Does supplementary feeding reduce predation of red grouse by hen harriers?

Journal of Applied Ecology

2001

38

, 1157–1168

© 2001 British Ecological Society

Blackwell Science LtdOxford, UKJPEJournal of Applied Ecology0021-8901British Ecological Society, 2001December 2001386000000

PRIORITY CONTRIBUTION

Supplementary feeding of hen harriersS.M. Redpath, S.J. Thirgood & F.M. Leckie

Does supplementary feeding reduce predation of red grouse by hen harriers?

STEPHEN M. REDPATH*, SIMON J. THIRGOOD† and FIONA M. LECKIE*

*

CEH Banchory, Hill of Brathens, Banchory, Aberdeenshire AB31 4BW, UK; and

†

Game Conservancy Trust, ICAPB, University of Edinburgh, West Mains Road, Edinburgh EH9 3JT, UK

Summary

1.

Hen harriers

Circus cyaneus

can reduce the numbers of red grouse

Lagopus lagopusscoticus

available for shooting. We conducted a supplementary feeding experiment onLangholm Moor, UK, in 1998 and 1999 to determine whether feeding hen harriers couldreduce the numbers of red grouse killed. The experiment was done at two distinct stages ofthe breeding cycle: prior to incubation (spring experiment) and after hatching (summerexperiment). In spring, Langholm Moor was divided into two areas, one with food andone without. In summer a number of birds were provided with food in both areas.

2.

Providing harriers with food in spring had no significant effect on the breeding densityof males or females, although feeding was associated with an increase in density on onearea in one year. In addition, over the 2 years of the experiment, there was no evidencethat feeding led to more chicks returning to breed in subsequent years. Fed harriers hadlarger clutches but did not lay earlier than unfed birds.

3.

A minimum of 78% of the radio-tagged grouse that were killed during spring werekilled by raptors. The mortality rates of adult grouse did not differ between the twoareas or between the two years despite the availability of supplementary food and thelarge differences in harrier breeding density between areas. We infer that other raptorswere responsible for much of the predation of adult grouse.

4.

During the nestling period, female harriers took supplementary food at a higher ratethan males. Females that were fed during the spring took more supplementary food insummer than those fed only during summer. Fed birds did not deliver more food overallto nests than those not provided with food.

5.

Both male and female harriers at nests where supplementary food was availablecaught grouse chicks at a lower rate than harriers at nests not provided with food. Forboth years combined, fed harriers delivered on average 0·5 grouse chicks to their nestsper 100 h, compared with 3·7 grouse chicks delivered to nests without supplementary food.

6.

We estimated that feeding all harriers at Langholm would cost approximately£11 000 per annum. In both 1998 and 1999, the numbers of grouse chicks lost were 10times higher than expected from harrier predation rates. Some other, unknown, factorhad a strong influence on grouse chick survival in these years. Feeding some of thebreeding harriers did not lead to an increase in grouse density at Langholm.

7.

The results suggest that supplementary feeding may provide a useful tool in reducingthe number of grouse chicks taken by harriers. Further experiments are now necessary tosee under what conditions this reduced predation will lead to increases in grouse density.

Key-words

:

Circus cyaneus

, gamebirds, heather moorland,

Lagopus l. scoticus

, raptors,wildlife management.


(2001)

38

, 1157–1168

Correspondence: Steve Redpath, CEH Banchory, Hill of Brathens, Banchory, Aberdeenshire AB31 4BW, UK (fax + 441330 823 303;e-mail [email protected]).†Present address: Centre for Conservation Science, Stirling University, Stirling FK9 4LA, UK.

JPE_683.fm Page 1157 Monday, November 19, 2001 4:47 PM

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S.M. Redpath, S.J. Thirgood & F.M. Leckie

© 2001 British Ecological Society,


,

38

,1157–1168

Introduction

Heather moorland is an internationally importanthabitat and its maintenance is viewed as a conserva-tion priority (Anonymous 1995; Thompson

et al

. 1995).Within this habitat red grouse

Lagopus lagopus scoticus

Lath. management represents an important form ofland use, the aim of which is to maximize the numberof grouse available for shooting, through heatherburning and the control of parasites and predators(Hudson 1992; Hudson & Newborn 1995). As part ofthe predator control, raptors have traditionally beenkilled, as have red foxes

Vulpes vulpes

L. and cor-vids. Despite the fact that raptors have received fulllegal protection since 1954, their persecution on grousemoors is still widespread (Etheridge, Summers & Green1997; Scottish Raptor Study Groups 1997; Green &Etheridge 1999).

Recent research strongly suggested that preda-tion during spring and summer by high densities ofgeneralist predators, particularly hen harriers

Circuscyaneus

L., was able to limit grouse populations atlow density and to reduce shooting bags (Redpath &Thirgood 1997; Redpath & Thirgood 1999; Thirgood

et al

. 2000a,c). These findings have highlighted theconflict between those who wish to manage grouseand those who wish to conserve raptors (Thirgood

et al

. 2000b). One possible short-term solution to thisconflict is to provide harriers with supplementaryfood throughout the breeding season, thus reducingpredation rates on grouse. The effects of feeding henharriers have not previously been investigated, althoughSimmons (1994) showed that African marsh harriers

Circus ranivorus

Daudin took supplementary food inspring.

In many raptors, breeding density is correlated withfood abundance (Newton 1998). Such correlations existfor harriers in Scotland and elsewhere (Hamerstrom1979; Korpimaki 1985; Redpath & Thirgood 1999;Redpath, Thirgood & Clarke 2002). It is possible,therefore, that additional food for harriers may increasetheir breeding density, thus reducing the possiblebenefits of any reduced predation on grouse chicks.In addition, supplementary feeding in other raptorstends to advance laying, increase clutch size (Dijkstra

et al

. 1980; Newton & Marquiss 1981; Aparicio 1984;Korpimaki 1987; Korpimaki & Wiehn 1998) and im-prove fledging success (Korpimaki 1987; Gehlbach& Roberts 1997; Wiehn & Korpimaki 1997). Wetherefore predicted that harriers provided withsupplementary food would show improved breedingperformance.

