CHANGES ATTRIBUTED TO PESTICIDES IN THE NESTING
SUCCESS OF THE SPARROWHAWK IN BRITAIN
BY I. NEWTON
12 Hope Terrace,Edinburgh
EH 9 2AS
NatureConservancy,
INTRODUCTION
(Accipiternisus(L.)) in Britainhas been
The recentpoor breedingof the sparrowhawk
on mainlycircumstantial
evidence,to contaminationof thepopulationwith
attributed,
1972).
(Ratcliffe1970; Prestt& Ratcliffe
used in agriculture
insecticides,
organochlorine
As partoftheevidence,thispapercomparesthenestingsuccessofthespeciesbeforeand
duringthetimewhenthesepesticideswerein widespreaduse. It also comparesnesting
level of pesticide
success since 1956 on two land types,each subjectedto a different
application.For thislast purpose,breedingrecordshave been examinedaccordingto
whethertheywere obtainedwithinan arbitrarydistance(20 km) fromarable land or
in the
fartheraway, on untitledareas used mainlyfor sheep,deer,grouseor forestry,
northand westof Britain.Particularnestswereplaced in one or othercategorybyreference to the OxfordAtlas(1963).
since 1947, and the more
In farmlandareas, DDT and BHC were used extensively
toxic cyclodienecompounds(aldrin,dieldrinand heptachlor)since 1956 (Cook 1964).
in the use of cyclodienescame from1962,whentheywerevoluntarily
Some restriction
bannedfromdressingson springsownwheat,buttheiruse forotherpurposescontinued.
pesticideswereneverappliedwidely,thoughDDT
On non-arableareas,organochlorine
in sheepdipsuntil1966,whenthelatterwas bannedfor
and dieldrinwereused regularly
thispurpose.All suchcompoundshavealso reachedremoteareas intheair,in rainwater
1972).Sparrowhawks
(Frazer& Neuberger1970)and in migrantbirds(Prestt& Ratcliffe
in Britain,and 61% of 226 recoveriesof birdsringedas
are highlysedentary
themselves
nestlingswere within20 km of the natal area (Newton 1974). On balance, therefore,
forheavy
individualsnestingon or near arable land should have greateropportunity
pesticidalcontaminationthan should more distantones. This view was supportedby
analysesof tissuesand eggs (Presst& Ratcliffe1972), and by the greaterpopulation
foundin easternfarmedareas thanin north-west
declineand thegreatershellthinning
hilldistricts(Presst1965; Ratcliffe1970).
SOURCES OF INFORMATION
from
The aim was to assembleunbiaseddata on clutchand broodsizesof sparrowhawks
stated dates and localities,and particularlyon the case historiesof individualnests.
Severalobvioussourcescould notbe used becausetheywouldbias theresultsin one way
or another,but thefollowingproveduseful.
the onlydetailedstudyof
(1) J. H. Owen's(1916-37) workin Essex was untilrecently
in Britain;it gave clutchsizes for100 nests,but recordedbrood sizes
the sparrowhawk
in thetextor in photographs.
fromeightnestsonlyincidentally
Diaries and notebooksofthefollowand naturalists.
(2) Diariesofearlyegg-collectors
95
96
Pesticideson thenestingsuccessof thesparrowhawk
ingwereconsulted,becausetheygave detailsof everynestfound:E. Blezard(thirty-two
nests,mainlyCumberland,1925-68),Rev. F. C. R. Jourdain(fifty-one
nests,mainly
Derbyshireand Berkshire,1886-1940),J. Walpole-Bond(ninetynests,mainlySussex,
clutchesand nine broods, South Yorkshire,
1893-1941),A. Whitaker(twenty-nine
and
C.
V.
clutches,mainly Co. Donegal, 1909-30).
Stoney
(sixty-three
1900-46),
were
more
useful
than
Diaries
egg-collections
because theseobserverskeptmainlythe
the
various
of
clutches
found.
