the status of inland-breeding great cormorants in england · breeding cormorant population in...
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289© British Birds 100 • May 2007 • 289–299
ABSTRACT Since the establishment of a tree-nesting colony of GreatCormorants Phalacrocorax carbo at Abberton Reservoir, Essex, in 1981, the
inland-breeding population in England has increased considerably andnumbered at least 2,096 breeding pairs in 2005.This population is thought to
have been founded by Continental birds of the race sinensis, although anincreasing proportion of Cormorants of the nominate race from coastalcolonies in England and Wales may have contributed to its development.
Increasing numbers of feeding Cormorants are now attracted to inland watersin England, intensifying the conflict between Cormorants and fisheries.This
prompted Defra to announce a ‘new’ policy in September 2004, whichincreased the number of Cormorants that could be killed under licence. It is
not known how the change in policy is affecting breeding populations.
The status of inland-breeding Great
Cormorants in EnglandStuart E. Newson, John H. Marchant,Graham R. Ekins and Robin M. Sellers
Alan Harris
Prior to 1981, Great Cormorants Phalacro-corax carbo (hereafter referred to simplyas ‘Cormorants’) in England rarely
attempted to breed away from coastal cliffs,stacks and offshore islands. This paper chartsthe development of nesting at alternative sites(termed ‘inland’, although a number are close toestuaries or open coasts). The first documentedrecord of inland tree-nesting by Cormorants inEngland occurred in East Anglia during the1540s (Coward 1928). Until the 1940s, inlandbreeding was reported from just six sites, inCumbria, Dorset, Kent, Norfolk (two) andSuffolk (Babington 1884–1886; Mansel-Pleydell1888; Seago 1977; Taylor et al. 1981; Stott et al.2002). Pinioned birds and their fully wingedoffspring are also known to have bred at StJames’s Park in London (Homes et al. 1957). Atseveral of these sites, human persecution isthought to have curtailed breeding activity. Therelative inaccessibility of coastal colonies inEngland probably allowed the coastal, cliff-nesting population to remain at a reasonablyhigh level during this period. Although histor-ical data are scarce, there were an estimated1,154 pairs of coastal-breeding Cormorants, allbelieved to be of the nominate race P. c. carbo,in England in 1969–70 (Cramp & Simmons1977). Repeat surveys in 1985–88 and1998–2000 suggested coastal populations ofapproximately 1,435 and 1,564 breeding pairs
respectively (Lloyd et al. 1991; Mitchell et al.2004).
Growth of the European populationIn continental Europe, where birds of the racePh. c. sinensis predominate, population levelswere low during the nineteenth and twentiethcenturies, and distribution restricted, mostlikely through a combination of habitat loss andpersecution (van Eerden & Gregersen 1995).Throughout the twentieth century there werebetween 1,000 and 1,200 pairs breeding in TheNetherlands, about 1,000 pairs in Denmark andfewer than 400 pairs in Germany (van Eerden &Gregersen 1995). In addition, pesticide contam-ination during the 1950s and 1960s is thoughtto have reduced breeding success, causing afurther decline in the Continental population(Russell et al. 1996). Persecution in other partsof Europe is also believed to have reducedbreeding numbers; for example, France hadfewer than 60 pairs at the turn of the nineteenthcentury (Marion 1991). Growing concerns forthese relatively small populations during thetwentieth century led to protective legislationbeing introduced, first in The Netherlands(1965) and Denmark (1971), and then widelythroughout Europe under Annex 1 of the ECBirds Directive (1979). Protective legislation forboth European races, carbo and sinensis, wasintroduced in Britain under the Wildlife and
Countryside Act(1981).
Once the birds wereprotected, populationgrowth was immediateand significant; for adetailed review, seeBregnballe (1996). InDenmark and TheNetherlands thebreeding populationincreased from 5,800pairs in eight coloniesin 1978, to 61,720 pairsin 116 colonies by2005 (Bregnballe &Gregersen 1997; vanEerden & Zijlstra 1997;Eskildsen 2005;SOVON Dutch Centrefor Field Ornithologyunpubl.). Similar population growth
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125. Until recently, the majority of Great Cormorants Phalacrocorax carbobreeding in England were of the nominate form Ph. c. carbo, and nested on
coastal cliffs, stacks and offshore islands; Ceann Leathad, Caithness, June 1996.
