Spatial and temporal associations between

recovering populations of common raven Corvus

corax and British upland wader populations

Arjun Amar1*, Steve Redpath2, Innes Sim1 and Graeme Buchanan1

1Royal Society for the Protection of Birds – Scotland, Dunedin House, 25 Ravelston Terrace, Edinburgh, EH4 3TP,

UK; and 2Aberdeen Centre for Environmental Sustainability, Aberdeen University & Macaulay Institute, Tillydrone

Avenue, Aberdeen, AB24 2TZ, UK

Summary

1. Recovering populations of predators and scavengers have often given rise to concerns about the

impact they may have on prey species. Particularly, this is the case when the prey species are of

economic or conservation importance.

2. Recovery of common raven Corvus corax populations in the UK and Europe has given rise to

a conflict with some stakeholders over their concerns for both the protection of livestock and the

possible detrimental impact on some upland bird species, particularly ground nesting waders. This

has led to demands by some land managers for licences to lethally control ravens to protect upland

breeding birds.

3. We used data from broad scale surveys of distribution and abundance of upland breeding birds

in the UK carried out in 1980–1993 and 2000–2002 to test whether variation in raven abundance or

change in raven abundance was negatively associated with changes in abundance of five species of

waders.

4. We found no significant negative spatial or temporal relationships between ravens and any of

the five species of waders. However, weak (0Æ05 < P < 0Æ1) negative relationships between raven

abundance and trends of curlew Numenius arquata and lapwing Vanellus vanellus may warrant

further investigation.

5. Synthesis and applications. Our study found no significant negative associations between raven

abundance and population changes in upland waders, and so does not provide support to justify

granting of licences for the lethal control of ravens in the interest of population-level conservation

of these upland wader species. However, the near significant negative associations with lapwing and

curlew merit further investigation. This study emphasizes the importance of making a thorough

evaluation of the evidence base beforemaking decisions regarding predator control.

Key-words: avian conservation, grouse moors, lethal control, population decline, population

limitation, predation, protected species

Introduction

Across Europe, many predatory species are recovering from

the adverse effects of persecution, over-hunting or toxic

pesticides. Whilst many organizations view these changes as

positive, increases in predator numbers do raise concerns

amongst other groups, because of their perceived impact on

prey species. This is especially the case for those predators

which take prey of either economic or conservation impor-

tance (Redpath & Thirgood 1997; Landa et al. 1999; Stahl

et al. 2002; Petty, Lurz & Rushton 2003; Amar et al. 2008).

Such concerns can lead to conflicts between land users and

conservation or statutory agencies, with land users often

demanding the right to kill predators (Linnell et al. 2005;

Treves &Naughton-Treves 2005). Therefore, licensing author-

ities must balance the need to safeguard populations of vulner-

able, recovering predators, whilst at the same time

acknowledging and addressing the concerns that farmers,

hunters or conservationists may have over increasing predator*Correspondence author. E-mail: arjun.amar@rspb.org.uk

Journal of Applied Ecology 2010, 47, 253–262 doi: 10.1111/j.1365-2664.2010.01772.x

� 2010 The Authors. Journal compilation � 2010 British Ecological Society

populations. Such decisions need to be informed by evidence,

and in particular the impact that predators are having on prey

populations and livelihoods.

In theUK,persecutionof commonravenCorvus corax (here-

after simply raven) by farmers and gamekeepers caused a rapid

contraction in breeding range by the 20th century (Gibbons

et al.1994).More recently, however, both in theUKandacross

Europe, raven populations have increased rapidly (BirdLife

International 2004). In theUK, theBreedingBirdSurvey shows

that from 1994 to 2007 the population increased by 134%,with

increases of 267%, 155% and 34% in England, Scotland and

Wales respectively (Risely, Noble & Baillie 2008). The species

remains restricted largely to the uplands, and upland bird sur-

veys carried between 1980 and 1993 and repeated in 2000 or

2002 found increases in ravens,althoughwithconsiderablevari-

ation between survey areas (Sim et al. 2005). Ravens also occa-

sionally kill lambs (Ratcliffe 1997) and in the UK, licences are

granted tokill ravens toprotect livestock. InScotland, thenum-

ber of licences granted increased by nearly 300% from 21 in

1998 to61 in2008 (ScottishGovernment,unpublisheddata).

Concurrent with the increases in ravens reported by Sim

et al. (2005), populations of many upland wader species have

declined. Sim et al. (2005) reported widespread declines from

1980–1993 to 2000–2002 for three species of waders: lapwing

Vanellus vanellus (L.), dunlin Calidris alpina (L.) and curlew

Numenius arquata (L). For these species large per annum

decreases, sufficient to cause at least a 50% population decline

over 25 years, were found in several survey areas. For two

other species of wader, golden plover Pluvialis apricaria (L.)

and snipe Gallinago gallinago (L.), population changes varied

between survey areas, with declines of over 50% in some areas,

but increases of the same extent found elsewhere.

Ravens are omnivorous scavengers and predators (Ratcliffe

1997). Their diet varies greatly between studies (reviewed by

Ratcliffe 1997), and although full grown birds occasionally

form part of their diet (Klicka & Winker 1991; Hendricks &

Schlang 1998) these are thought to be more often scavenged

than predated (Marquiss, Newton&Ratcliffe 1978). However,

ravens prey regularly on the eggs and young of birds. Several

studies in theUKhave found the frequent presence of eggshells

in raven castings, which included curlew and other wader spe-

cies (Marquiss et al. 1978; Ewins, Dymond & Marquiss 1986;

Marquiss & Booth 1986). Another study in southern Norway

found that ravens were responsible for most of the June losses

of golden plover clutches (Byrkjedal 1987).

Thus, there is evidence to suggest that ravens could be an

important predator of breeding wader, and some stake-

holders have expressed concerns that ravens may have

been responsible for upland waders declines. For example,

Scottish Natural Heritage (SNH), the licensing authority in

Scotland, received licence applications in both 2007 and

2008 for the control of ravens to protect upland breeding

waders and wild gamebirds from 15 estates (SNH, un-

published data). No such licences have yet been issued,

because SNH have not been satisfied that there is sufficient

evidence to indicate that any wild bird populations are

affected by increases in raven populations.

These issues have been the source of much controversy, and

were included within a petition submitted to the Scottish

Parliament (http://www.scottish.parliament.uk/business/peti-

tions/docs/PE449.htm), which led to a review on the impact of

predatory birds on waders, songbirds, gamebirds and fisheries

(Park et al. 2005). This review concluded that analyses to

investigate changes in wader abundance in relation to preda-

tory birds, including the raven, was a high research priority

(Park et al. 2005).