For
this
same reason,museumshellsare in general
larger
in
and
value
assessingclutch-size have not been used here.
of limited
Under a schemestartedin
of theBritishTrustfor Ornithology.
(3) Nest-record-cards
1939,anyonewhofounda nestcouldrecordthedetailsfromeach visiton a standardcard
and thenreturnthecardto a centralcollectionto awaitanalysis.The schemecoveredall
Britainand Ireland,but in practicerecordsweremostnumerousfromareas withmost
people. Up to 1970,370 nestswererecorded,usuallywithfewdetailsfromeach.
observers.
Many recordswerecollectedby myselfand
(4) Privaterecordsof individual
to thenationalscheme,and I am grateful
otherrecentobserverswho did not contribute
to the followingfor makingtheirrecordsavailable: B. Campbell (fivenests,mainly
nests,mainlyCumberland,1924-70),
Berkshire,1927-45), R. H. Brown (fifty-three
L. MacNally (fivenests,Inverness-shire,
1960-64), J. Mavrogordato(eighteennests,
nests,mainlyCumberland,
1925-40),D. A. Ratcliffe(fifty-seven
mainlyKent/Surrey,
1897-1932),
1943-70),G. Tomkinson(178 clutches,mainlyShropshire/Worcestershire,
1952-55),W. Ruttledge(fournests,Co. Mayo,
D. A. Robinson(ninenests,Lincolnshire,
nests,mainly
1968-70)and J. E. Wightman(fifty-seven
1947-49;fifteen
nests,Wiltshire,
north-west
broods,Dumfries/KirkcudYorkshire,1927-49),H. Ostroznik(sixty-eight
bright,1968-70).
in birdbooks publishedbefore1947,and available in the Alexander
(5) Photographs
Library,Oxford,gaveeighteenbroodsizes,aftercheckingthateach nestwas represented
only once. These recordsreferonly to well-grownyoung,because small youngwere
usuallyhuddledtogetherand could not be countedfrompictures.Brood sizes obtained
fromthoseobtainedin moreconventional
ways,buthelped
in thiswaywereno different
to increasethe sample.
In all thesedata, theamountof detailfromdifferent
nestsvariedwiththetimingand
existed
differences
numberofvisits,hencethevaryingtotalsin Tables 1-6. No significant
fromdifferent
betweeninformation
sources,however,exceptthosediscussedbelow.All
data are dividedaccordingto whether
theywerecollectedbefore1947(no organochlorines
in widespreaduse), 1947-55 (DDT, BHC and relatedcompounds,but practicallyno
cyclodienes)and 1956-70(DDT, BHC and relatedcompounds,and cyclodienes).I came
acrossno nest-records
fromnon-farmland
areas before1956,thoughsome presumably
exist.
FATE OF INDIVIDUAL CLUTCHES
Nestsin use longestweremostlikelyto be found,so an analysisof nestingsuccesswhich
includedall nestswouldbe biasedbysuccessfulones.In Table 1, onlyrecordsfromnests
whichwerefoundbeforelayinghad finishedand whichwerefollowedto the end have
beenused.All instancesoffailurethroughdirecthumanintervention,
suchas egg-taking,
shootingtheadultsand timberfelling,wereexcluded;as wererepeatclutcheslaid after
oflateclutcheswas important
because,accordtheloss ofan earlierone. The segregation
ing to Owen (1918), theywereless successfulthanfirstones, and containedmoreeggs
I. NEWTON
97
that failedto hatch.This lefta sample of eightynestsforfarmedareas before1947,
for1956-70.None oftheearliernestsfailedto
twenty-nine
for1947-55and eighty-eight
produceat least one young,yet3400 of nestsin 1947-55,and 42% of thosein 1956-70,
and fromthefailureof incubated
The losses werefromegg-breakage
failedcompletely.
eggs to hatch,seldom fromdeath of the young (in two out of threecases, a single
areas in 1956-70
youngster).
