harrier diet,although in some areas birds may also form animportant component of summer diet (Errington &Breckenridge 1936, Schipper 1973, Barnard et al.1987, Millon et al. in press).

The finding that vole abundance was correlatedstrongly with the breeding density of harriers wasconsistent with the fact that harriers do not breed onthe Scottish islands of Harris, Lewis and Shetlandwhere habitat is apparently suitable but voles areabsent (Gibbons et al. 1993). However, harriers dobreed in Ireland and on Hoy, in Orkney, where voles

–6

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1992 1993 1994 1995 1996 1997 1998 1999

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ce

Figure 3. Number of Field Voles caught per 100 trap nights in spring (dotted line), and residual numbers of harriers (n males; j females)attempting to nest from 1992 to 1999 at Langholm.

4.0

4.5

5.0

5.5

0.0 2.0 4.0

Field Voles caught per 100trap-nights in spring

Mea

n h

arri

er c

lutc

h s

ize

Figure 4. Relationship between harrier clutch size and thenumbers of Field Voles trapped per 100 trap nights in spring.


Field Voles and Hen Harriers E37

are absent. Similarly, even when vole populationscrashed at Langholm, harriers bred at relatively highdensity. A previous analysis found that variation inharrier densities between areas was correlated posit-ively with the relative abundance of Meadow Pipits(Redpath & Thirgood 1999). This implied that mini-mum densities of breeding harriers might be deter-mined by pipit abundance, but year-to-year changesby variation in vole abundance.

Vole abundance was correlated positively withharrier clutch size, as found for harriers elsewhere(e.g. Simmons et al. 1986). Our data indicated thatfledging success was also higher in years whennumbers of voles trapped increased from springto autumn than in years when numbers of volestrapped decreased. However, we found no signifi-cant correlation between fledging success and volesin spring or autumn. Schipper (1978) found thatHen Harrier fledging success correlated with voleabundance in the Netherlands. In contrast, fledgingsuccess and vole abundance were not correlated inNorth America and this was explained by the factthat harriers switched to eating more birds after theirchicks hatched (Simmons et al. 1986, Barnard et al.1987). In our study area, Meadow Pipits and RedGrouse chicks formed over half the nestling prey bynumber (Redpath & Thirgood 1999) and a relation-ship between numbers of these avian prey and thefledging success of harriers might have beenexpected. However, although these individual rela-tionships were positive they were not statisticallysignificant. A combination of all three prey types isprobably important in determining fledging successof harriers. Female age and status probably also influ-ence fledging success (Simmons et al. 1986), butsample sizes were too small to examine the possibleconfounding effects of these factors.

Food is not the only factor limiting the breedingdensity of harriers in Scotland. Harriers are largelyrestricted to nesting in tall heather Calluna vulgaris,so availability of this habitat will provide some limitto density (Redpath et al. 1998). However, evenwhere prey and habitat are available, gamekeepersmay limit harrier density (Etheridge et al. 1997). RedFoxes Vulpes vulpes also kill harriers but evidence isconflicting as to whether they limit harrier density(Potts 1998, Green & Etheridge 1999).

Our findings presented here and elsewhere(Redpath & Thirgood 1999) suggest that on heathermoorland, where persecution is absent, the highestdensities of harriers will occur in areas where volesand Meadow Pipits are abundant. Given that vole

populations tend to show cyclical dynamics, it ispossible that vole and grouse cycles could becomesynchronous through predation by harriers. Similarexplanations have been proposed for cycles of game-birds elsewhere (e.g. Angelstam et al. 1984, Smallet al. 1993). The likelihood of the population dynamicsof voles, harriers and grouse becoming synchronouscould be ascertained through testing the predictionsof modelling with further monitoring of harrierpopulations on grouse moors in the absence ofpersecution.

We thank Buccleuch Estates for allowing the work to bedone at Langholm. We are particularly grateful to themany assistants and students who have helped with datacollection. Beatriz Arroyo, Mick Marquiss, Steve Albon,Francois Mougeot, Rob Simmons & Karen Steenhof pro-vided helpful comments on the manuscript. The study wasfunded by Buccleuch Estates, Westerhall Estates, CEH,GCT and Scottish Research Trust, JNCC, RSPB & SNH.

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Received 10 August 2000, revised manuscript accepted 17 September 2001

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Field Vole Microtus agrestis abundance and Hen Harrier Circus cyaneus diet and breeding in Scotland