The greatest insights into the role of predators in limiting

prey populations come from replicated predator removal

experiments (Newton 1998). However, these types of experi-

ments are logistically difficult to conduct at appropriately large

spatial scales, particularly when the predators involved are

legally protected. Where such studies are impractical, useful

knowledge can be gained by examining correlations between

changes in prey and predator abundance (Newton, Dale &

Rothery 1997; Thomson et al. 1998; Amar et al. 2008; Cham-

berlain, Glue & Toms 2009). In this study, we take advantage

of data provided by the Repeat Upland Bird Survey (RUBS)

(Sim et al. 2005) to undertake such a study. The RUBS pro-

vides data from original and repeat surveys of upland breeding

birds from a range of plots throughout Britain, providing data

on both the abundance and population change of wader spe-

cies and ravens from a number of discrete survey areas.

In this study, we use the RUBS data to examine spatial and

temporal associations between populations and trends in

ravens and waders. Spatially, using data from the repeat

surveys, we explore whether wader abundance correlates nega-

tively with raven abundance. Temporally, we examine whether

the changes in the abundance of any wader species are nega-

tively associated with either raven abundance in the repeat sur-

vey or the change in raven abundance between the two survey

periods on a plot. However, because any such relationship

could in theory be as a result of difference in abundance or

changes associated with environmental variables, we

constructed full models incorporating broad habitat and topo-

graphical measures prior to examining the hypothesis that

raven abundance or change was associated with wader abun-

dance or changes in wader populations (Whittingham et al.

2006). Temporal associations, and in particular negative

relationships between changes in wader and change in raven

abundance, would provide the strongest correlative evidence

that increases in raven populations might be responsible for

declines in wader populations. This study presents the first

attempt to test whether there are any negative associations

between raven and upland wader populations with a view to

helping inform policy on whether licenced control of raven

populations is likely to benefit broader upland bird conserva-

tion objectives.

Materials and methods

THE REPEAT UPLAND BIRD SURVEY

Sim et al. (2005) conducted re-surveys of upland birds on plots

distributed in a series of regionally discrete survey areas (Fig. 1).

254 A. Amar et al.

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

Survey areas were not necessarily representative of the UK

uplands (Sim et al. 2005), because the rationale for survey area

selection in the original surveys varied, and was associated with

identifying areas with important upland breeding bird assemblages

for legal protection under UK and EU conservation legislation.

In many cases, these areas were simply known or suspected to

hold relatively high densities of upland breeding birds, particularly

waders. In others, they were areas directly threatened with large-

scale land use change, such as afforestation. They do, however,

represent plots from most of the main upland blocks of Britain.

Thirteen survey areas covering a total of 1713 km2 were originally

surveyed between 1980 and 1993, and were then resurveyed in

2000 or 2002. Thus, time between original and re-surveys varied

from 9 years (e.g. Exmoor) to up to 21 years (Migneint). Any

plots or areas of plots that were afforested between surveys were

not resurveyed; for partially afforested plots, we recalculated origi-

nal bird abundance within the plot removing those birds found in

the areas that were subsequently afforested. Bird surveys (see Sim

et al. 2005 for details) used either parallel transects (200 ⁄ 250 m

apart) or were based on the methods of Brown & Shepherd

(1993) which were broadly equivalent to 200 m transect surveys,

as they involve surveying to within 100 m of every point in the

plot. Surveys consisted of two visits, occurring between April and

July. For analyses, we used the maximum count from either of

the two visits (early and late) in both the original and re-survey.

Original and repeat surveys were always conducted using the same

methods.

USE OF RUBS DATA

Details of the number and average size of plots in the different survey

areas are given in Table 1. Survey areas were the same as Sim et al.

(2005) with a few exceptions. We split the North Wales area into two

(Berwyn and Migneint), because the original surveys took place in

different years. We excluded data from Staffordshire, South Scotland

and South-west Scotland because they lacked survey data for ravens

(Sim et al. 2005), and from East Flows because surveys used 500 m

transects and were therefore not comparability with themethods used

Fig. 1.Map of the UK showing and the

locationoftheplotsusedinthisstudy.Smaller

images to the right and top, show the survey

area locations in more detail with the key

place names to provide greater clarity on the

locations of plots. Please note that scales

vary.

Impacts of raven populations on upland waders 255

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

in the other survey areas. Where survey plots were contiguous or

where they were separated by gaps of <200 m from a neighbouring

plot, they were combined into a single plot for the purpose of this

analysis. Altogether, we used data from surveys in 10 survey areas,

from 118 plots covering 1285 km2.

We had sufficient data to examine population changes for five

wader species: curlew, golden plover, snipe, dunlin and lapwing.

Counts of ravens (including flocks) were also made during the bird

surveys, and we calculated raven abundance, for use in our analysis,

using the maximum count between the two visits per total area (km2)

of the plot.

ENVIRONMENTAL ATTRIBUTES OF PLOTS

Environmental data extraction was undertaken using Map Info 6Æ0(MapInfo Corporation 2000) or idrisi 32 (Clark Laboratories 2001).

Plot boundaries were digitized and the area of land surveyed was

calculated. Topographical information was extracted from a 50 m

digital terrain model (Panorama, Ordnance Survey, Southampton,

UK). The average altitude of each plot was calculated as the arithme-

tic mean of altitudes at all 50 m points across the plot. A slope model

was produced in IDRISI, and the proportions of each plot with a

slope of<10� was calculated.The proportions of different habitats in plots were extracted from

the UK Land Cover Map 2000 (LCM2000; Fuller et al. 2005), a UK

habitat map with 25-m resolution produced by classification of satel-

lite images. We used data from five habitat types classified from

LCM2000 subclass level 2: (i) dwarf shrub heath, (ii) open shrub

heath, (iii) bog, (iv) rough grass and (v) acid grass. These five habitats

types accounted, on average, for 82% of the area within each plot.

We combined Dwarf Shrub Heath and Open Shrub Heath into its

lower level subclass 1 – hereafter termed ‘heather’. We then con-

structed a Principal Components Analysis (PCA) to describe the

broad differences in habitat between plots. The first two axes of this

PCA accounted for 76% of the variation in the data. The first axis,

which explained 45% of the variance, described a gradient of plots

from those dominated by heather to those dominated by acid grass,

bog and rough grass (eigenvectors: Heather = )0Æ876, Bog = 0Æ233,Rough grass = 0Æ374, Acid grass = 0Æ196). The second axis, which

explained 31% of the variance, distinguished plots which were domi-

nated by bog habitat from those which were grass dominated (eigen-

vectors: Heather = )0Æ015, Bog = )0Æ880, Rough grass = 0Æ335,AcidGrass = 0Æ337).