By contrast,nearlyall twentynestsfoundin non-farmland
were successful.
failuresthrough
periods,excluding
ofnestsproducing
youngindifferent
Table 1. Proportion
directhumanintervention
No.((%) No.(%)
Causesofcompletefailure
No. clutches inwhich in which
foundbefore at least at least DesertedUnhatchedBrokenUnhatchedDeath
completion one egg oneyoung eggs (incubated) eggs and broken of
eggs young
eggs
hatched fledged
Farmland
Pre-1947
1947-55
1956-70
Non-farmland
1956-70
80
29
88
80 (100)
19 (66)
51 (18)
80 (100)
19 (66)
48 (55)
0
0
0
0
1
6
0
8
19
0
1
12
0
0
3
20
19 (95)
19 (95)
1
0
0
0
0
periods,excludingrepeatlayings
in different
Table 2. Clutch-sizes
Farmland
Pre-1947
1947-55
of
No. clutches No. ofclutch-sizes
6 7
3
4
5
1956-70
Non-farmland
1956-70
635
39
46
9
16 143 332 132 12
1 10 22
6 0
1
11
25
9
0
0
2
5
2
0
Mean
4-8
4-8
49
50
CLUTCH AND BROOD SIZES
Many of the earlyrecordsof clutchsizes were fromcollectors'diaries,and referred
to clutcheswhich,at most,had been only slightlyincubated.From 1947, when egg
specialcarewas neededto ensurethatonlyfullclutcheswere
breakagebecameprevalent,
included.This meantacceptingonlythosefromnestswhichwerevisitedrepeatedlyover
the layingperiodand in whichno depletionwas observed.Brood sizes weretakenonly
fromnestswhichwerevisitedin thelastquarterofthenestingperiodaftermostmortality
had occurred(see later),and whentheyoungwereold enoughto be ringed.
No changein initialclutchsizes occurredduringthe yearsconsidered,nor was any
areas (Table 2). Broods,in condifference
apparentbetweenfarmlandand non-farmland
trast,becamesmalleron farmedareas,averaging4 0 before1947,3 2 in 1947-55,and 2 9
morenumerous
in 1956-70(Table 3). Broods of morethantwoyoungweresignificantly
before1947 than after1955 (P<0f001). The analysisreferred
onlyto successfulnests,
between
wereincluded,thedifference
moreover,and ifclutcheswhichfailedcompletely
periodswas even greater,namely4 0 before1947,2 1 in 1947-55,and 1 6 in 1956-70.
areas.
changeoccurredon non-farmland
Again,no significant
98
Pesticideson thenestingsuccessof thesparrowhawk
INCIDENCE OF UNHATCHED EGGS
An assessmentof the importanceof unhatched(incubated)eggs in reducingbreeding
success should take into account, not only those clutcheswhich failed completely
(Table 1), but also thosein whichonlya proportionof theeggsgave riseto young.To
recordan egg whichis incubated,yetfailedto hatch,a clutchmustsurvivebeyondthe
end of incubation,so resultsare inevitablybiased in favourof successfulnests.Also,
since fewernestswere successfulafter1947 than before,any resultsobtained underestimatetheincidenceof embryodeathsin the laterperiod.In particular,manyrecent
eggswerebrokenwhichmightotherwise
nothavehatchedanyway.None thelessthetrend
is clear,and on averagethemean numberof unhatched(whole)eggsper nestincreased
Table 3. Brood-sizes(at 3-4 weeks) in different
periods,excludingthose
fromrepeatlayings
No. of brood-sizes
of
No. broods 1 2 3 4 5 6 Mean
Farmland
Pre-1947
1947-55
1956-70
Non-farmland
1956-70
117
29
219
27
7 3 15 54 35 3
2 5 9 10 3 0
37 51 68 38 18 7
0
3
8 12
3 1
4-0
32
29
37
Table 4. Incidenceofunhatched
(incubated)eggsindifferent
periods,excluding
repeatlayings
No. nests
Farmland
Pre-1947
1947-55
1956-70
Non-farmland
1956-70
81
23
76
13
No. unhatched
eggspernest
0 1 2 3 4 5
Mean
56 15 6 3 1 0
13 7 2 0 1 0
13 31 18 11 3 0
6
6
0
1 0 0
0-49
0 65
1-48
069
from0 49 before1947to 0 65 in 1947-55and 1 48 in 1956-70(Table 4). The increasein
theproportionofnestscontainingmorethanone unhatchedeggis statistically
significant
(P< 0.001). No suchmarkedincreaseoccurredamongthirteen
nestsfromnon-farmland
areas in 1956-70,butthesamplewas too smallto be conclusive.This analysiswas based
onlyon nestsvisitedwithina weekafterhatchingbecause,as theyounggrew,unhatched
eggswereoftenpushedout.
