The Decline of the Raven, Corvus corax, in Relation to Afforestation in Southern Scotland
and Northern England
Author(s): M. Marquiss, I. Newton, D. A. Ratcliffe
Reviewed work(s):
Source: Journal of Applied Ecology, Vol. 15, No. 1 (Apr., 1978), pp. 129-144
Published by: British Ecological Society
Stable URL: http://www.jstor.org/stable/2402925 .
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JournalofAppliedEcology(1978), 15, 129-144
THE DECLINE OF THE RAVEN, CORVUS CORAX, IN
RELATION TO AFFORESTATION IN SOUTHERN
SCOTLAND AND NORTHERN ENGLAND
BY M. MARQUISS*, I. NEWTON*
*
AND
D. A. RATCLIFFEt
Institute
of Terrestrial
Ecology,12 Hope Terrace,Morningside,
Edinburgh
EH9 2AS and
t NatureConservancy
Council,19 BelgraveSquare,LondonSWI
SUMMARY
(1) The breedingraven population of southernScotland and Northumberlandwas
inbothnumbersand distribution.
formerly
(pre-1960)characterized
byyear-to-year
stability
The 123 knownpairsoccupiedtraditionalnest-sites
on cliffsor trees.Breedingdensitywas
correlatedwithaltitude(greateston highground)and land productivity
(greateron basepoorgranitethanon sedimentary
rocks).Thiswas probablylinkedwitha greateravailability
of sheepcarrionon highgroundand base-poorground.
(2) The breedingpopulationhas beendecliningsincethe1960s,and in 1974-5only55% of
formerregularnestingareas werestilloccupied,withbreedingpairsreducedto 44%. This
was mainlyassociatedwiththeafforestation
offormersheepwalk.The argumentis based on
a geographicalparallel(greatestdeclinein areaswithmostafforestation),
and on a temporal
parallel(desertionofparticularnestingareas coincidentwithplantingin thearea surroundingthenestsites).
(3) The level of afforestation
at whichravens desertedvaried betweennestingareas.
Probablyit dependedon theoverallqualityoftheoriginalhabitat,and thealternative
food
sourcesavailable: good habitatcould take moreafforestation
beforeit became untenable
thancould poor habitat.
(4) Not all desertionofnestingareas could be attributed
to afforestation:
fourravenpairs
weredispossessedfromcliffnest-sites
bygoldeneagleswhichrecolonizedsouthernScotland
overtheperiodconsidered,and at leastfiveotherpairsbyrockclimbers.Sheepmanagement
also improvedovertheyears,and mayhave contributed
to thedesertionof some marginal
nestingareas,byleadingto a reductionin theamountofcarrionavailable.Organo-chlorine
compoundsand persecutionare unlikelyto have beeninvolvedin thedecline.
therewas
no afforestation,
(5) In theLake District,a comparablehillarea withpractically
no declinein thenumbersof breedingravensoverthesame period.
(6) Among the pairs studiedin 1974-6, those occupyingthe most heavilyafforested
groundshowedmorenon-breeding,
and producedlater,smallerbroods thandid thoseon
lessafforested
ground.Thosenestingareas stilloccupiedand wheremorethan25% offormer
sheepwalkwithin5 kmhad beenafforested,
producedlaterand smallerbroodsthantheydid
beforeafforestation.
(7) To judge frompellets,sheepcarrionformeda majorpartofthediet,buta greatvariety
of otherfoodsavailable fromgrasslandwas also eaten.The percentagefrequency
of sheep
remainsin pelletsdeclinedsignificantly
withincreasingafforestation
in thearea aroundthe
nest-sites.
(8) The predictionis made of a furtherdeclinein raven breedingnumbersif blanket
afforestation
continuesto expandoverformerupland sheepwalk.
INTRODUCTION
Ravens (Corvuscorax) have longbeen associatedwiththeuplandsof southernScotland
and northernEngland.They are year-roundresidents,occupyingtraditionalterritories
? 1978BlackwellScientificPublications
0021-8901/78/0400-0129$02.00
129
130
Declineof theraveninBritain
based on particularcliffsor treeswhichprovidenest-sites.Theirbreedingpopulations
fromyearto year(Ratcliffe1962),but
wereformerly
stablein numbersand distribution
have beendecliningin theareas concerned,mostlysince1960.The declinecoincidedwith
in
of formersheepwalk,and withimprovements
the steadilyincreasingafforestation
sheep husbandry.Sheep carcases are an importantsource of food forravensin these
areas, as are manyothersmall animalsassociatedwithsheepwalk(Ratcliffe1962). In
1974-6,therefore,
an attemptwas made to (a) documenttheextentofdeclinein breeding
and decliningsheepstocks.
ravens,and (b) assess itsconnectionwithafforestation
STUDY AREAS
be regardedas a singleupland
The Cheviotsand theSouthernUplandsmayconveniently
and southernScotland.The region
Northumberland
regionoccupyingmuchofnorthern
is divisibleintoseveralsectors,in threeof whichindividualsummitsexceed 800 m: the
Merrick(845 m)-Corserine rangesof Galloway in the west; the Broad Law (841 m)
Hills in thecentre;and the
-White Comb (822 m) groupsof the Moffat-Tweedsmuir
withlarge
Cheviot(816 m) in theeast. The geologyis varied,but mainlysedimentary,
greywackesand shales in the SouthernUplands, and of
areas of Ordovician-Silurian
Carboniferousgritstoneand shales in the Cheviots.There are localized igneouscomplexes,mainlygranitesin Galloway and andesitein Cheviot.Excepton thegraniteand
of theseuplandshas for
base-rich,and thegood fertility
gritstone,
thesoilsare relatively
centuriessustainedlarge sheep populations.Grouse moors underheatherhave been
maintainedat severalplaces,but theseusuallycarrysome sheepas well.