We examined the effectiveness of supplementaryfeeding as a management tool in reducing predation byhen harriers on adult grouse during the spring and ongrouse chicks during the summer. We examined theeffect of feeding on harrier breeding density and suc-cess, and we considered the financial costs and benefitsof such a feeding programme.

Methods

This study was based on Langholm Moor, located at55

°

10

′

N and 2

°

55

′

S, in south-west Scotland (Redpath& Thirgood 1997, 1999). The feeding experiment wasconducted in 1998 and 1999, but data from years before(1992–97) and after (2000) the experiment were alsoincluded. Throughout the paper we have divided thestudy into spring and summer sections. Spring refers tothe period between harriers returning to their breedingsites up to clutch completion (March–April), and sum-mer refers to the period between hatching and chickdispersal (May–August).

On the basis of the distribution of heather-dominantvegetation (Thirgood

et al

. 2000a) and harrier nests in1997, Langholm Moor was divided into two areas, Aand B (Fig. 1). During spring, harriers were providedwith food (treatment) in area A in 1998 and in area B in1999. Harriers on the other area were not fed (control).No food was provided in 2000. Males were locatedas they returned to their breeding sites in March andfeeding perches were erected in their territories in theareas where males were displaying and prospectingfor nests. Perches consisted of standard 1·5-m high10-cm diameter fence posts with a 30-cm section ofpost nailed across the top, to form a T-shape.

Supplementary food, primarily dead white rats andday-old poultry chicks, was placed daily on the perchesfrom late March until the start of incubation. Any foodremaining the next day was removed and disposed of.Because feeding attracted potential harrier egg pre-dators (see later) we did not continue feeding duringthe incubation period. We noted the number of malesestablishing territories on each area in spring, definedas birds consistently seen displaying and defending onearea of moorland. We also recorded the number of femalesbreeding (i.e. laying at least one egg) with each male.Despite the fact that juvenile (brown) males were oftenseen displaying in the spring, there was only one instanceof a juvenile male breeding. The remaining non-breedingjuvenile males were excluded from all analyses. In 1998–2000 some adult (grey) males held territories throughoutspring and summer but failed to breed. As some of thesemales were provided with food during 1998 and 1999,these birds have been included in analyses of the effectof feeding on the numbers of females breeding per male.

Nests were generally found during egg laying or earlyincubation, and both laying date (first egg) and clutchsize were recorded. Where possible female harrierswere aged as first year birds or older, based on wing-tags (colour-coded tags had been fitted to nestlings inprevious years) or eye colour (first year birds havebrown eyes). Harrier breeding status was recorded asmonogamous or polygynous (two or more females)for males, and as monogamous and either primaryor secondary for females. Polygynous females were


1159

Supplementary feeding of hen harriers



,

38

,1157–1168

classified according to laying date, with the earliestlaying female being considered the primary female.In 1998 there was one tertiary female classified foranalyses of breeding success as a secondary female.

The abundance of small mammals and adult grousewas estimated in all years using standard techniques.Small mammal abundance was estimated from 10 linesof 50 snap traps set for two nights in March, and grousewere counted on 10 0·5-km

2

areas in late March. Briefly,a trained dog quartered the ground in front of the obser-ver and flushed the grouse (for techniques see Redpath& Thirgood 1997). In both 1998 and 1999, some grousewere fitted with 15-g necklace radio-transmitters (BiotrackLtd, Wareham, UK): in area A, 51 grouse in 1998 and48 in 1999; in area B, 49 grouse in 1998 and 43 in 1999.Birds were caught at night in hand-held nets after dazz-ling them with a strong light. All birds were relocatedonce before 1 April and their survival was monitoredweekly until the end of June. From these data we cal-culated weekly survival rates on the basis of the springseason starting on 1 April (Thirgood

et al

. 2000c).Survival data were compared within experimental yearsand also with similar data collected in years before theexperiment. Raptor and mammalian predators couldbe distinguished from field signs (Thirgood

et al

. 1998).

Birds at a number of nests from each area were pro-vided with supplementary food from hatch to chick

dispersal, partly because harrier breeding density andlaying date varied between treatment and controlareas (see the Results) and also to avoid the effectsof the two experiments being confounded. Nests wereallocated randomly to experimental or control (not fed)groups, within the constraints that both groups showedsimilar median laying dates and breeding system.Supplementary food was provided to nine nests in 1998and five nests in 1999. No food was provided to sixnests in 1998 and to five nests in 1999. No food wasprovided in the years before or after the experiment.

Once harrier chicks had hatched, perches were placedon average 9

±

1 m (mean

±

SE) away from the nests.Food was placed on perches each morning, the amountvarying according to the food requirements of the chicks.Estimates of maximum food requirements were basedon previous observations of food deliveries to youngharriers at Langholm (S.M. Redpath & S.J. Thirgood,unpublished data). As in the spring, any food remain-ing the next day was removed. Harriers were providedwith food from hatching for 60 days, the estimated timeto dispersal (Redpath & Thirgood 1997).

To examine what the harriers were eating, we set uphides 5–7 m from each nest. We aimed to spend aminimum of two 6-h watches per nest per week, for 5weeks from hatching. In 1998 and 1999 all nests werewatched, with the exception of one nest in 1999 withno additional food, which was judged to be too closeto a public road. Nests were also watched in 1993–96(Redpath & Thirgood 1999). No nests were watched in

Area B

Area A

5 km

Langholm

Fig. 1. Location of Langholm Moor in Scotland and the division of the study area into two experimental areas. Symbols indicatethe distribution of harrier nests in 1998 (d) and 1999 (s). Nests joined by lines indicate polygynous males. Harriers in area A wereprovided with food during spring 1998, and harriers in area B were provided with food in spring 1999.


1160




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38

,1157–1168

1997 or 2000. During each watch we recorded the timewatched and the identity and quantity of food broughtto the nests by males and females. After fledging, somewing-tagged broods (12 unfed and six fed) were observedevery 2–3 days to record dispersal date, taken as the dayafter the last day on which the birds were seen on themoor.