INCIDENCE OF EGG-BREAKAGE
At the nestlingstage,it was uncertainwhethershellsin the nestcame fromhatchedor
brokeneggs,so onlythosenestsvisitedat theeggstagewereused to assesstheincidence
of breakage.The resultsin Table 5 confirmRatcliffe's
(1970) contentionthatbreakage
on a wide scale set in onlyafter1947. For the periodbeforethisthe recordsused were
from635 nestswitheggs,manyexaminedmorethan once, but onlytwo instancesof
breakage were recorded.Both were fromthe CumberlandSolway, whereE. Blezard
founda brokenegg beneatha nestin 1934 and anotherin 1938.Isolatedinstanceslike
I. NEWTON
99
thesecould have resultedfromtheaccidentaldisplacement
of an egg as thehen leftthe
nest. Since 1947, when the next instancewas recordedby Ratcliffe(1960), breakage
at least 19% of all clutches
occurredin farmlandareas throughout
Britain,and affected
seen. Its incidencewas almostcertainlyunderestimated
fromthe records,however,because nestswhichfailedat an earlystagewereless likelyto be recordedthanthosewhich
survivedlonger.Of 117 nestsfoundbeforethe end of layingin 1947-70,twenty-seven
throughegg breakage.It occurredmostoftenduringthelaying
(23%) failedcompletely
were then foundin the nest
period,but also duringincubation,and shell-fragments
or on the groundbelow. (The reasonswhysuch breakagecould not be attributedto
no breakageswere recordedin
predationweregivenby Newton (1973).) Significantly
1956-70fromseventeennestsin non-arableareas.
Table 5. Incidenceof egg-breakage
in different
periods
No. clutches Full clutchbroken* Partclutchbroken
Farmland
2
635
0
Pre-1947
90
11
6
1947-55
19
1956-70
160
12
Non-farmland
17
0
0
1956-70
* Includesnestswheresomeeggswerebroken,and therestdeserted,
so thatnonecouldhatch.
Table 6. Mortalityof nestlings
in different
periods
Farmland Total young
No. dying
No. broods Wholebrood Partbrood
hatched before4thweek involved
dying
dying
Pre-1947
179
14
46
0
10
71
0
4
1947-55
5
21
6
1956-70
136
19
3
39
Non-farmland
1956-70
18
15
4
0
2
MORTALITY OF YOUNG
Young sparrowhawks
normallystayin thenestforabout 4 weeks.An unbiasedassessmentof theirmortality
could be gained onlyfromnestswhichwerevisitedsoon after
hatchingand again in the last week. Many observersmade theirlast visitabout the
beginningof the fourthweek to ringthe youngand survivalto this age is countedas
fledging.This is reasonable,since almostall the deathsrecordedoccurredin the first
fewdays,evenin nestswatchedforthefullperiod.Deaths werefew,however,and onlya
slight(and insignificant)
increasein theirincidenceoccurredin recentyears(Table 6).
Butwhereasbefore1947theloss ofone or twoyoungnormallyrepresented
onlypartofa
brood, afterthis date it was oftenthe whole brood. Such youngusuallydisappeared
withouttrace,and werepresumably
eatenafterdeathby theirparentsand siblings.