STATIST ICAL ANALYSIS

Because data were collected from multiple plots clustered in survey

areas, with differing numbers of plots in each area, it was necessary

to incorporate this lack of independence in our analysis. Therefore,

where possible we fitted a Generalized Linear Mixed Model

(GLMM), with survey area fitted as a random term. Where models

failed to converge using this approach, we used a Generalized

Linear Model (GLM), with survey area fitted as a fixed effect. This

latter approach is more conservative with respect to finding an

association with the explanatory covariates, as more variation in

the data is accounted for by survey area as a fixed effect. Models

were fitted using either a Poisson error structure and a log link

function, when examining wader abundance; or a binomial error

structure and logit link function, when examining wader population

change. All models were scaled to correct for over-dispersion.

Denominator degrees of freedom in GLMM were estimated using

Satterthwaite’s formula (Littell et al. 1996). Additionally, as timeTable

1.Detailsofplotsandcovariatesusedin

theanalysisforeach

survey

area

Survey

area

Plots

Plotsize

(km

2)

Totalarea

surveyed

(km

2)

Originalraven

abundance

(per

km

2)

Repeatraven

abundance

(per

km

2)

Change

(%)

PCA1scores

PCA2scores

%<

10�

slope

Altitude(m

)

Berwyn

514Æ4

(±7Æ01)

71Æ9

0Æ18(±

0Æ10)

1Æ33(±

0Æ62)

656

)0Æ57(±

0Æ06)

0Æ03(±

0Æ02)

96(±

18)

442(±

11)

Exmoor

916Æ1

(±10Æ91)

144Æ6

0Æ00(±

0Æ00)

0Æ00(±

0Æ00)

0*

)0Æ13(±

0Æ06)

0Æ10(±

0Æ02)

60(±

6)

309(±

21)

LakeDistrict

12

5Æ9

(±0Æ75)

70Æ4

0Æ31(±

0Æ09)

0Æ50(±

0Æ11)

59

0Æ06(±

0Æ05)

0Æ15(±

0Æ03)

82(±

4)

335(±

34)

Lew

is&

Harris

18

6Æ0

(±0Æ40)

108Æ5

0Æ60(±

0Æ19)

0Æ58(±

0Æ11)

)3

)0Æ23(±

0Æ09)

)0Æ27(±

0Æ08)

97(±

1)

76(±

6)

Migneint

29Æ8

(±1Æ35)

19Æ6

0Æ06(±

0Æ05)

0Æ60(±

0Æ12)

907

0Æ03(±

0Æ12)

0Æ26(±

0Æ03)

82(±

5)

425(±

15)

NEScotland

12

18Æ2

(±3Æ97)

218Æ9

0Æ06(±

0Æ02)

0Æ31(±

0Æ11)

413

)0Æ38(±

0Æ08)

)0Æ00(±

0Æ02)

61(±

7)

509(±

44)

NorthPennines

12

6Æ1

(±0Æ04)

72Æ7

0Æ08(±

0Æ03)

0Æ12(±

0Æ04)

51

0Æ01(±

0Æ06)

)0Æ10(±

0Æ10)

77(±

6)

508(±

34)

NorthYorkshire

10

6Æ3

(±0Æ53)

62Æ8

0Æ04(±

0Æ03)

0Æ07(±

0Æ07)

58

0Æ00(±

0Æ09)

0Æ03(±

0Æ06)

81(±

3)

494(±

22)

South

Pennines

22

10Æ2

(±3Æ57)

223Æ3

0Æ00(±

0Æ00)

0Æ06(±

0Æ03)

NA

)0Æ08(±

0Æ05)

)0Æ03(±

0Æ04)

75(±

3)

355(±

13)

WestFlows

16

18Æ3

(±4Æ01)

292Æ1

0Æ00(±

0Æ00)

0Æ15(±

0Æ07)

NA

)0Æ43(±

0Æ06)

)0Æ14(±

0Æ05)

89(±

2)

205(±

23)

Data

shownare

mean(±

1SE).

PCA,principalcoordinatesanalysis;NA,notapplicable,since

ravenswerenotrecorded

inthefirstsurveys.

*Figuresgiven

inSim

etal.2005forraven

change

inExmoorwereincorrect.

256 A. Amar et al.

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

span between original and repeat surveys varied (from 9 to

21 years) between survey areas and because there were small differ-

ences in the distances between transects in different survey areas,

our ‘survey area’ term also controlled to some degree for the poten-

tial influence that these differences may have had on our measures

of abundance or population change. All analyses were carried out

in SAS version 9.1 (SAS Institute Inc 2004).

In each analysis, we constructed full models, as recommended for

hypothesis testing (Whittingham et al. 2006). Environmental vari-

ables, which included the first and second axis from the PCA describ-

ing habitat within each plot, and the average slope and altitude of

each plot were included in the model together with our raven term

(either abundance or change in abundance), the significance of which

was then examined from the results of a Type III analyses. Thus, our

models attempted to control for any additional influence of habitat or

topography on wader abundance or change before testing the

hypothesis that raven abundance was negatively associated with

wader abundance or changes. Mean values are presented as mean ±

1 SEunless otherwise stated.

Spatial association between wader and raven

abundance

For this analysis, we modelled wader density by using the counts of

each wader species on each plot during the repeat surveys as the

response variable and the log of the plot area as an offset. We

weighted the analysis by the square root of plot size to account for

our increased confidence in the abundance estimate on larger plots, as

larger plots were more likely to be representative of the overall abun-

dance and less susceptible to small-scale stochasticity. We carried out

these analyses within a GLM, fitting survey area as a fixed effect,

because of lack of model convergence when survey area was fitted as

a random effect in a mixed model. Raven abundance was included in

the full model, together with the habitat and typographical variables,

and we examined the effect of raven abundance using a Type III anal-

ysis.

Temporal association between wader change and

raven abundance or change

We used a binomial measure of wader population change on a plot as

our response variable, by fitting the repeat survey count in each plot

as the numerator, and the sum of the original and repeat counts in the

plot as the denominator. This model therefore examines the degree of

increase or decrease on a plot, and automatically weights appropri-

ately for small or large counts. In this analysis, we excluded all plots

with zero counts in the first episode because these ‘colonization

events’ would have a disproportionately higher value in the response

variable than plots showing large increases of pre-existing popula-

tions. However, numbers of plots excluded were relatively small, as

follows (excluded plots ⁄ total plots for the species): Golden Plover

n = 4 ⁄ 86; Lapwing n = 5 ⁄ 69; Dunlin n = 6 ⁄ 69; Curlew n = 7 ⁄ 95;Snipe n = 13 ⁄ 93). Plots that held no birds in both the original and

repeat surveys were also excluded. Raven abundance in the repeat

survey was included in the full model, together with the habitat and

typographical variables, and we examined the effect of ravens abun-

dance using a Type III analysis.