In May, radio-receivers were used to locate the nestsof grouse carrying transmitters, so that clutch size andthe number of chicks hatching could be recorded. Inearly June and late July, grouse broods were locatedwith trained pointing dogs on transects through countareas and the number of chicks in each brood wascounted. As there was approximately 41·5 km

2

of suit-able grouse habitat on the estate (Redpath & Thirgood1997), we were able to estimate the number of grousetaken by harriers per 0·5 km

2

. We thus obtained estimatesof the number of grouse chicks available to harriers andthe number present after the main period of harrierpredation (Thirgood

et al

. 2000c).Based on the watches at harrier nests we estimated

the number of grouse chicks taken by harriers as follows:grouse taken = grouse delivered nest

–1

h

–1

×

no. nests

×

15

×

X

(Redpath & Thirgood 1997), where 15 = numberof hours available for hunting per day (Watson 1977)and

X

= the number of days from hatching.Two calculations were made, for the period from

hatching to fledging (

X

= 42) and hatching to dispersal(

X

= 60). The percentage of grouse chicks in the pelletsof harriers during the first 4 weeks was not differentfrom the percentage during weeks 5–6 and from weeks7 up to dispersal (Redpath & Thirgood 1997). Thissuggests that predation rates on grouse chicks do notchange as harrier chicks get older.

Analyses were done in Minitab, version 13 (MinitabInc. 2000) and SAS version 6.12 (SAS Institute 1990).To test for the effects of supplementary feeding onharrier breeding parameters and on the provisioningrate of grouse chicks to harrier nests, we incorporateddata from years before and after the experiment. Unlessotherwise stated, we used generalized linear mixedmodels (GLMM) with both year and area included inall models as random effects. Parameters that had non-normal error structures were analysed with a Poissonerror structure and a log-link function; otherwise modelsincorporated a normal error function and an identityfunction. Extra dispersion in all the models was correctedfor by dividing the deviance by the residual degrees offreedom. The models were implemented using aGLIMMIX macro in SAS (Littell

et al

. 1996). Denom-inator degrees of freedom were calculated in SAS usingSatterthwaite’s formula (Littell

et al

. 1996). Models wereconstructed using a backward elimination procedurein a SAS type III analysis, dropping the least significantterm in the subsequent model, until only terms signific-ant at the 10% level remained.

A general linear model (GLM) was used to test fordifferences in the numbers of breeding harriers inareas A and B in relation to feeding. Numbers of maleshave increased over the course of the study and are cor-related with field vole

Microtus agrestis

L. abundance(Redpath, Thirgood & Clarke 2002). To examine theeffect of supplementary feeding on the numbers ofharriers, year and vole numbers in areas A and Bwere included as covariates, with area (A and B) andsupplementary feeding as fixed effects. We used anautocorrelation function to test for serial correlationamong the residuals of the fitted models. As sample sizewas small, we also conducted a randomization test, byreiterating the GLM for the 72 possible combinationsof feeding in the two areas in separate years.

For other models, the moor was divided up into 10nesting areas of approximately 200 ha (five each inareas A and B). These areas included the sites of all thenesting attempts and nesting occurred in each area in atleast one year. Nesting areas were included in GLMMmodels as random effects, to control for spatial effectsof nest location on harrier breeding performance andrates of grouse chick provisioning. Over the 8 years,six females failed and relaid a clutch. Clutch sizeand laying date were taken for the first clutch. Modelsexamining variation in provisioning of prey to harriernests incorporated the log of the hours watched as anoffset function along with female status and whether ornot birds were fed in spring and summer as fixed effects.For models of male provisioning, a male identificationnumber was included as a random effect. For models ofgrouse provisioning the density of grouse chicks (log)in any one year was also included.

When comparing adult grouse survival in relationto spring feeding, we used the Kaplin–Meier productlimit method and tested for end-point differences insurvival using a two-tailed

z

-test statistic (Redpath &Thirgood 1997). Sample sizes for testing the effects offeeding on adult grouse survival, harrier breedingsuccess and harrier chick dispersal were small andtherefore their power was rather low. Throughout thetext, means

±

1 SE. are presented and analyses otherthan those indicated above are noted in the text. Alltests were two-tailed.

Results

During spring, 193 kg of food was put out in area A oneight territories (eight males, 13 females) in 1998 and63 kg of food put out in area B on five territories (fivemales, three females) in 1999. Less food was put out in1999 partly because there were fewer females but alsobecause less food was removed in total. In 1998, 91% ofthe food had disappeared by the next day, whereas in1999 only 44% of the food had disappeared. A similarproportion of food was seen being taken in both years


1161

Supplementary feeding of hen harriers



,

38

,1157–1168

(4% in 1998, 3% in 1999), and of this harriers took asimilar proportion (15% in 1998, 16% in 1999). Theremainder of the food seen being taken (85% in 1998,86% in 1999) was removed by corvids (ravens

Corvuscorax

L., carrion crows

Corvus corone

L. and rooks

Corvus frugilegus

L.). In 1998, the first item seen takenby harriers was on 4 April, whereas in 1999 the firstitem seen taken was on 4 May.

In the 85 recorded male territories established during1992–2000, 97 females attempted to breed. In the last3 years, 11 grey males failed to attract a female (fourin A and seven in B). In all years area A had morebreeding male harriers than area B (area A 5·7

±

0·8,area B 2·6

±

0·6). During the years of the supple-mentary feeding experiment harrier numbers increasedin the area where food was provided (A) in 1998,but not in 1999 (Fig. 2). Both area (males,

F

1,13

= 38·2,

P

< 0·001; females,

F

1,13

= 25·3,

P

< 0·001) and voleabundance (males,

F

1,14

= 9·89,

P

= 0·008; females,

F

1,13

= 8·2,

P

= 0·013) had a strong statistical effecton the numbers of breeding harriers but, controllingfor these, supplementary feeding in spring had nodetectable effect on breeding harrier numbers (males,

F

1,13

= 1·20,

P

= 0·29; females,

F

1,13

= 2·35,

P

= 0·15).