DISCUSSION AND CONCLUSIONS
Before1947 the successof sparrowhawknestsin Britainwhichwerenot destroyedby
human agencywas extraordinarily
high.In all the eightyforwhicha fullrecordwas
100
Pesticideson thenestingsuccessof thesparrowhawk
obtained at least one youngwas reared.Natural predationwas lacking,presumably
becausethehen habituallystayson or nearthenestfromthestartof laying,and drives
away likelypredators.On the continentthe goshawkhas been recordedkillingmany
adultand nestlingsparrowhawks
(Tinbergen1946),butwas absentfromBritain.In most
an eggwhichfailedto hatchwas examined,itwas
nestsall theeggshatched,and whenever
No instanceswerenotedof
invariably
foundto be addled,withno obviousdevelopment.
and eggbreakagewas extremely
rare.Amongthepre-1947records
young'dead-in-shell',
used here,singleeggs were brokenin onlytwo of 635 clutchesseen; but in addition,
Campbell(1960) founda brokenegg below a nestin 1927,and Owen (1918) mentioned
clutch,
one instanceof a sparrowhawkbreakingand eatingpart of a repeat(infertile)
whichhad been incubatedbeyondthe normaltime.The highsurvivalof nestlingswas
and in markedcontrastto the situationin some otherraptors.Some 92%
also striking,
of all youngthathatchedsubsequently
fledged,and in 78% ofbroodsall youngsurvived.
Some nestsproducedsix fledglings.
thata markedand significant
Laterresultsconfirmed
declineinnestingsuccessoccurred
on farmedareas of Britainfrom1947.This was almostentirely
due to an increasein egg
breakagesand inthefailureofincubatedeggsto hatch.No changeoccurredinthenumber
of eggslaid and, once hatched,the youngsurvivedwell.The frequency
of egg breakage
inwas about the same in 1947-55as in 1956-70,but after1955therewas a significant
creasein thenumberofincubatedeggswhichfailedto hatch.Thiswas associatedwiththe
ofmany'dead-in-shell'
embryosin thelaterperiod(as wellas moreaddledeggs).
finding
The implicationis that,while shell thinningand egg breakage occurredthroughout
the period of DDT usage, deaths of partlyformedembryosoccurredonly afterthe
cyclodienescame in.
How muchhavethesechangesreducednestingsuccess?Before1947,in theabsenceof
humanintervention,
everyclutchcould be expectedto produceat leastone youngand the
meanbroodsizewas 4 0, on whichbasis 100clutchesproduced400 young.Undersimilar
out of 100 clutchesproduced young,and the
conditionsin 1956-70, only fifty-five
in
nests
was
2 9. Thus 100 clutchesproduced only 160
brood
size
successful
mean
from
a
the
level.
This percentageis minimal,moreover,
pre-1947
fledglings, 60% drop
it
account
of
takes
no
because
non-breeding
pairs,whichhave also increasedin recent
of
I
no
came
across
record
pairs before
nonbreeding
by establishedsparrowhawk
years.
in
but
southern
Scotland
in
a
found
at
out of 113
twenty
1971,
that
1955,
during study
built
or
a
nest
either
no
nest
was
was
built
but
in
not
lined
or
laid
(Newton
occupiedsites,
1973). Ratcliffe
(1960) likewisefoundthatsix out ofnineteennestsseenin Cumberland
beforethisdate. Thus
in 1956-70werenot laid in, comparedto none out of thirty-eight
all theemptynestsrecordedwereafter1955,so perhaps,likeembryonic
deaths,failureto
after
the
the
data
are reallytoo
only
cyclodienes
came
in,
but
layalso becamewidespread
sure.
fewto be
No changein nestingsuccesswas apparentfromthesampleofnestssituatedmorethan
20 kmfromarableland. The proportionofpairsproducingyoung,hatchingsuccessand
mean brood size was about the same on such land in 1956-70 as it was in the whole
thehawksin these
countrybefore1947,and no eggbreakageswererecorded.Presumably
whichweretoo low to influence
areas weresubjectedto levelsof organochlorines
their
The
few
of
made
from
suchareas confirmed
thatthe
analyses
eggs
breedingsignificantly.
levelstherewerelow (Prestt& Ratcliffe1972; I. Newtonand J. Bogan, unpublished).