We used this same model structure to examine whether changes in

wader abundance were associated with changes in raven abundance,

replacing raven abundance as the explanatory variable with changes

in raven abundance, thismeasure was simply the abundance of ravens

present in the re-survey minus the abundance of ravens present in the

original-survey.

Results

CHANGES IN RAVEN AND WADER POPULATIONS

The full data on wader abundance and changes in abun-

dance between the original and repeat surveys are presented

elsewhere (Sim et al. 2005), with only the key data included

here for completeness (Table 2). Raven populations showed

increasing trends in all our survey areas, apart from on

Lewis and Harris (Sim et al. 2005). On Exmoor, no ravens

were recorded during either the original or repeat surveys. In

the South Pennines and the West Flows, ravens were first

recorded on the repeat visits, where none were counted origi-

nally. Elsewhere, there were increases of between 50% to

60% in the three northern England survey areas (Lakes,

North Pennines and North Yorkshire), and increases of over

400% in northeast Scotland and in the two survey areas in

Wales (Table 1).

Table 2. Change (%) in the average number of eachwader species counted in each survey area

Survey area Golden plover Lapwing Dunlin Curlew Snipe

Berwyn )89% (1Æ8, 0Æ2) )100% (2Æ8, 0Æ0) NA (0Æ0, 0Æ0) )78% (28Æ8, 6Æ2) )56% (1Æ8, 0Æ8)Exmoor NA (0Æ0, 0Æ0) )100% (0Æ1, 0Æ0) NA (0Æ0, 0Æ0) )63% (0Æ8, 0Æ3) +167% (0Æ3, 0Æ8)Lake District )95% (2Æ0, 0Æ1) )63% (6Æ5, 2Æ4) )100% (0Æ1, 0Æ0) )39% (15Æ3, 9Æ3) +67% (4Æ6, 7Æ7)Lewis & Harris +60% (22Æ7, 36Æ3) )19% (1Æ6, 1Æ3) +11% (30Æ3, 33Æ6) +133% (0Æ9, 2Æ1) +46% (3Æ5, 5Æ1)Migneint )60% (5Æ0, 2Æ0) )100% (2Æ5, 0Æ0) 0% (0Æ5, 0Æ5) )100% (15Æ5, 0Æ0) )50% (1Æ0, 0Æ5)Northeast Scotland )46% (18Æ3, 9Æ8) +17% (5Æ3, 6Æ2) )34% (3Æ2, 2Æ1) +8% (20Æ7, 22Æ3) +90% (2Æ0, 3Æ8)North Pennines +2% (28Æ5, 29Æ0) )26% (10Æ5, 7Æ8) )14% (4Æ3, 3Æ7) )29% (44Æ5, 31Æ5) 0% (4Æ3, 4Æ3)North Yorkshire )29% (32Æ3, 23Æ1) )45% (14Æ5, 8Æ0) )56% (3Æ6, 1Æ6) )41% (37Æ2, 22Æ1) )5% (4Æ3, 4Æ1)South Pennines +19% (12Æ6, 15Æ0) )9% (3Æ4, 3Æ1) )62% (2Æ6, 1Æ0) +116% (9Æ2, 19Æ9) )7% (1Æ5, 1Æ4)West Flows +38% (25Æ4, 35Æ0) )14% (0Æ7, 0Æ6) )5% (16Æ9, 16Æ0) )36% (3Æ3, 2Æ1) )64% (2Æ5, 4Æ1)

Figures in parentheses show the average numbers counted in each survey area in the original and repeat surveys. No statistical tests of

these changes were made. However, tests based on these data are given in Sim et al. (2005). Data in bold, are those survey areas where

changes were significant according to Sim et al. (2005). The significance results from Sim et al.’s (2005) North Wales survey area are

applied to both our Welsh survey areas.

NA, not applicable, given that no birds were counted in either period.

Impacts of raven populations on upland waders 257

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

SPATIAL ASSOCIATIONS BETWEEN WADER AND RAVEN

ABUNDANCE

Where survey areas had plots with none or very few individuals

of a species recorded during both surveys, it was necessary to

exclude these survey areas from the analyses to allow the mod-

els to converge (Table 3). After controlling for the effects of

survey area, habitat and topographical variables, there were

no significant negative relationships between raven abundance

and any of the wader species (Table 3). However, there was a

significant positive relationship between lapwing abundance

and raven abundance (Table 3), indicating that lapwings were

more abundant on plots withmore ravens.

TEMPORAL ASSOCIATIONS BETWEEN WADER

POPULATIONS AND RAVEN ABUNDANCE

There were no significant relationships between abundance of

ravens in 2000–2002 and the change in abundance of any of

the wader species (Table 4). There was amarginal tendency for

curlew to have declined more and for dunlin to have declined

less on plots with more ravens in 2000–2002 (P = 0Æ09 in both

cases). Thus, from these analyses there was no substantive evi-

dence to suggest that any of the wader species showed a higher

level of decline at plots with higher raven abundance in 2000–

2002.

TEMPORAL ASSOCIATIONS BETWEEN WADER

POPULATIONS AND CHANGES IN RAVEN ABUNDANCE

There were no significant negative relationships between raven

population change and the changes in the abundance of any of

the wader species. However, for both lapwing (P = 0Æ06) andcurlew (P = 0Æ08), there was a marginal tendency for popula-

tions to decline on plots where ravens had increased (Table 5).

These analyses also revealed a strong positive association

between raven changes and dunlin changes, indicating that

dunlin were more likely to have increased on plots where raven

abundance had increased (Table 5). Overall, these results pro-

vide no substantive evidence that increases in raven abundance

are negatively associatedwith changes in breedingwader abun-

dance.