Robi
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llers
occurred in Sweden from 1980 (Lindell 1997),and in Germany and Poland in the early 1980s(Lindell et al. 1995). By the mid 1980s, Cor-morants were extending their breeding rangeinto central Europe and along the Baltic Seacoast (Lindell et al. 1995). During this period,numbers of wintering Cormorants in Englandwere increasing (Sellers 1991) and, in 1981, aninland tree-nesting colony was established atAbberton Reservoir in Essex (Ekins 1989).
The establishment of an inland-breedingpopulation of Cormorants in England between1981 and 1995 has been well documented(Sellers et al. 1997). Although a large propor-tion of inland colonies were monitored between1998 and 2002 during Seabird 2000 (Mitchell etal. 2004), the subsequent development of theinland population in England (from 1995onwards) has not been covered adequately. It isrelevant to point out that inland breeding hasalso taken place in Scotland and Wales (in thelatter country for centuries), and that there aretree-nesting Cormorants in Ireland. Thesecolonies are, however, believed to be of thenominate race carbo, and are not part of therecent development in England discussed here.In this paper, we update Sellers et al. (1997) bypresenting an overview of the colonisation andsubsequent range expansion of the inland-breeding Cormorant population in Englandduring 1981–2005. Ourcurrent understanding ofthe origins of inland-breeding Cormorants inEngland is discussed inrelation to recent litera-ture, and the findings ofnew analyses of ring-recoveries and colour-ringing data.
MethodsColony countsCounts of apparentlyoccupied nests (AON),defined as nests in use andsufficiently finished tohold one or more eggs(Bregnballe & Lorentsen2006), were obtainedthrough a number ofsources: (a) county birdreports and correspon-dence with County
Recorders; (b) the BTO Heronries Census; and(c) personal communication with birdwatchers,ringers and reserve or site managers. FollowingBregnballe & Lorentsen (2006), a colony isdefined here as a group or groups of nests thatare within 2 km of one another. Such groups areoften referred to as ‘sub-colonies’. A single nestis sufficient to be termed a colony as long as it isnot located within 2 km of other colonies.While considerable effort has been made tocompile a complete list of colonies, it is likelythat some breeding attempts have been missed,because these have not been reported or detailswere unavailable at the time of writing. Despitethe large and often conspicuous nest of thisspecies, counts of AON are not necessarilystraightforward. Where there was more thanone count for a particular site and year, thelargest count is reported here. The location ofmost sites referred to in this paper has alreadybeen published but locations are not disclosedin a few cases where observers or recorders haverequested that confidentiality is maintained.
Ring recoveriesAlthough there are many potential biases inrecoveries from metal rings and from colour-ringing data for Cormorants, ringing providesan invaluable tool for examining the extent towhich different populations have contributed to
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126. Established in 1981, the tree-nesting Great Cormorant Phalacrocorax carbocolony at Abberton Reservoir, Essex (photographed here in 2004),
grew from nine pairs to a maximum of 551 pairs in 1996.
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the development of the inland-breeding popu-lation in England. In this paper, we use datafrom two main sources: (a) recoveries frommetal rings placed on chicks at coastal coloniesin Britain & Ireland (1961–2005); and (b)recoveries and resightings of metal- and colour-ringed Cormorants ringed as chicks outsideBritain & Ireland (1961–2005).
Results and discussionDevelopment of the inland-breeding populationBetween 1981 and 2005, Cormorants bred suc-cessfully in one or more years at 58 inland sitesin England, with a maximum of 36 coloniesoccupied in any one year. While breeding wasactively discouraged at a number of these sites,the inland-breeding Cormorant population inEngland in 2005 is estimated to have been atleast 2,096 breeding pairs (fig. 1, appendix 1).