Aftercenturiesunder sheep, theseuplands presentedan essentiallyopen grassyor
heatherylandscape,theearliernativeforestshavingbeen almost totallydestroyed.In
withoutseeinga singletreeor bush; in
some places one can stillwalk severalkilometres
oftreesalongstreams,whilein
othersscatteredbushesoccuron somehillsidesand fringes
yetotherplaces are smallshelterclumpsat variousstagesof growthor decay.Crags are
in thewesternGallowayarea, sparserin thecentralMoffat-Tweedsmuir
locallyplentiful
area, and fewin theeasternCheviotarea. Ravensnestforthemostparton cragsbutalso
in isolatedtrees,shelterclumpsor on theedgesof matureforest.
The ForestryCommissionplantedsomeofitsearliestforestsin thestudyarea. At first,
onlysmallareas wereinvolved,but after1945,therewas an enormousexpansionin the
concernswere
of unenclosedhillland. After1960 severalprivateforestry
afforestation
establishedwhichalso boughtand plantedmostsheepfarmswhichcameontothemarket.
The principaltreesplantedwereall coniferous,mainlynon-nativespecies.
entailstheremovalof sheepstocks,and thefencingof theforestmargin
Afforestation
fromgroundstillcarrying
sheep.The groundis ploughedand thetreeseedlingsplantedin
absence
the upturnedsod. In the
of sheep,the originalshort-cropped
vegetationsoon
in thefauna
This
in
to
a
litter.
turn
leads
changes
becomesluxuriantand develops thick
After
about
10
in
ravens
and
the
and itsaccessibility
to
otherpredators.
years treecanopy
is
and
the
forest
young
developsintoa dense
closes, thegroundvegetation shaded out,
remains
on
herbaceous
roadsides
and ridesand
chiefly
vegetation
impenetrable
thicket;
are
or
sterile
for
on areas which too wet,too high too
trees.
METHODS
Ravens weremuchsoughtafterby earlynaturalistsand egg collectors,so manyof the
M. MARQUISS et al.
131
traditionalnestingareas of thesebirdshave been knownfordecades. A nestingarea is
heredefinedas thearea containingall thenestsitesknownto have been associatedwith
particularterritories,
thatis,withparticularpairsor theirsuccessors.Makinguse ofearly
information,
togetherwithotherlocal knowledgeand searchingotherpossiblenestsites
systematically,
D.A.R. covered much of the studyarea several timesin the period
1945-70. Ravens are conspicuousbirds;theirbulkynestsare builtin easily-recognized
sitesand oftenlastforyears.In general,thepairsare also regularly
distributed
(Ratcliffe
1962),so byplottingthepositionsofknownpairson maps,gaps becameapparentwhich
werethencheckedforextrapairs.Over theyears,a knowledgeof theirdistribution
was
builtup, and it was unlikelythat,by 1970,any regularlyused siteswereunknownto us
exceptperhapsfora fewtreenests,away fromotherbreedingareas.
All placeswherebreedinghad previouslybeenrecordedwerevisitedat leasttwiceeach
season in 1974 and 1975,and detailswerenotedof presenceand breedingperformance.
Pelletswerecollectedfromaroundnestand roostsitesand analysedto givean indication
ofdiet.Each pelletwas brokenup byhandand all partswereseparatedand identified.
The
hair and featherremainswereidentified
microscopically
usingthe textof Day (1965),
supplemented
bylocal reference
material.In 1976,areas occupiedin the2 previousyears
were again visitedto obtain more data on breedingperformanceand to collectmore
pellets.
The patternof afforestation
was examinedfromstandard1:63360OrdnanceSurvey
maps,whichwereupdatedto showall theland undertrees,and thedates thatparticular
areaswereplanted.It was thenpossibleto (a) investigate
theuse ofnestingareas,breeding
performance
and food of ravensin relationto theextentof local afforestation;
and (b)
comparetheyearsthatravenswereknowntohavedesertedparticularareaswiththeyears
of planting.
RESULTS
Formernumbers
and distribution
In thepast, 123 ravennestingareas wererecordedin thestudyarea, withthelowest
densityin theeasternCheviothillsand thehighestin thewesternGallowayhills.We used
thedistanceto nearestneighbouras an indexof spacingbetweennestingareas. For pairs
whichapparentlyalternatedbetweentwo or morenest-sites,
we took thecentralpoint
betweenthesesitesfromwhichto measurethe distanceto the nest-site(s)in the next
nestingarea. Two main trendswereapparent.First,nearestneighbourdistanceswere
negatively
correlatedwithaltitude(Fig. 1); nestingareaswereclosertogether
on highhills
and further
apart on low hillsand moorland.Secondly,forany givenaltitude,nesting
in theGallowayand Moniaivehillsthanin theSouthAyrshire
areas wereclosertogether
hills(Fig. 1). Theywereevenfurther
or theMoffat-Tweedsmuir
apartin theLeadhillsand
further
stillin Northumberland.
