THE PREDATION OF WILD BIRDS IN THE UK A review of its conservation impact and management Starlings David Kjaer (rspb-images.com) THE PREDATION OF WILD BIRDS IN THE UK A review of its conservation impact and management

Gibbons DW, Amar A, Anderson GQA, Bolton M, Bradbury RB, Eaton MA, Evans AD, Grant MC, Gregory RD, Hilton GM, Hirons GJM, Hughes J, Johnstone I, Newbery P, Peach WJ, Ratcliffe N, Smith KW, Summers RW, Walton P and Wilson JD (2007). The predation of wild birds in the UK: a review of its conservation impact and management. RSPB Research Report no 23. RSPB, Sandy.

ISBN-10: 1-905601-02-6 ISBN-13: 978-1-905601-02-8 Avocet chick David Tipling (rspb-images.com)

CONTENTS 1. Introduction 6

2. Summary 7 Crynodeb 10

3. A review of the effects of predation on bird populations 12 a A theoretical background 12 Birth, death and population regulation 12 The important concept of additive mortality 12 Specialist and generalist predators and predator ‘traps’ 12 Habitat modification and predation 14 b Predators and their populations 14 Predatory birds 14 Predatory mammals 16 c Evidence of impacts of predators on bird populations 17 Breeding wading birds 17 Curlew 17 Golden plover 17 Lapwing 18 Sport shooting and wading bird densities in the uplands 20 Hedgehogs and wading birds on the Outer Hebrides 20 Avocet 20

Gamebirds 21 Grey partridge 21 Capercaillie 23 Black grouse 24 Capercaillie and black grouse on Baltic islands 24 Red grouse 26

Songbirds 28 Impacts of agricultural intensification 28 Has sparrowhawk and magpie predation influenced changes in songbird numbers? 28 Sparrowhawk predation on songbirds 28 Magpie predation on songbirds 29 Predation on woodland songbirds by squirrels and woodpeckers 30 Cats and house sparrows 31

Seabirds 32 d Summary of the impacts of predation on bird populations 33

4. Mitigating the impacts of predators 36 a Predator removal 36 b Predator exclusion 38 c Habitat management to reduce predation and its impacts 39 d Diversionary feeding 42 e Conditioned Taste Aversion 43 f Warning prey of approaching predators 44