During the first eight years of inland coloni-sation, the breeding population at AbbertonReservoir grew rapidly from nine to 310 pairs
(fig. 2). Abberton was the onlysite at which a colony was estab-lished successfully between 1981and 1988, although confirmedbreeding was reported duringthese years from a further sixsites, in Cambridgeshire, Corn-wall, Middlesex, Norfolk andStaffordshire (fig. 3a). From1989 to 1994, when growth ofthe colony at Abberton wasshowing signs of slowing, afurther eight colonies wereestablished in England:Haweswater (Cumbria), LowerDerwent Valley (East Yorkshire),
Walthamstow Reservoirs (Essex), Stodmarshand Dungeness (Kent; the latter was a ground-nesting colony), Rutland Water (Leicestershire& Rutland), Deeping St James (Lincolnshire)and Besthorpe Gravel-pits (Nottinghamshire).Short-lived attempts at colonisation werereported from a further six sites between 1989and 1994 (fig. 3b).
The period between 1995 and 2000 wascharacterised by rapid growth of existingcolonies and further expansion, with newcolonies established at a further 13 sites. Thisincluded the formation of the following tree-nesting colonies: Harrold–Odell Country Park(Bedfordshire), Aldermaston Gravel-pits (Berk-shire), Chain Corner, Ouse Washes (Cam-bridgeshire), Drakelow Wildfowl Reserve(Derbyshire), Rye Harbour (East Sussex), Whel-drake Ings (East Yorkshire), Swithland Reservoir(Leicestershire & Rutland), Holkham (Norfolk),Earls Barton Gravel-pits (Northamptonshire),Stanton Harcourt (Oxfordshire), a confidential
site in Staffordshire, LoompitLake (Suffolk) and CoombeAbbey Country Park (Warwick-shire). In addition, successful butshort-lived breeding wasreported from a further 17 sitesbetween 1995 and 2000 (fig. 3c).These included WillingtonGravel-pits (Derbyshire), wherebreeding on a pylon was reportedfor the first time in England, in1998 ( James & Key 2001),although breeding here was sub-sequently discouraged. Illegalshooting of Cormorants at thecolonies of Besthorpe and at
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81 83 85 87 89 91 93 95 97 99 01 03 05
Fig. 1. Population growth (line) and number of inland Great CormorantPhalacrocorax carbo colonies (columns) in England between 1981 and 2005.
40
35
30
25
20
15
10
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0
no.c
olon
ies
2,500
2,000
1,500
1,000
500
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81 83 85 87 89 91 93 95 97 99 01 03 05
Fig. 2. Colonisation and development of the Great CormorantPhalacrocorax carbo colony at Abberton Reservoir, Essex,
from 1981 to 2005.
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500
400
300
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no.n
ests
Deeping St James, in 2000 at least, is thought tohave influenced breeding numbers at thosesites.
During 2001–05, growth at the oldercolonies, including Abberton Reservoir, PaxtonGravel-pits (Cambridgeshire) and Besthorpe,stabilised or declined, while growth continuedat the new colonies established during the pre-vious five years. During this time, furthercolonies became established, at Rostherne Mere(Cheshire), at a confidential site in Norfolk andat Castle Howard (West Yorkshire). Short-livedbreeding was reported from a further 14 sitesbetween 2001 and 2005 (fig. 3d).
Origin of inland-breeding CormorantsPopulation modelling work examining thegrowth rate of the Abberton colony during1981–88 showed that there must have been sig-
nificant immigration into the colony at thistime (Newson 2000). Evidence for immigrationto this and other inland sites in England fromthe Continent during both the breeding and thenon-breeding season is provided by recoveriesand resightings of metal- and colour-ringedCormorants ringed at sinensis colonies, princi-pally in The Netherlands and Denmark (fig. 4).These include 16 Cormorants ringed at coloniesin The Netherlands, six in Denmark, one inGermany and one in Sweden, which have beenpresent or reported breeding at established tree-nesting colonies in England (between April andJune).
Although there is evidence that Continentalbirds (sinensis) have influenced the develop-ment of an inland-breeding Cormorant popu-lation in England considerably, ringedCormorants of the nominate form carbo, origi-
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Inland-breeding Great Cormorants in England
Fig. 3. Inland Great Cormorant Phalacrocorax carbo colonies in England with successful breeding in one or more years.These maps show the extent to which the number of colonies increased during the periods
1981–88 (a), 1989–94 (b), 1995–2000 (c), and 2001–05 (d). Dot size indicates number of Apparently Occupied Nests at each site. Confidential sites are shown centrally within their counties.