There was no statistically significant interaction betweenfeeding and area (GLM males,

F

1,12

= 0·89,

P

= 0·36;females,

F

1,12

= 2·92,

P

= 0·11). We found no signific-ant serial correlation among the residuals of the fittedmodels (males

r

= 0·03, females

r

= 0·15,

n

= 18,

P

> 0·1),suggesting that harrier numbers in one year could beconsidered independent from those in the previous year.

In the randomization test the

F

-values for the effectof feeding on breeding numbers were not unusual foreither males or females, with 28% of the randomizedvalues being higher for males and 14% being higherfor females. When we considered the effect of supple-mentary feeding on the numbers of females breedingper male, there was no evidence for a statistically sig-nificant effect of feeding, either including (

F

1,68

= 0·66,

P

= 0·42) or excluding (

F

1,57

= 0·74,

P

= 0·39) malesthat failed to attract females. We concluded that therewas little evidence for an effect of supplementary feed-ing on breeding harrier numbers.

In years before and after the feeding experiment, anaverage of 44% of 55 breeding females that could beaged were first years (range 80% in 1993 to 0% in 1995).During these years, there was no relationship betweenthe percentage of first year birds and vole abundance(

F

1,4

= 0·6,

P

= 0·5). In 1998, of 16 females of knownage, four (25%) were first years, which all bred in areaA. In 1999, three (25%) of 12 females of known agewere first years, two of which bred in area A. There wasthus no apparent increase in the proportion of youngfemales breeding in the population during years whenbirds were fed. Of birds tagged as chicks in 1998 and1999, six females returned to breed in the next year.Three of these were from nests provided with food andthree were from control nests. No chicks reared in 1998bred in 2000. No young males reared in 1998 and 1999bred during 1999 or 2000. Thus feeding did not leadto an increase in young females entering the breedingpopulation in 1999 or 2000, nor did feeding lead to anincrease in the return of chicks reared by fed birds.

Laying date varied considerably between years (GLM

F

7,79

= 6·7,

P

< 0·001) and was on average 9 days earlierin area A than B (

F

1,79

= 17·1,

P

< 0·001). Controlling forarea and year as random effects in the GLMM model,and female age, female status and field vole abundance(log) as fixed effects, we tested whether feeding in springcould account for any variation in laying date, but wefound no effect (

F

1,64

= 0·88,

P

= 0·35). The model estim-ate for effect of feeding was –0·03

±

0·03. In the areaswhere food was provided, median laying date variedfrom 21 April in 1998 to 13 May in 1999, a difference of23 days. This was similar to the difference in dates whenharriers were first seen to take food from the perches.This suggests that the use of food in the spring maybe a consequence of female condition, rather than theavailability of supplementary food.

0

1

2

3

4

1992 1993 1994 1995 1996 1997 1998 1999 2000

Fie

ld v

oles

per

100

trap

nig

hts

0

5

10

15

Har

rier

num

bers

0

2

4

6

8

10

Har

rier

num

bers

Area B

Area A

Fig. 2. Changes in the number of field voles caught per 100 trapnights in areas A and B combined and the number of male (s)and female (d) harriers breeding in area A and B from 1992 to2000. Arrows indicate year of feeding.


1162




,

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,1157–1168

Clutch size varied from two to seven eggs, with anoverall mean of 5·0

±

0·1. Clutch sizes were larger inyears when field voles were abundant (GLMM

F

1,6

= 17·8,

P

= 0·006) and harriers that were provided with foodlaid more eggs (Fig. 3;

F

1,74

= 4·5,

P

= 0·04). As therewas a strong negative relationship between clutch sizeand laying date (linear regression

F

1,91

= 12·8,

P

= 0·001),we included laying date in the mixed model. After con-trolling for laying date, the effects of spring feeding onclutch size were still significant (

F

1,73

= 4·1,

P

= 0·05).So, feeding during the spring had no apparent effect onlaying date but led to larger harrier clutches.

Raptors were the main source of mortality for adultgrouse during April and May in 1998 and 1999, killinga minimum of 18% of the grouse overall and a minimumof 78% of those grouse which died (Table 1). Survivalrates of radio-tagged grouse did not differ significantlybetween areas A and B in either 1998 or 1999 (Table 2;1998, Z = 0·48,

P

> 0·5; 1999, Z = 0·36,

P

> 0·5). Sim-ilarly, survival rate (area A, Z = 0·92,

P

> 0·1; area B,Z = 0,

P

= 1·0) did not differ between years.We compared survival rates of all radio-tagged grouse

at Langholm in 1998 and 1999 with survival during thespring of 1995 and 1996 when no supplementary feed-ing was conducted (Redpath & Thirgood 1997). Springsurvival rates in 1998 and 1999 tended to be higher,but not significantly (Table 2; 1995 vs. 1998, Z = 1·04,

P

> 0·1,

P

> 0·1; 1996 vs. 1998, Z = 1·72,

P

> 0·1; 1996vs. 1999, Z = 1·18,

P

> 0·1).

Of the 17 female harriers that laid clutches in 1998,three failed at incubation or during hatch. A mamma-lian predator (probably a stoat) ate one clutch, anotherclutch disappeared and one young brood apparentlystarved despite both adults being present. Two of thesenests were in area A and one (which relaid) was in areaB. Of the 13 females that laid in 1999, four failed due tofox predation in area A during incubation. Two of thesefemales were killed on the nest by foxes and one of theother two relaid.

There was no significant effect of spring feeding onthe number of harrier chicks that hatched (

F

1,29

= 0·06,

P

= 0·81). Controlling for female age and status didnot improve the level of significance (

P

= 0·95), andexcluding nests that failed during incubation had littleeffect (

F

1,52

= 0·22,

P

= 0·64). Of those nests whereyoung were successfully hatched, there was a tendencyfor birds fed during the nestling period to fledge morechicks in 1998 and 1999 (3·4 young for fed broods vs.2·4 young for broods not fed). However, this differencewas not significant, when controlling for clutch size andfemale status (

F

1,51

= 1·02,

P

= 0·32).

In 1998 and 1999 we obtained data on estimated dis-persal dates for six nests provided with food and com-pared these to dates from four nests without food inthese years and eight nests without food in previousyears. Median dispersal dates did not differ (Mann–Whitney U = 101, P = 0·24) and were 60 days for unfedbroods (range 59–64) and 62 days for fed broods(range 50–66).

