the status of inland-breeding great cormorants in england · breeding cormorant population in...

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

290 British Birds 100 • May 2007 • 289–299

Inland-breeding Great Cormorants in England

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.

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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

291British Birds 100 • May 2007 • 289–299


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



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.

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Fig. 2. Colonisation and development of the Great CormorantPhalacrocorax carbo colony at Abberton Reservoir, Essex,

from 1981 to 2005.

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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-



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

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b. 1989–94

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c. 1995–2000

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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-



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.

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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



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.

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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-



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.

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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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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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role in the establishment and development of other inland colonies in Englandduring the period 1989–94. Colour-ringed birds from Abberton have also beenseen during the breeding season in Cormorant colonies in The Netherlands,Belgium and France, and it appears likely that some birds from this colony

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the status of inland-breeding great cormorants in england · breeding cormorant population in...

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