0–910–5051–100101–250251–600
a. 1981–88
0–910–5051–100101–250251–600
b. 1989–94
0–910–5051–100101–250251–600
c. 1995–2000
0–910–5051–100101–250251–600
d. 2001–05
nating from British colonies, have also beenobserved at inland colonies during the breedingseason (between April and June). These haveincluded three birds from St Margaret’s Island(Pembrokeshire), two from Grune Point(Cumbria) and one from the Farne Islands(Northumberland). The origin of all colour-and metal-ringed birds breeding at inlandcolonies in England is shown in fig. 5. Consid-ering the small number of Cormorants thathave been colour-ringed at coastal colonies inEngland and Wales, the influence of British
carbo is likely to be far greater than theselimited data suggest.
Further confirmation that mixed colonies ofcarbo and sinensis occur at inland sites inEngland comes from DNA analysis. Goostrey etal. (1998) used microsatellite markers tocompare the genotypes of individuals, andanalysed feather samples from 78 chicks in1997; they found both genotypes in the samecolony, at Abberton Reservoir and at at leastfour other, more recent colonies. Similar find-ings have been provided through mitochon-
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127. Between 1981 and 2005, Great Cormorants Phalacrocorax carbo have bred successfully in one or moreyears at 58 inland sites in England, with a maximum of 36 colonies occupied in any one year. Although breedinghas been actively discouraged at a number of these sites, the inland-breeding population was estimated to be inthe region of 2,096 breeding pairs in 2005.This photograph shows an adult and a juvenile in Norfolk, June 2003.
Bill
Bast
on
Fig. 4. Recoveries and resightings of Great Cormorants Phalacrocorax carbo ringed in predominantly Ph. c. sinensis colonies in continental Europe and reported in Britain or Ireland outside the breeding season,between July and March. Birds marked with metal rings are shown in fig. 4a, while colour-ringed birds are shown in fig. 4b.These maps include Cormorants on spring and autumn passage as well as wintering birds.
a. Recoveries of metal-ringed Cormorantsfrom continental
Europe
b. Recoveries of colour-ringed Cormorants
from continental Europe
drial-DNA sequencing (Winney et al. 2001). In1998, field assessment based on physical differ-ences between the two races also suggested amixed population at Abberton (Newson 2000);museum work examining anatomical differ-ences between carbo and sinensis was describedby Newson et al. (2004). However, there is evi-dence that the proportion of carbo and sinensisbreeding at Abberton, and now at other inlandcolonies in England, may have changed overtime. The percentage of carbo at six inlandcolonies in 1998 shows a strong correlation withthe age of the colony, with older colonies suchas Abberton having a higher proportion ofcarbo (fig. 6). This may suggest a mechanismwhereby inland colonies are founded bysinensis, but an increasing pro-portion of carbo join thesecolonies as they develop. Withoutmonitoring the change in theseproportions over time, however,it is not possible to prove thatthere was not a difference fromthe outset.
Analyses of colour-ringingdata from Abberton have shownthat birds fledged from thiscolony at least have played animportant role in the establish-ment and development of otherinland colonies during theperiod 1989–94 (Ekins 1996).Cormorants are faithful to theirnatal colony, but as a colonynears its carrying capacity anincreasing proportion of
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Fig. 5. This shows the origins of Great CormorantsPhalacrocorax carbo ringed at coastal Ph. c. carbo
colonies in Wales, England or Sweden (red lines),or at Ph. c. sinensis colonies in The Netherlands
or Denmark, and found at inland colonies in Englandduring the breeding season, April–June (blue lines).
(mostly) younger birds breed elsewhere, eitherby moving to existing colonies or establishingnew ones. Between 1993 and 1996, about 7% ofCormorants hatched at Abberton dispersed(Newson 2000), but the proportion breeding(or attempting to breed) away from the colonyincreased to 12% in 1997 and 18% in 1998(Newson 2000).
The establishment of new inland coloniestends to be at sites already used by Cormorantsas night-time roosts and often increasinglyduring the summer months by immature birds(Newson 2000). Observations of Cormorantsdisplaying and carrying sticks and of nest-building attempts by young birds are oftenmade in years prior to successful breeding. Inaddition, perhaps because of the similarity inbreeding requirements between Cormorantsand Grey Herons Ardea cinerea, a large propor-tion of Cormorant colonies have been estab-lished within or alongside heronries.