Such remoteareas,however,compriseonlya smallfractionof thetotal land surfaceof
Britain,and supportonlysmallnumbersof sparrowhawks.
I. NEWTON
101
oforganoManychangesoccurredon farmlandafter1946,as wellas thedissemination
chlorinepesticides.Hedgerowswereremoved,agrochemicalsof othertypesapplied on
increasingscale and, throughgreatermechanisation,operational procedureswere
changed. Some of these changesmightalso have influencedsparrowhawkbreeding.
But theseotherchangesoccurredgraduallyover the period considered,not suddenly
and egg-breakage
from1946. Also, thetypesof failurerecorded,namelyshell-thinning
of incubatedeggs,have been associatedwithcontaminatedand
and the non-hatching
the NorthernHemisphere
decliningraptorpopulationsin varioushabitatsthroughout
(variouspapers in the book edited by Hickey (1969)). They have also been induced
in individualAmericankestrels(Falco sparveriusL.) and prairiefalcons
experimentally
(F. mexicanusSchlegel),whichwerefedon dietscontaminatedwithDDT and dieldrin,
but not in individualsof thesame speciesfedon dietslackingthesecompounds(Porter
& Wiemeyer1969; Endersoa & Berger1970).
of
the restriction
Ratherthetimingof depressednestingsuccessin the sparrowhawk,
withthehypofailuresto farmedareas,and thetypesof failurerecorded,are consistent
thesisthatorganochlorine
compoundswerethecause. Taken withtheanalyticalresults
on otherspecies(Hickey
fromtissues and eggs (Prestt& Ratcliffe1972),the findings
evidencejust mentioned,the case forthe
1969; Ratcliffe1972), and the experimental
compelling.
breedingis extremely
involvement
of thesechemicalsin thesparrowhawk's
ACKNOWLEDGMENTS
I am especiallygrateful
to all those observerslistedon p. 96, who willinglyand kindly
made theirpersonalrecordsavailable. The diariesof Rev. F. C. R. Jourdainand A.
Whitakerin theAlexanderLibrary,I examinedby courtesyof Dr D. Lack, thoseof J.
Walpole-Bondthroughthe kindnessof Dr S. Sloan Chesser,and those of E. Blezard
those of C. V. StoneythroughL. W.
throughMrs Blezard and Dr D. A. Ratcliffe,
and thoseof G. TomkinsonthroughJ.W. Tomkinson;I am also grateful
Montgomery
to theBritishTrustforOrnithology
foraccess to theirnestrecordcards,to I. Presttfor
forconstructive
helpfuladvice and to R. J. O'Connor, D. Jenkinsand D. A. Ratcliffe
commentson themanuscript.
SUMMARY
(1) The nestingsuccessof the sparrowhawkin and near arable areas of Britainhas
and the failureof incubatedeggsto
declinedsince 1947,due to increasedegg-breakage
and
hatch.In consequence,fewernestswere successfulin recentyearsthan formerly,
successfulbroodsweresmaller.
(2) The meanbrood size in successfulnestswas 4 0 up to 1947,3 2 in 1947-56and 2 9
in 1956-70.No changewas notedin theinitialnumberof eggsin each clutch,norin the
overallmortality
of theyoungafterhatching.
100 clutchescould be expected
(3) Before1947,in theabsenceof humanintervention,
to produce400 young,but in 1956-70only 160 young,a drop of 60%.
(4) No change was apparentin the success of birdsnestingin areas remotefrom
arable land,thoughthe samplefortheseareas was small.
to contamination
of thesparrowhawks
(5) The changein farmedareas was attributed
organochlorine
pesticides.
nestingtherewithpersistent
102
Pesticideson thenestingsuccessof thesparrowhawk
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