Formerstability
ofpopulationand recentdecline
From 1946 onwards,thirty-six
ravennestingareas in Galloway wereexaminedfrewereused everyyearuntildesertion,and eightwereused
quently.Of these,twenty-eight
onlyin 1, 2 or 3 years.The twenty-eight
regularlyused areas wereexaminedin a totalof
245 nestingarea-years(between1946and thefinalyearofuse foreach nestingarea). The
nestin anyone nestingarea-yearwas notfoundon onlyfiveoccasions.On thisbasis,by
Declineof theraveninBritain
132
12
I(a)
(b)
8-
0
0.0
0-
0-O~~~~~~~~~~~
*
~~~00
pp
(C)
o 4a3
.
%
(d)
8
~~~~~~~~~~~~~~0
a)
>'
12''-
'
|
4-4
|
_-O
0
a)S
C
12
(f)
(e)
*4
0~~~~~~~~~
6
4-.
0'~~~~~~~~~~~Attd
..
0
0
0
.I .
0
4060
I
(m)I
0~~~~~~~
0O
00
a..sS1.
Altiude
FIG. 1. Nearest-neighbour
distancesbetweennestingareas plottedin relationto altitudeforsix
partsof thestudyarea. Each nestingarea heldone ravenpair. Some nestingareas werecloserto
one anotherthanto anyothernestingarea, so thesamenearest-neighbour
distanceheldforboth.
cannottherefore
be consideredstatistically
and in thefigure
The twomeasurements
independent,
thesepointsare connectedbylines.(a) S. Ayrshire,
(b) Galloway,(c) Moniaive,(d) Leadhills,(e)
Northumberland,
(f) Moffatand Tweedsmuir. -
of theregularlynesting
located
98wr
examiningall knownnestingareas we shouldhave
ravenpairsin thefirstyear,and all of themin the2 yearsof thesurvey.
For the purposesof our survey,it was essentialto separate those areas thatwere
regularlyused in thepast fromthosethatwereonlyoccasionallyused. By questioning
local observers,shepherdsand gamekeepers,
itwas establishedthatat leasteighty-one
of
the 123 nestingareas werein regularuse. The restwereeitherused sporadically(twentyfour)or wereof uncertainstatus(eighteen).
In 1974 and 1975,forty-four
of theformerregularareas, one of theoccasional areas
and threeofthosewhosepastuse was uncertainwerestilloccupied.The situationwas the
sameinbothyearsand no nestingarea was desertedbetweenthefirst
yearand thesecond.
Thus at least45% of theformerravennestingareas wereno longeroccupiedin 1974-5.
et al.
M. MARQUISS
133
The situationvariedbetweenregions,however,witha 750 declinein theeasternCheviot
Hills, and a 42% declinein the
area, a 38% declinein thecentralTweedsmuir-Moffat
thatsomeravensmerelyshiftedto breed
westernGallowayHills.Therewas no possibility
elsewherein the studycounties,with no overall decline in numbers.As some pairs
disappearedhowever,otherssometimesextendedtheirrange,and in one instancetwo
used by two pairsbecame alternativenest-sitesof thesame pair; i.e. two
cliffsformerly
groupsof ravens
nestingareas became one. Nor during1974-5 wereany non-breeding
in
but
occurred
as large
former
years,
were
in
study
area
even
birds
rare
the
seen. Such
flocksin otherpartsof Britain(Ratcliffe1962) and stilldo so.
Therelationship
betweenthepopulationdeclineand afforestation
Most ravenpairswereseen within3 km of knownnestsites,but a fewwereseen on
sheepwalkup to 5 kmaway,a distancegreaterthanhalftheaveragedistancebetweenthe
have had its
shouldtherefore
nestsitesof adjacentpairsin thesame area. Afforestation
but mightalso
greatesteffectson ravenswhereit occurredwithin3 kmof thenest-sites,
calculatedthe percentageof former
have had some effectat up to 5 km. We therefore
ofthenestsites
(up to 1975)within3 kmand 5 kmrespectively
sheepwalknow afforested
in everynestingarea.
within3 km
Onlysixteennestingareas(13%) out ofthetotalof 123had no afforestation
and onlyone (088%) had nonewithin5 km. In general,forformerregularlyused nesting
the less likelythe area was still to be
areas the greaterthe amount of afforestation,
occupied (Fig. 2). As predicted,this tendencywas strongerusing the figuresfor
afforestation
within3 km (P<0 001) thanwithin5 km (P=0 05). Some nestingareas
(in themostextremeinstance
continuedto be occupied despiteextensiveafforestation
within
5 km,had beenplanted),and
of
3
km,
and
sheepwalk
within
the
former
63%
76%
one totallyunafforested
nestingarea had been deserted.
The only formerirregularlyused nestingarea that was occupied in 1974-5 was
Similarly,of theareas whose formerstatuswas unknown,thethreeoccuunafforested.
ground(all less than20%). Nineteenof
littleafforested
pied in 1974-5wereon relatively
by
the thirty-six
unoccupiednestingareas in thesetwo categorieshad been afforested
morethan20% within5 km.The probabilitythatoccupancyoftheseareas was unrelated
to afforestation
was low (P= 0066, Fisher'sExact Test,in Siegel 1956).
For twenty-two
nestingareas,itwas knownexactlyin whichyearoccupationceased.