The future for breeding Cormorants in the UKCormorants are perceived as a threat to inlandcommercial fisheries, particularly where groupsof birds gather to spend the winter, and thegrowth of the inland Cormorant population inEngland has heightened this problem. Until2004, licences were issued to fisheries to killsmall numbers of Cormorants in England(between 200 and 500 in total each year), tohelp to reinforce scaring measures. Followingfurther pressure from fisheries, however, theDepartment for Environment, Food and Rural
0 2 4 6 8 10 12 14 16 18colony age (years)
Fig. 6. Based on field observations at six inland Great CormorantPhalacrocorax carbo colonies in England during the 1998 breeding season(including Abberton Reservoir in Essex, the oldest colony studied), thisshows the relationship between number of years since establishment of
and percentage of breeding cormorants identified as Ph. c. carbo (asopposed to Ph. c. sinensis). Based on these data, a Spearman rank-order
correlation coefficient of r = 0.98 (P <0.001) was obtained.
40
30
20
10
0
%P.
c.ca
rbo
Affairs (Defra) announced a new policy in Sep-tember 2004 for controlling Cormorants. Thisallows licences to be issued to cull Cormorants(i.e. to reduce the population level); the numberof birds that could be culled was increased to3,000 for two years following the announce-ment, after which an annual control of up to2,000 birds is to be maintained. Each applica-
tion for a licence is considered on itsown merits, but it is undoubtedlyeasier to obtain a licence now thanin previous years.
The Government’s decision tointensify the cull was based on thefindings of population modelling,which suggested that increasedcontrol would decrease the inlandwintering population (CentralScience Laboratory 2004). However,Defra has not commissioned workto explore how increased levels ofcontrol will affect breeding popula-tions. The UK Government has aninternational responsibility underthe EU Birds Directive to ensure afavourable conservation status ofbreeding Cormorants in the UK. Aswe show here, inland waters in
England support Cormorants from a number ofbreeding populations outside the breedingseason. These include coastal-breeding carbo,mainly from England and Wales, sinensis fromthe Continent, and both races from the devel-oping inland-breeding population in England.At present, we do not know the likely influenceof the increased level of control on the inland-
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129. Following pressure from fisheries, Defra announced a new policy in September 2004 for the control ofGreat Cormorants Phalacrocorax carbo.This increased the number of Cormorants that could be controlled to3,000 in the first two years, after which an annual control of up to 2,000 birds will be maintained. Currently,
it is not known how this change in policy will affect the inland- or coastal-breeding populations.This photograph shows a Cormorant of the form Ph. c. sinensis, Quinta do Lago, Algarve, Portugal, February 2004.
Ray T
ippe
r
128. Feather samples taken from Great Cormorant Phalacrocoraxcarbo chicks at Abberton Reservoir and at four other, more recently
founded inland colonies in 1997 have confirmed that both thenominate form and the Continental race Ph. c. sinensis are
breeding at inland colonies in England.
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k Co
llier
and coastal-breeding popu-lations, for which annualmonitoring is required.
While the inland-breeding Cormorant popu-lation in England,predominantly sinensis asshown here, has grownconsiderably since 1981,our long-establishedcoastal-breeding popula-tion of nominate carbo hasremained relatively stableand is declining in parts ofits range; and Great Cor-morant remains an Amber-listed species in the UK(Debout et al. 1995; Bud-worth et al. 2000; Mitchellet al. 2004). Simple popula-tion models suggest thatcoastal colonies, whichfledge considerably fewerchicks per brood (Newsonet al. 2005) and have lowerannual survival rates thaninland colonies (Newson2000), are more susceptibleto natural variation in theseparameters, without anartificially increased level ofmortality. At this time, wehave little understanding ofthe level of culling at whichlong-term population decline in the coastal-breeding carbo population in the UK would beapparent.