Discussion

Our analysis did not find any strong evidence to indicate spa-

tial or temporal links between recovering raven populations

and the declines of five upland wader species. Within the limi-

tations of a broad scale correlative study, and accepting that

the re-survey data used are now several years old, we suggest

that these analyses do not provide support for controlling

raven populations in the interest of conservation of upland

breeding waders. However, the near significant negative

relationships between temporal changes in raven and lapwing

and curlew populations is interesting and supports the need for

further research on the impacts that ravens might be having on

these species. Whether an absence of significant negative Table

3.Outputs

(parameter

estimates,Fvalues

andsignificance)from

fullGeneralizedLinearModels,controllingfortheinfluence

ofhab

itat

[principalcoordinatesan

alysis(PCA)1

andPCA2]an

d

topographicalvariables(altitudeandslope),before

testingforaspatialassociationbetweentheabundan

ceofthefive

wad

erspeciesan

dtheab

undance

ofravensduringtheresurvey

Golden

Plover

Lapwing

Dunlin

Curlew

Snipe

Variable

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Intercept

)6Æ621

***

)8Æ325

***

)8Æ439

***

)8Æ429

***

)6Æ304

***

PCA1

0Æ635

F1,94=

3Æ87*

)0Æ427

F1,91=

0Æ58

0Æ466

F1,80=

2Æ14

)0Æ214

F1,102=

0Æ31

0Æ095

F1,102=

0Æ07

PCA2

0Æ245

F1,94=

0Æ41

)0Æ307

F1,91=

0Æ23

0Æ031

F1,80=

0Æ00

0Æ091

F1,102=

0Æ05

)0Æ149

F1,102=

0Æ10

Altitude

0Æ051

F1,94=

17Æ76***

)0Æ036

F1,91=

4Æ44*

0Æ051

F1,80=

6Æ83**

)0Æ023

F1,102=

4Æ39*

)0Æ019

F1,102=

2Æ31

Slope

1Æ391

F1,94=

3Æ08p=

0Æ07

1Æ350

F1,91=

1Æ87

2Æ739

F1,80=

3Æ68*

2Æ306

F1,102=

12Æ28***

0Æ636

F1,102=

0Æ65

Raven

abundance

0Æ200

F1,94=

0Æ55

1Æ238

F1,91=

4Æ70*

0Æ064

F1,80=

0Æ07

0Æ147

F1,102=

0Æ12

0Æ154

F1,102=

0Æ23

Survey

area

F8,94=

16Æ59***

F7,91=

7Æ08***

F6,80=

9Æ29***

F9,102=

21Æ72***

F9,102=

11Æ82***

Survey

areawasfitted

asafixed

effect.Resultsin

bold

weresignificant(*P

<0Æ05,**P

<0Æ01,***P<

0Æ001).Toenable

modelsto

convergeweexcludedata

from

studyarealackingthespecies,

orwherecounts

wereverylow

inboth

surveys,

thusweexcluded

data

from

Berwynplots

forlapwinganddunlin,Exmoorplots

forgolden

plover,lapwinganddunlin,Lakes

plots

fordunlin,

andMigneintplots

forcurlew

.

258 A. Amar et al.

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

Table

4.Outputs(parameter

estimates,Fvalues

andsignificance)from

fullGeneralizedLinearMixed

Models,controllingfortheinfluence

ofhab

itat

[principalcoordinatesanalysis(PCA)1

andPCA2]and

topographicalvariables(A

ltitudeandSlope),b

efore

testingforatemporalassociationbetweenchanges

inwad

erab

undance

ofthefivewader

speciesandtheabundance

ofravensduringtheresurvey

Golden

Plover

Lapwing

Dunlin

Curlew

Snipe

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Main

effects

Intercept

0Æ424

NS

)0Æ857

*)0Æ397

NS

)0Æ903

NS

)0Æ571

NS

PCA1

)0Æ383

F1,51=

2Æ51

)0Æ300

F1,57=

0Æ38

0Æ138

F1,26=

0Æ40

)0Æ592

F1,72=

3Æ660Æ05

)0Æ383

F1,35=

0Æ66

PCA2

0Æ738

F1,63=

5Æ08*

0Æ832

F1,51=

1Æ59

)0Æ104

F1,42=

0Æ07

0Æ331

F1,71=

0Æ81

0Æ206

F1,53=

0Æ13

Altitude

)0Æ011

F1,3=

4Æ82

0Æ012

F1,18=

1Æ71

)0Æ008

F1,4=

0Æ88

0Æ020

F1,4=

4Æ10*

0Æ009

F1,20=

0Æ78

Slope

NA

NA

NA

NA

0Æ378

F1,14=

0Æ13

)0Æ158

F1,73=

0Æ05

0Æ392

F1,42=

0Æ15

Raven

abundance

0Æ044

F1,43=

0Æ04

)0Æ137

F1,21=

0Æ68

0Æ318

F1,34=

2Æ940Æ09

)0Æ727

F1,76=

2Æ950Æ09

0Æ187

F1,29=

0Æ22

Random

effect

Survey

area

0Æ047

0Æ160

0Æ051

0Æ720

0Æ072

Survey

areawasfitted

asarandom

term

.Resultsin

bold

weresignificant(N

S,non-significant;*P<

0Æ05).

NA,notapplicable,asslopecovariate

hadto

beremoved

toallow

modelsto

converge.

Table

5.Outputs(parameter

estimates,Fvalues

andsignificance)from

fullGeneralizedLinearMixed

Models,controllingfortheinfluence

ofhabitat[principalcoordinatesan

alysis(PCA)1

andPCA2]and

topographicalvariables(A

ltitudeandSlope),b

efore

testingforatemporalassociationbetweenchanges

inwader

abundance

ofthefivewaderspeciesandthechangeinraven

abundance

betweenthesurveys

Golden

Plover

Lapwing

Dunlin

Curlew

Snipe

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Estim

ate

Fvalue

Main

effects

Intercept

)0Æ514

NS

)1Æ242

*0Æ283

NS

)1Æ111

NS

)0Æ942

NS

PCA1

)0Æ286

F1,34=

1Æ37

)0Æ243

F1,59=

0Æ24

0Æ463

F1,57=

3Æ740Æ05

)0Æ597

F1,72=

3Æ610Æ06

)0Æ272

F1,50=

0Æ31

PCA2

0Æ672

F1,46=

4Æ57*

0Æ822

F1,56=

1Æ85

)0Æ263

F1,49=

0Æ47

0Æ287

F1,71=

0Æ62

0Æ292

F1,57=

0Æ28

Altitude

)0Æ006

F1,4=

1Æ78

0Æ023

F1,28=

3Æ870Æ05

)0Æ013

F1,7=

1Æ90

0Æ020

F1,76=

4Æ00*

0Æ017

F1,21=

2Æ44

Slope

0Æ981

F1,11=

3Æ07

NA

NA

)0Æ151

F1,20=

0Æ02

)0Æ027

F1,75=

0Æ00

0Æ589

F1,52=

0Æ33

Raven

change

14Æ24

F1,70=

1Æ70

)91Æ40

F1,52=

3Æ460Æ06

19Æ98

F1,52=

8Æ53**

)67Æ73

F1,73=

2Æ980Æ08

)56Æ17

F1,55=

1Æ82

Random

effect

Survey

area

0Æ005

0Æ178

0Æ134

0Æ634

0Æ179

Survey

areawasfitted

asarandom

term

.Resultsin

bold

weresignificant(N

S,non-significant;*P<

0Æ05,**P

<0Æ01).

NA,notapplicable,asslopecovariate

hadto

beremoved

toallow

modelsto

converge.

Impacts of raven populations on upland waders 259

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

relationships between raven increases and wader populations

continues to hold in the intervening years since the resurveys

were undertaken is unknown. However, given that raven

changes in this analysis span a range of values varying from a

small decline to over 500% increases in some areas, it seems

unlikely that any such negative association would necessarily

emerge, even with the observed, continuing overall increase in

raven numbers (Risely et al. 2008).