(These weremostlyin thelast 10 years,thoughmanyotherareas fellintodisusein the
previous10 years,withno recordof theprecisedate.) Aroundsomenestingareas,sheep
wereremovedtheyearbeforeplantingand aroundothersduringtheyearofplanting.For
before,immediately
all exceptone nestingarea, theyearof desertionwas immediately
after,or duringa yearoftreeplantingwithin3 kmofthenestsite(s)(Fig. 3). Aroundsome
of these nestingareas treeplantingalso occurredlong beforeand aftercessation of
occupancy,buttheprobabilityofthecoincidenceofthetwoeventsbychancealone is low
(P=0 002).
inrelationto afforestation
Breedingperformance
Non-breeding
At eightnestingareas,ravenpairswerepresentbutdid notnestin at leastone of the3
figuresfortheseareas werecomparedwiththoseof
yearsof survey.The afforestation
pairswereon areas that
eighteenareas wherepairsbredin all 3 years.The non-breeding
134
Declineof theraveninBritain
.-
e
U)o4
Co0
o
C5
0
z
8-
0-r
20
40
O-_--
A
60
_
-
% sheepwalk
100 afforested
80
.
4
within3km
''4 C
z
FIG. 2. Occupationofdifferent
nestingareas inrelationto extentofafforestation
within3 kmand
5 km of thenest-sites.
Year
1945
M
-
A
B
CY
1950
1965
1970
17
wihn5k
0
_
H
1960
CL
C
F
G
1955
_
-
-_
__
FIG.3. Thecdesetion
ofdforerregula
areas in relation
toexntoafforestationdae
nesting
-0within
- 3 kman
I
~~~~~~~~~~~
shwyaso
plnig
an afildcrlsso
steltyar
hnte
mof
the
nest-sites.Lie
A
0__
B
U
V
FG3.Te
SR dsrino
oftensFie.Lnsso
G~~~~nsigae
VU
omrrglrnsigaesi
er
fpatnan
a tl
-il
nesting area w
_
nw
eaint
foetto
ildcrlssostels
ob
cuid
ae
T
ihn3k
erwe
h
135
M. MARQUISS et al.
(Fig. 4). The difference
was moremarkedusing
wereon averagemoreheavilyafforested
thefiguresforafforestation
within5 km thanthosewithin3 km.The eightnestingareas
were once regularlyoccupied by breedingravens,so that inclusionof thesewith the
thirty-seven
desertionsincreasesthedeclinein breedingpopulationofregularpairsfrom
450 to 56%.
Layingdate,clutchsize and broodsize
The date on whichthefirsteggofa clutchwas laid was calculatedbyback-datingfrom
partiallylaid clutchesand partiallyhatchedbroods(incubationperiodassumedconstant
at 21 days and a 1 day intervalbetweenthelayingofeggs).The approximatelayingdate
(to thenearestweek)forsomeclutcheswas calculatedfromtheages of smallyoung(less
clutchesvariedfrom19Februaryto
thana weekold). Thus thedateoffirst
eggindifferent
23 March,withmostin thefirstweek of March. Clutchsize variedfromfourto seven
(overallaverage5 1), and fledgedbrood size fromzero to six (overallaverage2 7). The
onlybroodsofnilincludedin thiscalculationwerethosewhichstartedwitheggsand were
not destroyedby human agency(see later),i.e. thosewherefailurewas attributableto
'natural'causes.
Laying date was inverselycorrelatedwith clutch size using exact laying dates
(r= -0 70; d.f.= 10; 0 02>P>0 01) and using dates estimatedto the nearestweek
(r= -0 56; d.f.= 15; 0 02 > P > 0 01). The earliestlaid clutcheswerethelargest.
within5 kmbutless
Layingdatewas strongly
correlatedwiththeextentofafforestation
correlatedwiththeextentof
so within3 km (Table 1). Clutchsize was not significantly
afforestation,
evenwhendata fromall threeyearsweregrouped.Brood sizewas inversely
in 1974 and 1976 but not in 1975. All six
correlatedwiththe extentof afforestation
correlation
coefficients
forbroodsizewerenegative,and inall 3 yearsthecorrelationwith
a) 4z
Z
0
Oc0,
0
*-_
20
s
40
S
|
60
s
s
80
ffi
% sheepwalk
100 afforested
Iwithin
3 km
._
FIG. 4. Incidenceof non-breeding
in different
occupiednestingareas in relationto theextentof
afforestation
within3 km(P= 0082) and 5 km(P=0 0017)of thenestsites.
Declineof theraveninBritain
136
within5 kmwas betterthanthatwithin3 km.Summarizing,
afforestation
ravensshowed
more non-breedingand produced later, smallerbroods in areas that were heavily
afforested
thanin areas lightlyafforested.
TABLE
1.Correlation
coefficients
andthe
('r' values)between
breeding
performance
extent
ofafforestation
data
Percentageafforestation
Within5 km
Within3 km
data
Breedingperformance
(n)
Layingdates
1975
1976
Clutchsizes
1975
1974,5 and 6 (combined)
-0 414
-0 129
-0 354
-0 184
12
26
Brood sizes
1974
1975
1976
-0 330
-0 295
-0714****
-0.482***
-0 300
-0772****
23
21
18
0 254
0.512*
0 495**
0 602**
19
12
* 0 10> P>0 05, ** 005> P>0 02, *** 0 02> P>0 01, **** 0-001> P.
For somenestingareas therewereenoughdata to comparethemeanlayingdates (to
thenearestweek)and broodsizes,beforeand aftersomeofthesheepwalkwithin5 kmwas
planted(Fig. 5). At thosesiteswherelessthan25% offormersheepwalkwas plantedthere
was no significant
difference
in mean layingdate (t=1 03; d.f.= 7; P> 0 10) or mean
brood size (t = 089; d.f.= 8; P> 0 10) betweenthe two periods.At thosenestingareas
whereover25% of theformersheepwalkwas planted,theravenslaid eggslater(t= 4 51;
d.f.=6; 0005 > P> 0 001, one-tailedtest)and rearedsmallerbroods (t= 1b98;d.f.=7;
0-05> P> 0 025, one-tailedtest)thanformerly.