Acknowledgments
We are extremely grateful to the large number ofindividuals who contributed counts of Cormorant nestsfor this paper. In particular we are extremely grateful to M.Alibone, P. Almond, S. Armstrong, A. Banthorpe, K. F. Betton,J. J. Bowley,T. Bregnballe, P. Burnham, S. Busuttil, M. Calvert,S. E. Christmas, J. S. Clark, T. Dixon, G. E. Dunmore, B. Ellis,R. Field, I. Fisher, R. M. Fray, T. Garton, F. C. Gribble, A. Hall,A. V. Harding, R. Harold, T. N. Hodge, S. Holliday, J. Hughes,R.W. Key,W. N. Landells, S. M. Lister, P. Martin, I. McKerchar,C. Melgar, J. Oates, N. Pomiankowski, C. Ralston, C. Raven,H. E. Rose, A. Self, D. Shackleton, R. S. Slack, J. Taylor, M. W.Tyler, H. Vaughan, T. Watts, J. J. Wheatley, S. White and I.Winfield.Thanks also to M. J. Grantham, M. Roos, J. Sterupand S. van Rijn for extraction of ring recoveries andresightings of ringed and colour-ringed Cormorants fromnational datasets. The BTO kindly allowed access to datafrom the long-running Heronries Census. We thank J. Hughes for useful comments on the manuscript. We aregrateful to Defra for funding this work and to D. Parrot atCSL for ongoing discussions and comments.
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299British Birds 100 • May 2007 • 289–299
Inland-breeding Great Cormorants in England
App
endi
x 1.
Num
ber
of b
reed
ing
pair
s of
Gre
at C
orm
oran
ts P
hala
croc
orax
car
boat
inla
nd c
olon
ies
in E
ngla
nd,s
umm
aris
ed b
y co
unty
,for
the
per
iod
1981
–200
5.T
he n
umbe
r of
est
ablis
hed
colo
nies
in e
ach
coun
ty is
list
ed in
the
rig
ht-h
and
colu
mn.
Tota
ls fo
r si
tes
whe
re b
reed
ing
was
uns
ucce
ssfu
l are
exc
lude
d.
8182
8384
8586
8788
8990
9192
9394
9596
9798
9900
0102
0304
05C
olon
ies
Bed
ford
shir
e1
44
1617
2431
3127
2B
erks
hir
e2
914
1626
3434
3332
462
Bu
ckin
gham
shir
e3
66
2C
ambr
idge
shir
e2
01
01
19
1835
7613
318
518
422
323
522
922
519
918
715
119
818
424
85
Ch
esh
ire
722
3C
ornw
all
10
00
00
00
00
00
00
05
00
00
01
Cu
mbr
ia1
216
1313
2641
500
11
05
011
2D
erby
shir
e4
2446
5586
9696
100
2D
evon
10
00
00
01
11
11
2E
ast
Suss
ex2
635
8075
110
125
128
140
158
2E
ast Y
orks
hir
e2
25
112
3247
4338
4530
62
Ess
ex9
2546
7995
151
210
310
355
356
366
444
584
546
617
727
636
679
598
623
627
619
680
689
558
5G
reat
er L
ondo
n1
05
20
01
Her
tfor
dsh
ire
22
02
21
12
3K
ent
3188
9414
014
615
115
115
415
519
423
823
22
Leic
este
rsh
ire
215
1828
4053
4538
1Li
nco
lnsh
ire
1755
7112
087
3013
612
688
8897
9379
451
Mid
dles
ex1
00
00
00
00
02
611
1520
3337
4242
2N
orfo
lk4
414
00
00
00
014
3554
5745
5583
964
Nor
tham
pton
shir
e7
313
2727
3031
302
Nor
thu
mbe
rlan
d1
11
00
00
00
00
01
Nor
th Y
orks
hir
e2
22
61
Not
tin
gham
shir
e10
3374
9211
414
018
117
318
017
898
7811
012
010
014
01
Oxf
ords
hir
e1
00
00
05
818
2525
251
Ru
tlan
d6
1320
3346
2751
3968
8313
615
014
512
21
Staf
ford
shir
e1
10
11
10
00
00
00
00
00
75
810
1015
2626
2Su
ffol
k1
1930
6466
8375
791
War
wic
ksh
ire
210
1115
1917
1718
302
Wes
t M
idla
nds
10
00
00
1W
iltsh
ire
10
00
00
00
00
00
1A
nn
ual
tot
al10
2648
8098
152
212
315
368
398
435
619
893
986
1,19
81,
394
1,30
31,
589
1,57
41,
603
1,71
31,
841
2,09
72,
126
2,09
658