During summer 1998, 6499 food items were put onperches next to nine harrier nests, of which 85% had

3

3·5

4

4·5

5

5·5

6

Clu

tch

size

1998 1999

4

13

10

3

Fig. 3. Relationship between hen harrier clutch size and supple-mentary feeding in the two experimental years (1998 and 1999).Data presented as means ±1 SE with sample size above. Blackbars indicate birds not provided with food in spring and openbars those birds with food provided in spring.

Table 1. Number of grouse radio-tagged in areas A and B in 1998 and 1999, and numbers killed by raptors, mammals and othercauses. Number lost indicates cases where contact was lost with the birds

1998 1999

Totals (%)Area A Area B Area A Area B

Number radio-tagged 51 49 48 43 191Number killed before 1 June 10 12 13 10 45 (24)Number killed by raptors 10 10 8 7 35 (18)Number killed by mammals 0 0 2 0 2 (1)Number killed by unknown 0 2 3 3 8 (4)Number lost 1 0 2 3 6 (3)


1163Supplementary feeding of hen harriers

© 2001 British Ecological Society, Journal of Applied Ecology, 38,1157–1168

disappeared by the next day. Harriers took 92% of thefood seen being taken, while crows and lesser black-backed gulls Larus fuscus L. took the remaining 8%. In1999, 4069 items were put out at five harrier nests, ofwhich 69% had disappeared by the next day. Harrierstook 83% of the food seen taken with the remaining17% taken by lesser black-backed gulls.

Over the 2 years 2007 h were spent in hides (83 hper nest with food and 84 h per nest without food).We only saw supplementary prey items delivered tonests where food was provided nearby. Over the 2 years,females provisioned 40 supplementary items per 100 hcompared with four supplementary items by theirmales (Fig. 4; t-test, t = 6·3, 19 d.f., P < 0·001). Thereappeared to be an effect of mating system on provision-ing of supplementary food by males. The two bigamousmales in 1998 brought in 34 and 11 supplementaryitems per 100 h, respectively. In contrast, the remainingfive monogamous males in 1998 delivered on average0·6 items in 100 h (range 0–2), and the five monogamousmales in 1999 delivered on average 0·2 items in 100 h(range 0–1).

For females, the use of supplementary food variedaccording to whether or not they were fed during thespring. For both years combined, females fed in springdelivered more (GLM controlling for year F1,11 = 9·01,

P = 0·012) supplementary items (50·3 ± 5 100 h–1)compared with females not fed in spring (23·3 ± 7100 h–1).

The availability of supplementary food had a large effecton the provisioning rates of natural prey by females(fed 30 ± 6 vs. unfed 8 ± 2 items 100 h–1, F1,43 = 18·0,P = 0·0001). For males, the provisioning rates ofnatural prey were slightly higher at those nests whereno food was provided (fed 62 ± 4 vs. unfed 51 ± 7).However, differences in provisioning rates betweenmales were not significant (F1,40 = 2·3, P = 0·13). So therate at which females delivered wild prey was affectedby feeding, but this was not the case for males.

Interestingly, although provisioning rates of all prey(including supplementary items) were slightly higherat nests where supplementary food was provided (fed97 ± 8, unfed 81 ± 7), this difference was not significantonce the effects of area, year and female status werecontrolled (F1,43 = 2·18, P = 0·15). Despite the fact thatfed females provisioned food at a higher rate (F1,39 =6·9, P = 0·01), fed males did not (F1,40 = 1·2, P = 0·28).This suggested that the additional food acted as asubstitute rather than a supplement to natural prey.

In 1998, 23 grouse chicks were seen delivered to the15 harrier nests in 1088 h of observation. Of these,only two (9%) were delivered to the nine nests wherefood was provided. In 1999, 16 grouse chicks were seendelivered to nine harrier nests in 919 h of observation,of which five (31%) were delivered to the five nestswhere food was provided. There was a significant effectof providing supplementary food during the summeron the rate at which both male and female harriersdelivered grouse chicks to their nests (Fig. 5; males,F1,31 = 4·8, P = 0·03; females, F1,35 = 6·10, P = 0·02;both combined F1,41 = 11·8, P = 0·001). Over both yearscombined, harriers at nests where food was provideddelivered 0·5 grouse chicks every 100 h, or 4·5 chicksfrom hatching to dispersal. In contrast, harriers atnests where no food was provided delivered 3·7 grousechicks every 100 h, or 33·3 chicks from hatching to dis-persal. Other factors that were significant in the modelswere spring feeding for females (F1,36 = 6·03, P = 0·02),status of females (F2,39 = 6·49, P = 0·004) and grouseabundance for males (F1,4 = 8·6, P = 0·04) and bothsexes combined (F1,4 = 23·5, P = 0·008).

In 1998 and 1999 harriers delivered grouse chicks totheir nest at a lower rate than during 1993–96. In yearsprior to the experiment males delivered grouse chicksat 7·4 100 h–1, compared with 2·3 in 1998 and 1999 (datafor nests without supplementary food). For females therate dropped from 6·7 to 1·9 grouse. These differences

Table 2. Survival rates of radio-tagged grouse on treatmentand control areas at Langholm Moor, Scotland, during spring1998 and 1999, in comparison with spring 1995 and 1996. Survivalrates (S) and 95% confidence intervals (CI) calculated usingthe Kaplin–Meier method with staggered entry. *Areas providedwith supplementary food during the spring

Year Area n S 95% CI

1995 Total 75 0·71 0·60–0·811996 Total 99 0·67 0·58–0·771998 Area A* 51 0·80 0·69–0·91

Area B 49 0·76 0·64–0·87Total 100 0·78 0·70–0·86

1999 Area A 48 0·72 0·59–0·85Area B* 43 0·76 0·62–0·89Total 91 0·75 0·66–0·84

0

10

20

30

40

50

1998 1999

Year

Sup

plem

enta

ry it

ems

100

h–1

Male

Female

7

9

5

5

Fig. 4. Mean provisioning rates (±1 SE) of supplementaryfood by male and female hen harriers in 1998 and 1999.Numbers indicate sample sizes.