Otherfactorspossiblyaffecting
ravennumbers
Eagles
Golden eagles (Aquila chrysaetos)wereprobablyeliminatedfromthe studyarea by
humanpersecution,
butfrom1945wereallowedto recolonize.The appearanceofthefirst
0
10
20
30
40
sheepwalk
within
5kmafforested
%/ of former
50
60
FIG. 5.The meanlayingdatesand broodsizes(youngat least 18 daysold) before(o) and after(u)
afforestation
aroundparticularnestingareas.
M. MARQUISS et al.
137
pair of eagles in thisyear was at once followedby non-breedingof the ravenswhich
formerly
used thesamecliffs.The arrivalofa secondeaglepairin 1952coincidedwiththe
closelyfollowedbynon-breeding
ofanotherravenpairin thesamevicinity,
non-breeding
of a thirdravenpair at a cliffabout 1.5 km fromtheeagles' nestingcliff.Ravens have
excludedfromtheanalysis
neverbredat thesenestingareas sincethenand weretherefore
afforestation.
with
occupancy
comparing
A thirdeagle pair settledin about 1960,displacinga fourthravenpair,buttheseeagles
movedto anothersite5 kmawayin 1970.The ravensdidnotreturnand thesiteis included
in the above-mentionedanalysis. No other eagles were presentin 1974-5 at sites
previouslyoccupiedby ravens,and so eagles alone werenot responsiblefortheoverall
declinein ravens.
Rock-climbers
ravensnested)are now regularlyused by rock climbers.
Five cliffs(whereformerly
in
breedingsites.Ravenshave been seenfrequently
regular
Threewereknownto be once
areas
nesting
these
None
of
no
has
occurred.
nesting
cliffs
but
recentyearsat twoofthese
withafforestation.
was includedin theanalysesof occupancyand breedingperformance
None of themwas afforested
by morethan30%.
Egg robbingandpersecution
and tworepeatclutchesofeggswereknownto havebeentakenbyegg-collecFour first
negligiblein 1974-6compared
torsin the3 yearsof thestudy.Egg robbingwas therefore
clutchesinthe
(about 42% offirst
withthelargescalerobbingthatoccurredintothe'fifties
extensiveunpublisheddata of D.A.R.). In formeryearsravenswerepersecutedat times
by shepherdsand almostalways by gamekeepers,who shot or poisoned the adults or
destroyedthenestcontents,butgamekeepersoperatedoveronlysmallpartsof thestudy
destructionof adults ceased. In thepresent
area. When land was boughtforforestry,
land, and one case of nest
surveythereweretwo cases of poisoningon non-afforested
destruction.
Otherhumandisturbance
was once
byhumandisturbance.The cliffnest-site
One othernestingarea was affected
regularlyused,but not sincethebuildingof a smallreservoirand pumpinghouse on the
riverbelow.
Pollutants
Persistent
pesticideshave been implicatedin thedeclineof some raptorpopulations,
from
(Ratcliffe1970). In 1975eggswere-taken
partlythroughcausingegg-shellthinning
six raven nestsin the studyarea for chemicalanalysis.The levels of organochlorine
(Newton &
compoundswere verysmall (Table 2) and, by analogy withbirds-of-prey
or
Bogan 1974; Peakall 1976), would have had no influenceon breedingperformance
occurred.This agreed with earlierdata on
population. No significantshell-thinning
levelsin foureggs takenfromthe SouthernUplands in the 1960s,and
organochlorine
with the lack of thinningin 205 shells taken fromvarious parts of Britainafterthe
of DDT (Ratcliffe1970).
introduction
138
Declineof theraveninBritain
2. Organochlorinelevels in undevelopedeggs fromsix raven nestsin the
SouthernUplands, 1975.Resultsare givenas ppmin wetweight(and ppm in lipid)
TABLE
Nest
DDE
PCB
HEOD
1
0023(28)
1 4(17)
002(02)
2
3
4
5
6
0-09 (1-3)
0 11 (2 7)
0.23 (45)
0 29 (4 8)
0 16 (1 7)
1 4 (18)
0 7 (19)
1.2 (27)
0 8 (13)
0 5 (5)
0 04 (0 6)
0 05 (1 2)
006(09)
0003 (004)
0 06 (0 7)
DDE isthemainterminal
metabolite
ofDDT, PCB isfrom
industrial
polychlorinated
andHEOD
biphenyls,
isfrom
aldrinanddieldrin.
Food
Neithertheincidencenor thevolumeofvariousremainsin ravenpelletsweredirectly
proportionalto thequantitiesofvariousfoodsingested,so we could notassess theexact
dietfrompelletsalone.However,pelletsindicatedquiteaccuratelythequalitativecompositionof thedietand enabledbroadcomparisonto be madebetweenthedietsofdifferent
pairs.