Our data come from multiple survey areas throughout Brit-

ain, but these areas were not randomly selected. In theory,

therefore, if the plots used in this study were not representative

of the British uplands as a whole, we may have failed to detect

a significant effect of ravens when one really exists (i.e. a type II

error). Our plots could have differed from the wider uplands in

a few respects. First, some of our plots were originally selected

on the basis that they were particularly good for waders, and

so are likely to have had more waders than the average. Such

plots might be more resilient to the impacts of ravens, which

could in theory explain why no significant negative relation-

ships were found. Sim et al. (2005) also identified another

potential bias, plots where access was refused (and which were

therefore not re-surveyed) tended to have higher wader abun-

dance. To explore whether either of these issues could have

influenced our ability to find significant relationships, we tested

for an interaction between the effect of raven change on wader

changes, and the abundance of the wader species during the

original survey, by fitting the original abundance of eachwader

species (birds per ha) and the interaction between this and

raven change. For dunlin, we found a weak but significant

interaction between raven change and the original abundance

of dunlin on a plot (F1,51 = 4Æ89, P = 0Æ03), with the positive

relationship between dunlin change and ravens change being

stronger at sites where dunlin abundance was originally lower.

However, no other interactions were found, and thus we con-

sider it unlikely that these possible biases have had a strong

influence on the outcome of this study.

Tharme et al. (2001) found higher numbers of golden plo-

ver, lapwing and curlew on moorland managed for red grouse

shooting compared with moorland not managed for grouse

shooting. However, they found no difference in the abundance

of ravens between these two land management classes, adding

some support to the findings reported in the current study, par-

ticularly with respect to our spatial analysis. Some studies have

documented a negative effect of ravens on avian breeding suc-

cess. For example, Brambilla, Rubolini & Guidali (2004)

found a negative effect of raven occupancy on peregrine falcon

Falco peregrinus breeding success, Gaston & Elliot (1996)

found that risk of raven predation on Brunnich’s guillemot

Uria lomvia may influence nesting locations, and Kelly,

Etienne & Roth (2005) found higher predation rates on great

egret Ardea alba nests at heronries where raven presence and

productivity were highest. However, another study examining

the influence of nesting ravens on songbird breeding abun-

dance in Poland, found a positive effect of breeding ravens on

songbird densities, and in particular skylark Alauda arvensis

densities (Tryjanowski 2001). The positive relationship

between dunlin and raven changes in themain study is unlikely

to be causal, a more likely explanation is that another as yet

unknown and unmeasured variable is correlated with both

raven and dunlin change. For example, raven recovery may

have been greatest on grousemoors, where predator control of

red foxes Vulpes vulpes and carrion crows Corvus carone may

have been beneficial for dunlin populations (Jonsson 1990,

1991; Sotherton et al. 2009); therefore, this might be one expla-

nation for the apparent positive relationship between ravens

and dunlin.

So, why is it that raven increases have had little detectable

effect on upland waders in our analyses? Although we know

that ravens predate wader eggs and young chicks, we know

little about the extent of this predation. It may be that preda-

tion rates are not particularly high, or predation may tend to

occur early in the season, therefore allowing waders to re-lay

and still successfully fledge young (Ratcliffe 1997), as has been

documented for other species (Moorcroft & Wilson 2000). An

alternative explanation is that ravens do have an impact on

wader breeding success, but that these upland wader popula-

tions are drawing birds in from areas where breeding success is

much higher, thereby acting as sinks to the detriment of the

national populations (Pulliam 1988; Baillie et al. 2002). To

explore this issue would necessitate examining wader breeding

success in relation to varying raven abundance combined with

survival estimates of wader species to estimate if populations

were self sustaining in the presence of higher raven predation,

which was clearly outside the scope of this study. Another

potentially important issue, but one not considered within this

study is whether the effects of ravens on waders differs depend-

ing on whether the raven population in an area is made up of

breeding birds or non-breeding flocks. Predation by crows on

upland birds is known to differ between breeding and non-

breeding birds, with predation on willow ptarmigan Lagopus

lagopus being largely attributable to territorial birds (Erikstad,

Blom & Myberget 1982). Risk of predation by ravens of the

threatened desert tortoise Gopherus agassizii in California,

USA, was determined by either proximity to successful nests

or the spatial distribution of non-breeding flocks, suggesting

that both groups were important predators (Kristan & Boar-

man 2003). Findings from another study on raven diet on the

Italian island of Vulcano suggested that breeding ravens were

more predatory than the non-breeding flocks present on the

island (Sara & Busalacchi 2003). Thus, it would be useful for

any further research on raven predation in the British uplands

to examine the relative influence of breeding and non-breeding

individuals.

Although the negative relationships between raven change

and lapwing and curlew declines were non-significant, these

associations are still worthy of further investigation. We know

that lapwing populations on lowland wet grassland may be

susceptible to corvid predation, and that in areas with high

densities of predators, crow and fox control can improve their

breeding success (Bolton et al. 2007a). Furthermore, prelimin-

ary results from a predator removal experiment on moorland

habitat in northern England, found that lapwing and curlew

were the only two wader species to show a significant positive

numerical response in response to predator control, suggesting

260 A. Amar et al.

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

that these speciesmight be particularly sensitive to any increase

in predators (Fletcher, Jones & Baines 2009). Any further

research should focus on establishing the causes of nesting fail-

ures of lapwing and curlews breeding in the uplands, and the

importance of ravens as a cause of failure. This can now be

achieved relatively simply with the use of remote camera tech-

nology which has been recently pioneered for examining the

causes of nesting failure in waders (Bolton et al. 2007b).

The issue of scale must also be considered when interpreting

the results from this study. Our study was carried out at a

national scale using data from multiple plots in multiple study

areas. The lack of any negative relationships at this large scale

does not mean that ravens could not have an impact at a smal-

ler scale or at some individual sites; however, if such local

effects are frequent then we would still have expected to see

such signals in our analyses. Indeed, thismight explain the near

significant results for lapwing and curlew.Where opportunities

exist, ravens are known to sometimes specialize on eggs and

young of certain bird species, for example black-legged kittiwa-

kes Rissa tridactyla (Tella, Torre & Ballesteros 1995) or great

egrets (Kelly et al. 2005) and there is evidence to suggest that

individual ravens may specialize on wader eggs or chicks. For

example, Whitfield (2002) suggested the decline of dotterel

Charadrius morinellus at one intensively monitored site in the

Scottish highlands was a local effect because of increased pre-

dation by ravens. On this site, numbers of breeding dotterel

correlated negatively with sighting rates of ravens in spring.