1164S.M. Redpath, S.J. Thirgood & F.M. Leckie


were significant (t-tests, male, t = 2·6, 19 d.f., P = 0·018;female, t = 4·24, 25 d.f., P < 0·001).

From 1993 to 2000 there was a significant decline inthe number of grouse counted in autumn, from 36·60·5 km–2 to 9·7 0·5 km–2 (Fig. 6) and in the numberscounted in spring (Thirgood et al. 2000c). Since 1997,

spring numbers have declined from 15·1 0·5 km–2 to8·0 0·5 km–2. Feeding harriers in 1998 and 1999 did notlead to an increase in grouse density.

The average losses of grouse chicks from hatch tolate July increased from 45% in 1995–96 to 62% in1998–99 (Table 3). However, this increased loss was notdue to increased predation by harriers. Harriers tookonly 6·5% of the available grouse chicks in the yearsof the feeding trial, compared with 28% in 1995–96.The reduction in grouse chick losses to harriers in thelast 2 years was partly as a result of a lower density ofgrouse chicks on the moor.

In 1998, we estimated that harriers took 187 ± 80grouse chicks, or 2·2 ± 1·0 0·5 km–2. Yet observed chicklosses from hatching to late July were 21·7 chicks0·5 km–2, or 10 times higher than the expected value.Similarly in 1999, we estimated that harriers took 107 ±53 grouse chicks, or 1·3 ± 0·6 0·5 km–2. Yet observedlosses were 12·7 chicks 0·5 km–2, or 10 times higherthan expected had harriers been the sole mortalityagent.

Costs of feeding will vary depending on the numberand location of harriers requiring feeding, the numberof additional staff employed, the type of food used andthe transport costs of getting to the harriers each day.The figures provided here are a rough estimate of thecosts associated with the technique.

Had all harriers been fed, the total weight of foodneeded would have been 1020 kg in 1998 and 639 kgin 1999. If rats (160 g) only were used, the cost (£0·30each) would have been £1912, or £127 per harrier, in1998, and £1198, or £120 per harrier, in 1999. Hadchicks (40 g) only been used, the cost (£0·025 each) in1998 would have been £637 (£42 per harrier) and in1999 would have been £399 (£40 per harrier). Thesecosts could potentially be reduced through the pro-vision of other prey such as locally caught rabbits.Transport costs varied between years, as most nestswith food were reasonably close to public roads in 1998,but not in 1999. In 1998, we estimated costs at £875.In 1999 we hired an all-terrain vehicle for £2000 andcovered an estimated 6000 miles, equivalent to £1500.In total therefore travel costs were £3500 in 1999.

The main cost of supplementary feeding of henharriers will lie in wages. Feeding all the harriers on

0·01·02·03·04·05·06·07·0

Total Male Female

FedUnfed

0·01·0

2·03·04·0

5·0

6·07·0

Gro

use

chic

ks

100

h–1

FedUnfed

1998

1999

Total Male Female

Fig. 5. Mean rate (±1 SE) at which harriers delivered grousechicks to their young in 1998 and 1999. Data shown separatelyfor males and females at nests with and without food.

05

1015202530354045

1992

1993

1994

1995

1996

1997

1998

1999

2000

Year

Gro

use

0·5

km–2

SpringAutumn

Fig. 6. Change in spring and autumn grouse density onLangholm Moor, Scotland, from 1993 to 2000. Data as means(±1 SE) from 10 0·5-km2 count areas within areas A and B.

Table 3. Changes in grouse brood size and chick density per 0·5 km2 from May to July in comparison with the number of grousechicks estimated to have been taken by harriers from hatching to fledging. Data from two periods are given: 1995–96, when nofeeding of harriers occurred, and 1998–99 when some harriers were provided with food

YearGrouse brood size at hatch

May chicks 0·5 km–2

July chicks 0·5 km–2

Difference May–July 0·5 km–2 (%)

Grouse brood size in July

Harrier nests (%)

Taken by harriers 0·5 km–2

1995 8·4 39·1 22·7 ± 3·5 16·4 (42) 4·5 8 (8) 11·2 (29)1996 8·5 35·7 18·4 ± 2·5 17·3 (48) 4·7 14 (12) 9·7 (27)1998 8·8 31·7 10·3 ± 1·3 21·4 (67) 2·8 17 (15) 2·2 (7)1999 8·7 21·7 9·5 ± 1·5 12·2 (56) 3·8 13 (10) 1·3 (6)




Langholm from March to August would be equivalentto one full-time job, plus additional casual help. Anapproximate cost of such staff-time for 5 months wouldbe £7500. Had all the harriers been fed at Langholm,we estimated that the cost per annum would have beenbetween £10 000 and £11 500 (Table 4).

Discussion

Providing harriers with supplementary food greatlyreduced the rate at which grouse chicks were deliveredto harrier nests. Over both years combined, harriers atnests where food was provided delivered one grousechick to their nest every 200 h, whereas harriers with-out supplementary food delivered one grouse chickevery 27 h. This alone suggests that supplementaryfeeding provides a useful tool in reducing the numberof grouse chicks taken by hen harriers.

The two areas of Langholm Moor used for the experi-ment varied considerably in the numbers of harriersthat bred there, with area A consistently attractingmore birds. Redpath & Thirgood (1997) attributed thispattern to differences in elevation, with birds breedingat higher density and laying earlier on low-lying heatherareas. Although feeding was associated with an increasein harrier density in one year, overall we found no statis-tical evidence that feeding harriers in spring led toincreases in breeding density. After controlling for theeffects of year and vole abundance, there was no appar-ent increase in breeding males in areas where foodwas provided. Nor did these fed males breed with morefemales than unfed males. In addition, we found noevidence that chicks from fed broods were more likelyto return to breed in subsequent years. While severalstudies have found a strong correlation between raptordensity and food supply, few studies of raptors haveexamined the effect of artificial feeding on density,although this issue has been addressed in a varietyof other species (Newton 1998). In hen harriers, breed-ing density is correlated with food supply, both inScotland (Redpath & Thirgood 1999; Redpath, Thirgood& Clarke 2002) and elsewhere (Hamerstrom 1979;Korpimaki 1985). However, the finding that feedingdid not increase male density is perhaps unsurprising,given that feeding was only directed at males that

had already established a territory. Although thesemales fed their females with the supplementary food,birds did not take this extra food until females wereapproaching egg laying, when their food requirementswere increased.