A totalof697 pelletswereexaminedfromtwenty-nine
different
nestingareas,buttenof
thesewererepresented
byless thantenpellets(Appendix1). Food-itemswereassessedin
in a sample.
termsof theirpercentagefrequency
thananyotheranimalremains,and was
Overall,sheepwool occurredmorefrequently
recordedin pelletsfromall nestingareas examined.About 3% of thepelletscontaining
sheepwool also containedtherubberringsused to castratelambs and to removetheir
tails.Fine particlesofvegetabledebrissometimesaccompaniedsheepwool and probably
representedthe remainsof sheep dung. Pelletscontainingthe hair of large mammals
usuallycontainedsome sliversof bone. Many of thelagomorphboneswerealso broken
bits. Small mammalswere frequently
recordedand wereparticularly
prevalentin one
sample.Thispairnestedin a recently
afforested
area wherevoleswereabundant,and was
seenon severaloccasionsto rob short-eared
owls (Asioflammeus)ofvoles.
Birdremainswerelargelyfeathertraces,and thebonesoftheneck,head,wingsand feet
of duck, pigeon and grouse.These are the pieces thatremainafterperegrines(Falco
have killedand eatentheirprey.The percentagefrequency
of birdremainsin
peregrinus)
thepelletsof ravenpairswas negativelycorrelatedwiththeirdistancefromthenearest
of eggshell
peregrinepair (r= - 0496; d.f.= 22; 0 05 > P> 002). Identifiablefragments
werechieflyfromcurlew(Numeniusarquata) and red grouse(Lagopus lagopusscoticus)
eggs.The chitinousexoskeletonofinsectsand arachnidswas mostoftenpresentas small
particles;wherelargerparticleswere present,thesewere usually the legs or elytraof
beetles.Pelletscontainingchitinfragments
also containedlarge amountsof vegetable
material.The latterappeared in nearlyall pelletsbut it is unlikelyto have contributed
muchto nutrition
becauseitwas cast up largelyintact.The samewas trueofcerealgrain,
whichwas recoveredfrompelletscontainingpigeonor duckremainsand so was probably
ingestedincidentallywith the digestivetracts(commonlydiscarded by peregrines).
Articlesofrefusewerescarcebutoccurredinfivesamples.Raven featherswereprobably
swallowedduringpreening.Raven eggshellincastingscould be theresultofeithereating
theshellsafterhatchingor eatingtheeggsthemselves.
On severaloccasions,singleeggs
disappearedfromravennestsduringincubationand thesemaywellhave been eatenby
theadult.
M.
MARQUISS
et al.
139
The percentagefrequency
ofsheepremainsinpelletswas negatively
correlatedwiththe
percentageof afforestation
within3 km (r=-0616; d.f.=17; 001>P>0001)
and
within5 km (r= -0 668; d.f.= 17; 0 01 > P > 0 001) (Fig. 6). Whereafforestation
levels
werehigh,thecastingscontainedremainsfroma varietyoffoods,butlargelygoat,deer,
lagomorphand birdcarrionand,inrecently
plantedareas,voles.Bothhighand low levels
ofchitinand eggshellwerefoundin samplesfromareasoflow afforestation,
whereasonly
low levelswerefoundin samplesfromhighlyafforested
areas.
1000
_
80-
0~~~~~~~~~~~~~
3 00
4) 60
*
-CL
0
0
40-
o
20-
0
0
10
20
*0
U
30
40
50
60
% sheepwalk
within
afforested
5 km
FIG. 6. Relationshipbetweenfrequencyof sheep wool in pelletsand the level of afforestation
within5 kmof thenest-sites
of particularravenpairs.
The generalpictureof ravenfeedinghabitsgainedfromour analysisis similarto that
gainedfromsmallersamplesofpelletsgivenby Bolam (1913) forWales, Ratcliffe
(1962)
forthe EnglishLake District,and Harlow et al. (1975) forVirginia,United States of
America.In all the Britishareas, sheep formeda staple food, a findingborne out by
observationsof sheep/raven
interactions
made by M. A. Ridpath(in Murton1971).
DISCUSSION
Whilstravenpairstendedto be mostnumerousin themostruggedpartsofthestudyarea,
theirformerdensitycould not be explainedsolelyby the availabilityof nest sites.In
general,cragsweremoreabundanton higherhillsand inareas ofigneousrock.However,
in manydistrictsravensbreedreadilyin treesand treesiteswereabundantthroughout
mostofthestudyarea,particularly
at lowerelevationswhereravensweresparser.Also in
Northumberland
(a low densityarea) somepairsused,as alternative
nestingplaces,crags
severalkilometres
apart,butin otherpartsof thestudyarea (withhighdensity),separate
pairsnestedwithinthisdistanceofone another.In a fewpartsofthestudyarea,thelackof
breedingravenswas associatedwiththelack of anysuitablenestsite,but thisfactorhad
muchmoreinfluence
on presence/absence
thanon nearestneighbourdistances.
The mostreasonableexplanationofthedispersionand densityofbreedingravensinthe
studyarea is one based mainlyon foodsupply.Therehas longbeena government
subsidy
on hillsheep,tendingto maximizetheirnumbers,oftenabove thecarrying
capacityofthe
grazing.The grazingis poorestat highaltitudesand in areas overlyingacidic rock,so
undertheseconditionsone wouldexpectthehighestsheepmortality.