The impact of specialist predatory behaviour by ravens is illus-

trated by the fact that in one instance a pair was believed to

account for 53% of dotterel clutches (Sue Holt, unpublished

data, cited in Ratcliffe 1997). However, Whitfield (2002) using

paired observations on the same sites in different years

concluded that the more widespread reductions in breeding

dotterel were not related to changes in numbers of ravens.

Requests to control recovering populations of predators

and scavengers to protect populations of some prey

species (http://www.scottish.parliament.uk/business/petitions/

docs/PE187.htm), needs to be tackled within a logical frame-

work. In deciding whether to allow recovering predator popu-

lations to be controlled, it is important that policy makers

consider the evidence of whether the specific predator is indeed

having any negative impact on the prey of interest, and

whether the predator population can be controlled without

risks to its long-term conservation status. This correlative

study has suggested that there is at present little substantive

evidence to justify the control of raven populations for the

large-scale conservation benefit of breeding wader popula-

tions, although the negative temporal association for lapwing

and curlew are worthy of further research. The data do not

show strong indications that wader populations were more

likely to have declined where ravens had increased, the logical

corollary of which suggests that removing ravens from these

upland sites would not benefit wader populations. However,

because these analyses were correlative, experiment removals

would still be required where impacts are suspected to ulti-

mately establish cause and effect. Further research is urgently

needed to understand what factors are actually driving these

population declines. This study therefore highlights the need to

obtain robust evidence on the effects that protected predators

have on their prey, prior to initiating lethal control, and may

provide a framework for the types of analyses that should be

undertaken to help decision makers decide on whether to issue

control licences as future conflicts arise. If decisions are made

with out such information, resources could be targeting inap-

propriately away from the real cause of any prey decline, and

could potentially and needlessly jeopardize the conservation

status of the protected predator involved.

Acknowledgements

We are very grateful to Ellen Wilson for help with digitizing the plot data and

extracting information on area, altitude and slope, and to Helen Mitchell for

map making. We thank Richard Hasting at the Scottish Government for data

on the number of licences granted for lethal ravens control. The original RUBS

project was funded by RSPB, EN, Defra and CCW. We thank JNCC, SNH

and the Lake District and Yorkshire Dales National Parks Authorities for help

and assistance. Particular thanks are due to David Stroud (JNCC), David

Smallshire (Defra), Sian Whitehead (CCW), Phil Whitfield (SNH), Phil Taylor

(LDNPA), Ian Court (YDNPA) and David Noble (BTO) for their help and

advice on the RUBS data. This project was a partnership project funded by

SNH, RSPB and ACES, and we are grateful to Andy Douse and Ben Ross

(SNH) for his help with this project. We are grateful to JeremyWilson,Murray

Grant and James Pearce-Higgins for advice throughout this project, and to the

Associate Editor, Dave Parish and an anonymous reviewer for suggestions

which greatly improved themanuscript.

References

Amar, A., Thirgood, S., Redpath, S. & Pearce-Higgins, J. (2008) The impact of

raptors on the abundance of upland passerines and waders. Oikos, 117,

1143–1152.

Baillie, S.R., Sutherland, W.J., Freeman, S.N., Gregory, R.D. & Paradis, E.

(2002) Consequences of large-scale processes for the conservation of bird

populations. Journal of Applied Ecology, 37, 88–102.

BirdLife International (2004)Birds in Europe: Population Estimates, Trends and

Conservation Status. BirdLife Conservation Series No. 12. Birdlife Interna-

tional, Cambridge.

Bolton, M., Tyler, G., Smith, K. & Bamford, R. (2007a) The impact of preda-

tors control on lapwing Vanellus vanellus breeding success on wet grassland

nature reserves. Journal of Applied Ecology, 44, 534–544.

Bolton, M., Butcher, N., Sharpe, F., Stevens, D. & Fisher, G. (2007b) Remote

monitoring of nests using digital camera technology. Journal of Field Orni-

thology, 78, 213–220.

Brambilla, M., Rubolini, D. & Guidali, F. (2004) Rock climbing and raven

Corvus corax occurrence depress breeding success of cliff-nesting peregrines

Falco peregrinus.Ardeola, 51, 425–430.

Brown, A.F. & Shepherd, K.B. (1993) Amethod for censusing upland breeding

waders.Bird Study, 40, 189–195.

Byrkjedal, I. (1987) Antipredator predator behaviour and breeding success in

GreaterGolden Plover and EurasianDotterel.Condor, 89, 40–47.

Chamberlain, D.E., Glue, D.E. & Toms, M.P. (2009) Sparrowhawk Accipiter

nisus presence and winter bird abundance. Journal of Ornithology, 150, 247–

254.

ClarkLaboratories (2001) Idrisi 32.2. ClarkUniversity.

Erikstad, K.E., Blom, R. & Myberget, S. (1982) Territorial hooded crows as

predators on willow ptarmigan nests. Journal of Wildlife Management, 46,

109–114.

Ewins, P.J., Dymond, J.N. & Marquiss, M. (1986) The distribution, breeding

and diet of RavensCorvus corax in Shetland.Bird Study, 33, 110–116.

Fletcher, K., Jones, C. & Baines, D. 2009. The Game and Wildlife Conservation

Trust Review of 2008. The Game andWildlife Conservation Trust, Fording-

bridge, Hampshire, UK.

Fuller, R.M., Cox, R., Clarke, R.T., Rothery, P., Hill, R.A., Smith, G.M.,

Thomson, A.G., Brown, N.J., Howard, D.C. & Stott, A.P. (2005) The UK

land cover map 2000: planning, construction and calibration of a remotely

sensed use-oriented map of broad habitats. International Journal of Applied

Earth Observation and Geoinformation, 7, 202–216.

Impacts of raven populations on upland waders 261

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262

Gaston, A.J. & Elliot, R.D. (1996) Predation byRavensCorvus corax on Brun-

nich’s Guillemot Uria lomvia eggs and chicks and its possible impact of

breeding site selection. Ibis, 138, 742–748.

Gibbons, D.W., Gates, S., Green, R.E., Fuller, R.J. & Fuller, R.M. (1994) Buz-

zards Buteo buteo and Ravens Corvus corax in the uplands of Britain: limits

to distribution and abundance. Ibis, 137, S75–S84.

Hendricks, P. & Schlang, S. (1998) Aerial attacks by common ravens, Corvus

corax, on adult feral pigeons, Columbia livia. Canadian Field Naturalist, 112,

702–703.

Jonsson, P.E. (1990) The Dunlin Calidris alpine schinzii as a breeding bird in

Scania in 1990 – numbers, hatching success and population trends. Anser,

29, 261–272.