In a supplementary feeding experiment with sparrow-hawks Accipiter nisus L., Newton & Marquiss (1981)found that all fed birds laid clutches whereas 27% ofunfed birds failed to lay eggs. Similarly, in a furtherfeeding experiment with harriers in Orkney, Amar &Redpath (in press) found that feeding led to increases inthe number of females that laid a clutch. This did notappear to be the case in the present study, as apparentlyall females seen consistently on territory in spring wenton to breed. The two other studies occurred in areaswhere food was considered scarce, so a difference innatural prey availability may account for the differencein breeding attempts.

Several studies have found an effect of supplement-ary feeding on clutch size and laying date (Dijkstraet al. 1980; Newton & Marquiss 1981; Aparicio 1984;Korpimaki 1987; Simmons 1994; Korpimaki & Wiehn1998). In addition, Redpath, Thirgood & Clarke (2002)found a close correlation between vole abundanceand clutch size, so a relationship between supple-mentary feeding and clutch size was expected. In ourstudy, fed females laid larger clutches but laying datedid not appear to be influenced by feeding. The timingof food removal suggested that supplementary itemswere taken only when females were approaching layingdate, rather than the food causing a change in layingdate.

Adult grouse survival was slightly higher in 1998 and1999 than it was in 1995 and 1996, although the differ-ences were not statistically significant. This patternmay have reflected the decrease in grouse density andan associated decline in density-dependent mortality(Thirgood et al. 2000c). Despite the fact that there werelarge differences in harrier breeding density betweenthe areas and that some birds were fed in each year,there was no significant difference in adult grouse sur-vival between the two areas in either year, or betweenthe 2 years in either area. Overall, the results indicatedthat providing harriers with food in spring did notgreatly improve adult grouse survival. This could eitherbe because feeding did not stop harriers killing grouseand they were killing grouse over the whole moor, orbecause harriers were killing few adult grouse anyway

Table 4. Estimates of the financial cost of feeding all breeding harriers at Langholm Moor, Scotland, in 1998 and 1999

Cost 1998 1999 Years combined

Food £637–£1912 £399–£1198 £1036–£3110Transport £875 £3500 £4375Miscellaneous £540 £40 £580Wages £7500 £7500 £15 000Total £9552–£10 827 £11 439–£12 238 £20 991–£23 065Total per harrier nest £637–£722 £1143–£1224 £840–£923




at this time of year. We know little about the rangingbehaviour of harriers in spring, but females appearedto spend most of their time on territory, being fed bytheir males with small prey for 2–3 weeks prior to lay-ing (Redpath & Thirgood 1997). In addition, pelletanalysis suggested that voles, not grouse, were the prin-cipal food of harriers during this period (Redpath,Thirgood & Clarke 2002). Given that most springmortality was due to raptor predation, the most likelyexplanation for the apparent lack of an effect of springfeeding on adult grouse mortality rates is that muchof the mortality at this time of year may have beendue to other predators, particularly raptors such asperegrines Falco peregrinus Tunstall and goshawksAccipiter gentiles L.

Providing harriers with supplementary food during theharrier nestling period had a marked effect on harrierprovisioning. Females delivered much more supple-mentary food than males and decreased their deliveryrate of natural prey. In addition, females used consider-ably more of the supplementary food during summerwhen they were provided with food during the spring.Males did not alter their overall delivery rates whensupplementary food was provided, and it was only thebigamous males (in 1998) that delivered any of thesupplementary food.

Two other studies have examined the effects of sup-plementary feeding on parental provisioning rates inraptors (Wiebe & Bortolotti 1994; Wiehn & Korpimaki1997). Both found that provisioning rates were reducedat nests with supplementary food. However, only Wiehn& Korpimaki (1997) distinguished between maleand female deliveries, finding that only female kestrelsFalco tinnunculus L. reduced their provisioning rates inresponse to feeding. They suggested that male parentaleffort was inflexible within a season and was fixed ata level that maximized lifetime reproductive success.This idea was supported by the finding that male hunt-ing effort in kestrels was not influenced by brood sizemanipulations and males were therefore not sensitiveto brood demands (Tolonen & Korpimaki 1996). Theresults of the present study support the idea that maleparental effort is fixed. However, while provisioningrate might not be flexible, our results suggest that malesare able to alter the types of prey they deliver to nests.Notably, fed males reduced the rate at which theydelivered grouse chicks to their nests.

Grouse chicks were seen delivered to only 29% ofnests where food was provided, compared with 70% ofnests where no food was provided. Overall, sevenfoldmore grouse chicks were delivered to nests where nosupplementary food was available. So, providingharriers with food at Langholm clearly reduced thenumber of grouse chicks taken by harriers. Three otherfactors appeared to be important in determining therate at which harriers took grouse chicks. First, the

density of grouse chicks themselves, with fewer chicksbeing taken in later years as grouse density declined(Redpath & Thirgood 1999). Secondly, the breedingstatus of the birds, with females mated with bigamousmales taking more grouse chicks than monogamousones. Thirdly, the provision of supplementary food inthe spring reduced the number of grouse chicks takenby female harriers. From a management point of view,these relationships are important. They suggest thatin order to maximize the benefits of supplementaryfeeding: (i) birds should be provided with food duringthe pre-laying period; (ii) bigamous birds should havepriority over monogamous ones; and (iii) the greatestreductions in the numbers of grouse chicks removed byharriers are likely at high grouse chick densities.