Thismayexplainthe
140
Declineof theraveninBritain
higherdensitiesof breedingravensat higheraltitudes,and in middleGalloway,theonly
regionwheretheunderlying
rockis largelygranite.
in the availabilityof sheep
Otherfactorswould tend to accentuateany differences
carrionbetweenareas. Thus someof thebesthillgroundin thestudyarea was managed
primarilyfor red grouse (e.g. in the Leadhills and parts of Northumberland)and
supportedfarfewersheep than othergroundmanaged forsheep alone. Secondly,the
mostacidic substrate(granite)withinthestudyarea was associatedwitha moreprecipitous terrain,and some sheep were killedaccidentallywhen theybecame 'cliff-bound'
whilstforagingon therelativelyluxuriantvegetationof cliffledges.(On one occasion
D.A.R. foundas manyas ninedead sheepbelowa Welshprecipice.)Thirdly,theground
at loweraltitudeswas usuallylessremote,so sheephusbandrywas possiblybetterand the
sheepthatdied weremorelikelyto be buriedbytheshepherdthanthoseon moredistant
ground.
Most bird species that have been studied show a positiverelationshipwith land
productivity,
withthe densestpopulationson the richestground(Jenkins,Watson &
Miller1967forredgrouse;Ratcliffe1969forperegrine;
von Haartman1971forvarious
song-birds;
Newtonet al. 1977forsparrowhawk
Accipiternisus).In thesespecies,density
thetwo
probablyadjustsnot to land-productivity
as suchbut to available food-supply,
oftenbeingcloselycorrelated.In contrast,theravenis a specieswhose food (through
humanintervention)
improveswithdeterioration
in land productivity.
SimilarlyLockie
& Stephen(1959) thoughtthatgoldeneaglepopulationsin someimpoverished
areaswere
also maintainedlargelyby theabundanceof sheepcarrion.
The formeroccupancyof ravennestingareas may also have been relatedpartlyto
in thatmanyirregularly
food-supply,
occupiedareaswereon relatively
low ground,or on
grousemoor wherecarrionwas probablyscarcer.However,anotherfactormay have
beenthepoor quality(in termsofheightand remoteness)
ofmanyofthelow groundcliffs
used fornesting.As a parallel,thefrequency
withwhichparticularperegrineterritories
wereoccupiedineasternNorthAmericawas directly
relatedto theheightand remoteness
of the nestingcliff;the reduceduse of 'poor quality'cliffsapparentlyarose fromthe
reducedprotectiontheyofferedagainsthumanand otherpredators(Hickey1942;Hagar
1969). Hence, in an era when ravens were heavilypersecuted,theymay have been
generallyreluctantto use the less safe and more disturbedsites.We suggestthisas a
theformer
possiblefactorinfluencing
occupancyofnestingareasand nottheirdispersion;
includingirregularly
used ones,nestingareas wereas evenlyspaced on low groundas on
highground.
A smallnumberofravenpairshave beendispossessedbyeaglesand rockclimbers,but
thelargerpartoftherecentdeclineinnumberswas probablylinkedwiththeafforestation
of sheepwalk.In 1974-5theoccupancyofnestingareas was associatedwiththeextentof
afforestation
chieflywithin3 km of thenestsites.Ravens are groundforagers,feeding
mainlyon largeitemsofcarrionbutalso eatinganyanimalmaterialdownto thesizeofa
beetleor snail.Itemssmallerthan100g areprobablyoftentakenalive.Afforested
ground
had no sheep carrionand in additionthe highvegetationobscuredothercarrionand
madeitmoredifficult
forravenstoforageforeggs,groundinsects,frogs,lizardsand small
werefeedingon
mammals.Those pairsstillpresentin 1974-6despiteheavyafforestation
thelatterapparentlyscavengedfrom
goat,deerand lagomorphcarrionor bird-remains,
peregrines.On some recentlyafforested
groundtherewas an abundance of voles, and
theseprovideda temporary
sustenanceforravens,sometimesfor2 or 3 yearsaftersheep
had beenremoved.
M. MARQUISSet al.
141
The stage at whichnestingareas were desertedprobablydependedon the balance
between(a) theoriginalqualityof thearea forsheepcarrion,(b) theamountof afforesfood.Ifsheepcarrionwas scarce,due tolow sheep
tationand (c) theamountofalternative
stocks,highqualitygrazingor good sheep husbandry,even small amountsof afforestationmayhaveremoveda criticalamountoffood.This probablyexplainedthelack ofa
and occupancybyravens.In addition,
veryconsistentassociationbetweenafforestation
werethosemostlikelyto
thoseareas of sheepwalkwhichhad thehighestsheepmortality
The greaterthe amount of existingforest,the more likelywas a
be sold forforestry.
Some nestingareasweredeserted
nestingarea to be desertedon subsequentafforestation.
even the year beforetreeplanting,but coincidentwiththe removalof sheep. Mostly,
however,desertionwas coincidentwithplanting.
was associatedchieflywiththeextentof afforestation
within5
Breedingperformance
km of thenestsites,whereasoccupancywas mostcloselyassociatedwithafforestation
eventhoughthe
within3 km.Ravensprobablyuseda largearea fortheirgeneralforaging,
tenancyof a nestingarea seemedto have beenmainlydependenton theamountof food
immediately
around thenest-sites.Ravens bredearlyin theyear,probablyto coincide
at theend of thewinterand duringlambingin
withtheperiodof majorsheepmortality
earlyspring.Feral goats also had theirkids in earlyspring.Ravens on more heavily
poorer
afforested
groundbredlater.Thiscould havebeena responseeitherto a generally
food supply or to an alternativefood supply:one late breedingpair fed largelyon
lagomorphcarrionwhichwas most abundantlaterin the springthan sheep and lamb
carrion.