Jonsson, P.E. (1991) Reproduction and survival in a decline population of the

Southern Dunlin Calidris alpine schinzii. In Waders Breeding on Wet

Grasslands (Supplement, 61) (ed. H. Hotker), pp. 56–58. Wader Study

GroupBulletin, Nottingham.

Kelly, J.P., Etienne, K.L. & Roth, J.E. (2005) Factors influencing the

nest predatory behaviours of common ravens in heronries. Condor, 107,

402–415.

Klicka, J. &Winker, K. (1991) Observations of ravens preying on adult kittiwa-

kes.Condor, 93, 755–757.

Kristan W.B. III & Boarman, W.I. (2003) Spatial pattern of risk of common

raven predation on desert tortoises.Ecology, 84, 2432–2443.

Landa, A., Gudvangen, K., Swenson, J.E. & Roskaft, E. (1999) Factors associ-

ated with wolverine Gulo gulo predation on domestic sheep. Journal of

Applied Ecology, 36, 963–973.

Linnell, J., Nilsen, E.B., Lande, U.S., Herfindal, I., Odden, J., Skogen, K.,

Andersen, R. & Breitenmoser, U. (2005) Zoning as a means of mitigating

conflicts with large carnivores: principles and reality. People and Wildlife:

Conflict and Coexistence? (eds R.Woodroffe, S. Thirgood&A.Rabinowitz),

pp. 162–175. CambridgeUniversity Press, NewYork.

Littell, R.C., Milliken, G.A., Stroup, W.W. & Wolfinger, R.D. (1996) SAS

System forMixedModels. SAS Inst. Inc., Cary,NC.

MapInfo Corporation (2000) MapInfo Professional 6. MapInfo Corporation,

Troy, NY.

Marquiss,M. & Booth, C.J. (1986) The diet of RavensCorvus corax in Orkney.

Bird Study, 33, 190–195.

Marquiss, M., Newton, I. & Ratcliffe, D.A. (1978) The decline of the Raven

Corvus corax in relation to afforestation in southern Scotland and northern

England. Journal of Applied Ecology, 15, 129–144.

Moorcroft, D. &Wilson, J.D. (2000) The ecology of LinnetsCarduelis cannabi-

na on lowland farmland. Ecology and Conservation of Lowland Farmland

Birds (eds N.J. Aebischer, A.D. Evans, P.V. Grice & J.A. Vickery), pp.

173–181. BritishOrnithologists’ Union, Tring, Herts.

Newton, I. (1998)Population Limitation in Birds. Academic press, London.

Newton, I., Dale, L. & Rothery, P. (1997) Apparent lack of impact of Sparrow-

hawk on the breeding densities of some woodland songbirds. Bird Study, 44,

129–135.

Park, K.J., Calladine, J.R., Graham, K.E., Stephenson, C.M. & Wernham,

C.V. (2005)The Impacts of Predatory Birds onWaders, Songbirds, Gamebirds

and Fisheries Interests. A Report to Scotland’sMoorland Forum. BTO Scot-

land &Centre for Conservation Science, Stirling.

Petty, S.J., Lurz, P.W.W. & Rushton, S.P. (2003) Predation of red squirrels by

northern goshawks in a conifer forest in northern England: can this limit

squirrel numbers and create a conservation dilemma? Biological Conserva-

tion, 111, 105–114.

Pulliam, H.R. (1988) Sources, sinks, and population regulation. American

Naturalist, 132, 652–661.

Ratcliffe, D. (1997) The Raven: A Natural History in Britain and Ireland. T & A

DPoyser, London.

Redpath, S.M. & Thirgood, S.J. (1997) Birds of Prey and Red Grouse. The Sta-

tionaryOffice, London.

Risely, K., Noble, D.G. & Baillie, S.R. (2008) The Breeding Bird Survey 2007.

BTOResearchReport 508. British Trust forOrnithology, Thetford.

Sara, M. & Busalacchi, B. (2003) Diet and feeding habits of nesting and non-

nesting ravens (Corvus corax) on a Mediterranean island (Vulcano, Eolian

archipelago).Ethology Ecology & Evolution, 15, 119–131.

SAS Institute Inc. (2004) SAS ⁄ STAT 9.1 User’s Guide. SAS Institute Inc.,

Cary, NC.

Sim, I.M.W., Gregory, R.D., Hancock, M.H. & Brown, A.F. (2005) Recent

changes in the abundance of British upland breeding birds. Bird Study, 52,

261–275.

Sotherton, N., May, R., Ewald, J., Fletcher, K. & Newborn, D. (2009) Manag-

ing uplands for game and sporting interests – an industry perspective.Drivers

of Change in Upland Environments (eds A. Bonn, K. Hubacek, J. Stewart &

T. Allott), pp. 209–227. Routledge, Oxon.

Stahl, P., Vandel, J.M., Ruette, S., Coat, L., Coat, Y. & Balestra, L. (2002) Fac-

tors affecting Lynx predation on sheep in the French Jura. Journal of Applied

Ecology, 39, 204–216.

Tella, J.L., Torre, I. & Ballesteros, T. (1995) High consumption rate of Black-

LeggedKittiwakes.ColonialWaterbirds, 18, 231–233.

Tharme, A.P., Green, R.E., Baines, D., Bainbridge, I.P. & O’Brien, M. (2001)

The effect of management for red grouse shooting on the population density

of breeding birds on heather-dominated moorland. Journal of Applied Ecol-

ogy, 38, 439–457.

Thomson, D.L., Green, R.E., Gregory, R.D. & Baillie, S.R. (1998) The

widespread declines of songbirds inBritain donot correlatewith the spreadof

avianpredators.ProceedingsoftheRoyalSocietyLondonB,265,2057–2062.

Treves, A. & Naughton-Treves, L. (2005) Evaluating lethal control in the

management of human-wildlife conflict. People and Wildlife: Conflict or

Coexistence? (eds R.Woodroffe, S. Thirgood &A. Rabinowitz), pp. 86–106.

CambridgeUniversity Press, NewYork.

Tryjanowski, P. (2001) Proximity of raven (Corvus corax) nest modifies

breeding bird community in an intensively used farmland.Annales Zoologici

Fennici, 38, 131–138.

Whitfield, D.P. (2002) Status of breeding Dotterel Charadrius morinellus in

Britain in 1999.Bird Study, 49, 237–249.

Whittingham, M.J., Stephens, P.A., Bradbury, R.B. & Freckleton, R.P. (2006)

Why do we still use stepwise modelling in ecology and behaviour? Journal of

Applied Ecology, 75, 1182–1189.

Received 29 July 2009; accepted 9 January 2010

Handling Editor:MarkWhittingham

262 A. Amar et al.

� 2010 The Authors. Journal compilation � 2010 British Ecological Society, Journal of Applied Ecology, 47, 253–262