In both years, delivery rates of grouse chicks werelow, even at nests where no food was provided, and thisreflected the scarcity of grouse on the moor. In July2000 there were only 19 grouse km–2, compared with 73grouse km–2 in 1993. Harriers were estimated to havetaken 7% of available grouse chicks from hatching tofledging in 1998 and 6% during the same period in1999. This compared with predation rates of up to 29%in previous years. Yet despite the reduced predationby harriers, losses of grouse chicks were higher than inprevious years (67% in 1998, 56% in 1999, comparedwith 42% in 1995 and 48% in 1996). We infer that someother factor was having a strong influence on grousechick survival in these 2 years, but whether this wasrelated to weather, food quality, parasites or other pre-dators was unknown because we did not focus ourstudies on grouse chick survival and we have no data onchanges in predator or parasite abundance. Brood sizesat hatching were comparable between areas during theyears of the experiment, suggesting that neither femalecondition nor clutch predation was responsible for thehigher chick losses. The loss of two adult harriers andtwo further clutches to foxes suggested that there mayhave been more foxes on the moor in 1999, which mayin turn have reduced grouse brood size. Also, the sum-mer of 1998 was exceptionally wet and cold and therewas a severe outbreak of heather beetle over LangholmMoor during 1999 and 2000, which may have contrib-uted to grouse chick losses.

A further possible explanation for the high grousechick losses is that feeding increased the abundanceof corvids and gulls on the moor, which subsequentlypreyed on grouse chicks. Crows, in particular, are thoughtto be important predators of grouse chicks (Hudson &Newborn 1995). At Langholm, grouse brood size athatch and therefore clutch size was not reduced andwe saw no removal of grouse chicks by these predators.However, increased predation by corvids and gullscannot be excluded as a possible reason for poor grousebreeding success and should be tested in any future trialof supplementary feeding.

Previously, data have suggested that most chick pre-dation by harriers was additive and not compensatedfor by other sources of chick mortality in June and July




(Redpath 1991; Thirgood et al. 2000c). In the 2 years ofthe experiment, however, unexplained chick mortalitywas considerably higher than it had been in other years,suggesting that there may be variation between years inthe amount of potential compensation. The only wayto test the extent of compensation is through an experi-ment, whereby harrier predation in one area is reducedand grouse chick survival in this area is compared withsurvival in a similar area with harrier predation.

Other studies have found that food supplementationat raptor nests increased the number of fledglings anddecreased nestling mortality (Wiebe & Bortolotti 1994;Gehlbach & Roberts 1997; Wiehn & Korpimaki 1997).Although not statistically significant, there was atendency for harrier breeding success to be improvedby summer feeding, with fed broods rearing on averageone more young than unfed broods. One possiblereason why the results were not more marked was thatoverall provisioning rates were not significantly higherfor fed broods, so food intake rate was similar forboth groups of birds. As for the number of breedingattempts, it is likely that the impact of feeding onharrier fledging success will be greater in areas wherenatural food is scarce. Nestling survival may also beimproved through the increased nest defence of theadult female as she is able to spend more time near thenest and less time hunting (Ward & Kennedy 1996).

Unlike some studies of other raptors involvingsupplementary food, we found no evidence that feed-ing increased the time that young harriers spent onterritory before dispersing (Kenward, Marcstrom &Karlbom 1993; Frumkin 1994). However, in contrastto these studies, which fed the young until they dis-persed, we only put food out up to a previously deter-mined dispersal date (Redpath & Thirgood 1997). Wefound no evidence that summer feeding increased theprobability that young birds would return to breed atLangholm.

We estimated that the costs of this feeding experi-ment were in the region of £11 000 per annum, thoughthis amount will vary according to a variety of factors(see the Results). It is unclear who would pay forsupplementary feeding, but it is worth noting thecosts of feeding relative to the costs of grouse manage-ment. Buccleuch Estates, who own Langholm Moor,have estimated that the annual cost of maintaininggrouse moor management at Langholm was £99 500(Redpath & Thirgood 1997). This compares to approx-imately £900 per harrier nest, or an increase in the costof approximately 1% per nest.

In conclusion, providing harriers with food duringthe nestling period had a clear impact on harrier pro-visioning. Both male and female harriers reduced therate at which they took grouse chicks when providedwith supplementary food. Despite this, there was noclear improvement in grouse breeding success com-pared with previous years, and grouse density declinedover the course of the experiment. Our experimentwas conducted over a 2-year period on one grouse

moor. Over such a relatively short time it is difficult toquantify the long-term impact of feeding on harriersurvival and recruitment. Similarly, our experiment wasnot designed to test the effect of feeding on annualchanges in grouse density. To address these issuesproperly, future experiments will require a longer timespan of 5–10 years. Measuring the impact on grousedensity would also require increased replication, whereall birds were fed throughout the spring and summer,paired with control areas where no birds were fed.Ideally such areas would be geographically isolatedto prevent control birds hunting in the experimentalareas. Our results imply that the benefits of supple-mentary feeding will be greater when improvementsin grouse chick survival are not compensated for byincreased losses to other factors. Future experimentsshould specifically address the extent to which com-pensation can occur.

Acknowledgements

We are grateful to Buccleuch Estates for allowing usto conduct this work on their land, and in particularto Brian Mitchell and the late Gareth Lewis for alltheir help. Thanks to Ian Bainbridge, Colin Galbraith,Ian Newton and Des Thompson for constructivecomments throughout the project, and to NicholasAebischer, Steve Albon, Arjun Amar, Dave Elston,Rhys Green, Pete Hudson, Mick Marquiss, Dick Pottsand Steve Tapper for their guidance and helpful com-ments on the first draft. The work could not have beencompleted without the efforts of Lucy Bellini, KarenBouwman, Steve Campbell, Eric Donnelly, Chris Gall,Kerry Lock, Mark Mainwaring, Andrew Walton and anumber of enthusiastic volunteers. We also thank BrianEtheridge, Mike Henry, Malcolm Henderson and espe-cially John and Bettina Halliday for their help. Thework was led by the SNH Moorland Working Groupand funded by Scottish Natural Heritage, BuccleuchEstates, The Game Conservancy Trust, The GameConservancy Scottish Research Trust, The Centre forEcology and Hydrology, The Royal Society for theProtection of Birds and The Scottish Landowners’Federation.

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Received 15 February 2001; revision received 14 September 2001


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Does supplementary feeding reduce predation of red grouse by hen harriers?