Anotherfactorcomplicatedthepatternof ravendecline,namelythegeneralimprovementinsheephusbandry.Overtheyears,someareasweredrained,fertilized
and reseeded
for
of
winter
more
the
the
became
to improve forage sheep, provision supplementary
feed
used to preventdisease.The netresultis likelyto
prevalentand drugswereincreasingly
have beenlesscarrion.The extentofthissortoffooddepletionforravenswas impossible
withinthelimitedscope ofour study,butsevenor eightunoccupiednesting
to determine
in 1974-6,and mayhavebeenrendereduntenable
areaswerestilllessthan20% afforested
throughloweredsheepmortality.
Immediatelyto the south of the SouthernUplands and Cheviots,the EnglishLake
Districtrepresents
a controlarea forthisstudy,becauseitshillsarestilllargelysheepwalk.
Therehas beenlittleafforestation,
and thismainlyaroundtheedges.The Lake district
has
to seventypairs since 1900.
held a stableravenbreedingpopulationof about sixty-five
knownnestingareas in thisregionwereexamined:of these,
During 1974-6,forty-eight
heldbreedingpairs,althoughsixofthesenestingareaswereformerly
regardedas
forty-six
irregularin occupation.One nestingarea was occupiedin 1974butdesertedin 1975and
1976. This nestingarea and one of theothertwo desertedones werein localitieswhere
within3 km of thenestsites.The third
therehad been substantialrecentafforestation
nestingarea was desertedforunknownreasons.Whiletheravencan probablysurviveat
low densityin afforested
uplands,thiswill be likelyto depend on the extentof sheep
ifblanketafforesgroundleftunplanted.Almostcertainlythespecieswilldeclinefurther
tationcontinuesto expandoverformerupland sheepwalk.
-
ACKNOWLEDGMENTS
For permissionto workon theirground,forplacingeveryfacility
at our disposaland for
142
Declineof theraveninBritain
fullco-operationand helpin otherways,itis a pleasureto thankthestaffof theForestry
Commission,South Scotland Conservancy.Mr S. Petty of ForestryCommission,
NortheastEnglandConservancy,kindlyprovidedus withup-to-dateplantingmaps for
For historicalinformation
we are grateful
Northumberland.
on ravendistribution,
to the
late E. Blezard,thewidowof thelateCaptain R. Cross,to J.Hutchinson,W. R. Laidler,
B. Little,E. Meek, W. Murdock, the late W. Murray,R. Nelson, D. Watson, R.
Roxburgh,G. Shaw, thelate G. W. Temperleyand T. Todd. For helpingwiththesurvey
in 1974-6, we are especiallygratefulto B. Little,E. Meek and othermembersof the
NorthumbriaRingingGroup,G. Carse,G. Horne,R. Mearns,thelateW. Murray,and
R. Roxburgh.We are also grateful
to variousshepherds,
forestrangers,gamekeepersand
otherlocal observers,too numerousto mentionindividually.
For drawingthefigureswe
thank Mrs S. Adair, for statisticalhelp P. Rothery,and for helpfulcriticismof the
manuscriptDr D. Jenkinsand Miss J.Rowe.
REFERENCES
in Wales.London.
Bolam,G. (1913). Wildlife
ofhairand featherremainsin thegutand faecesofstoatsand weasels.Journal
Day, M. G. (1965). Identification
ofZoology,147,201-17.
Hagar, J. A. (1969). Historyof theMassachusettsPeregrineFalcon population.1935-57.Pp. 123-31.In: PeregrineFalcon Populations,theirBiologyand Decline (Ed. by J. J. Hickey). Univ. of WisconsinPress,
Madison.
H. S. (1975). Some winterand nestingseason
D. R. & Crawford,
Harlow,R. F., Hooper,R. G., Chamberlain,
foodsof thecommonravenin Virginia.Auk,92, 298-306.
Haartman,L. von(1971). Populationdynamics.AvianBiology,Vol. 1 (Ed. byD. S. Farnerand J.R. King) Pp.
391-459. AcademicPress,London and New York.
Hickey,J. J. (1942). Easternpopulationof theDuck Hawk. Auk,59, 176-204.
in theredgrouse(Lagopus 1.scoticus).
Jenkins,
D., Watson,A. & Miller,G. R. (1967). Populationfluctuations
JournalofAnimalEcology,36, 97-122.
Lockie,J. D. & Stephen,D. (1959). Eagles,lambsand land managementon Lewis. JournalofAnimalEcology,
28, 43-50.
Murton,R. K. (1971). Man and Birds.Collins,London.
Newton,I. & Bogan, J. (1974). Organochlorineresidues,eggshellthinningand hatchingsuccess in British
Nature,London,249, 582-3.
sparrowhawks.
Journalof
nestingterritories.
Newton,I., Marquiss,M., Weir,D. N. & Moss, D. (1977). Spacingofsparrowhawk
AnimalEcology,46, 425-41.
90,
Peakall, D. B. (1976). The peregrinefalcon(Falco peregrinus)and pesticides.Canadian Field-Naturalist,
301-7.
and ravenCorvuscorax. Ibis, 104,
D. A. (1962). Breedingdensityin theperegrineFalco peregrinus
Ratcliffe,
13-39.
D. A. (1970). Changes attributedto pesticidesin egg breakagefrequencyand eggshellthicknessin
Ratcliffe,
some Britishbirds.JournalofAppliedEcology,7, 67-115.
Statisticsfor theBehaviouralSciences.McGraw-Hill,New York.
Siegel,S. (1956). Non-parametric
(Received25 August1977)
143
M. MARQuiss et al.
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