5. Future research needs 45

6. Implications for conservation 46

7. References 47 INTRODUCTION 6 pheasants andred-leggedpartridges. kills, orbyreleasinglargenumbersofnon-native with extrafood,forexamplefromlandfillsitesandroad predators andprey;byprovidingsome by alteringtheclimatewithunpredictableimpactson of wildhabitatssuchasforestsandfloodplainmarshes; country; byreducingthenumberandsizeofpatches cats, ottersandpolecatsfromallorlargepartsofthe removing somenativepredatorssuchaswolves,wild ways inwhichwehaveaffectedthisbalance:by defences tocope.Butthereareother, moresubtle, prey thathavenoexperienceofthesepredatorsor can haveafielddayandrapidlydepletethenumbersof countryside. Underthesecircumstances,anewpredator such asratsontoislands,orAmericanminkintothe predators intoplaceswheretheyarenotnaturallyfound– and damaging,oftheseinterventionsarereleases the eatenandeaters.Someofmostobvious, interventions thatupsetthebalanceofnaturebetween been forthousandsofyears.Inmostcases,itishuman sparrows butbothwouldstillbearound–astheyhave as now. Sparrowhawkswouldstillbemuchrarerthan roughly thesamenumbersofpredatorandpreyspecies generally similarinnature,wewouldexpecttosee years henceandif,it’s abigif,thecountrysidewere forwards intimetotheUKcountrysideof20,40or60 remain insomesortofbalance.Ifwewereabletotravel Generally speaking,thenumbersofpredatorsandprey then, afactoflife. possibility ofdeathatthehandsanotherspeciesis, keen senses,greaterspeedandpower. The of adaptationstotrywinthisevolutionaryarmsrace– Predators, inturn,haveevolvedasimpressiveanarray armoury, toavoidendingupassomeoneelse’s dinner. defences, suchascamouflage,vigilance,speedand real, dailythreat.Animalshaveevolvedawiderangeof practically everyotherspeciesonearth,predationisa and eatenbyanotherpredatoryspecies.However, for few peopleonthisplanetareworriedaboutbeingkilled Despite thetrialsandtribulationsofourdailylives,very 1. Introduction RSPB Director, Conservation Dr MarkAvery I hopethatyoufindthereviewinterestingandinformative. with awiderangeofpartnerorganisations. keen tocontinuebeinvolvedinbothareasofresearch seems tobesadlyneglectedatpresent.TheRSPBis non-lethal meanstoreducepredation–thislatteraspect done onhabitatrestorationandmanagement,butalso resort topredatorcontrol.Moreresearchneedsbe fences tolimitfoxaccess.Sometimes,however, we lapwings andoncreatingphysicalbarrierssuchaselectric are poor, weconcentratefirstonimprovingthehabitatfor lapwing productivityandnumbersonournaturereserves more difficultforthepredator. Forexample,where measures thatfavourthepreyspeciesandmakelife The RSPB’s strongpreferenceistoconcentrateonhabitat another thingtodecidewhat–ifanythingdoaboutit. population levelsofspeciesconservationconcern,and It isonethingtosaythatpredatorsmaysometimesaffect very weak. such assparrowhawksinthedeclinesofsongbirdsis also concludesthattheevidencetoimplicatepredators manager ofover130,000hectaresintheUK.Thereview the RSPB’s considerablepracticalexperienceasaland birds, forexamplelapwings.Thisconclusionaccordswith are toconservepopulationsofthreatenedground-nesting levels oftheirprey, andthatthisisagrowingworryifwe such asfoxes,cansometimesreducethepopulation The reviewconcludesthatgeneralistgroundpredators, science inthisareaisnotascompleteonemightwish. considerably tothestudiespresentedinthisreview, the wild birdsintheUK.WhileRSPBhascontributed the scientificevidenceonimpactsofpredators identify, butthisreportattemptstodosobyreviewing more naturallandscape.Thesesituationsarenoteasyto the populationsoftheirpreythanwouldbecaseina cases wherepredatorsarehavingagreaterinfluenceon So, inthishighlyalteredcountrysidetheremaywellbe 7 SUMMARY However, demonstrating that bird populations have demonstrating that bird populations However, have been kept low because declined, or their numbers straightforward. Simply of predation, is not so of predation is generally documenting high levels may be taking similar numbers insufficient, as predators died for other reasons anyway, of birds as would have have evolved to cope with high and many bird species needed that levels of predation. Instead, studies are times) where compare numbers of bird prey at sites (or as experiments predator numbers differ; those performed are generally most convincing. the UK and Growing evidence from such studies in of some elsewhere suggests that breeding populations and ground-nesting birds, such as wading birds by predation than gamebirds, are more likely to be limited or young are other groups, perhaps because their nests

Numbers of many predators of birds have increased Numbers of many predators of birds have from in the UK in recent decades. Many are recovering human the deleterious impacts of pesticide pollution, persecution or both. Changing land management some, and practices may have boosted numbers of into others are spreading following their introduction native. Over the UK or parts of it to which they are not of the bird a similar time-period, populations of some to the prey of these predators have declined, leading by predation. suggestion that these declines were caused Predation is a natural phenomenon to which all bird Predation is a natural and predators have lived alongside species are subject, natural bird prey for millennia most of their current bird populations However, without eliminating them. can be held low where can decline or their numbers them are high. levels of predation on 2. Summary

Peregrine falcon Peregrine Mark Hamblin (rspb-images.com) Hamblin Mark SUMMARY 8

Grey partridges Bob Glover (rspb-images.com) conditioned tasteaversion. habitat management,diversionary feedingand interventions includeremoval or exclusionofpredators, managers mayintervenetoreduce itsimpact.Such If predationisthoughttobelimitingbirdnumbers,wildlife densities byhabitatdeterioration. be limitedbypredationafterbeingreducedtolow For example,greypartridgepopulationsaremostlikelyto decline, habitatchangemaystillbeultimatelyresponsible. predation maybetheapparentcauseofaspecies’ these twopotentialcausesareoftenlinked.Thus,while environmental causes,suchaschangestotheirhabitat, populations aredueeithertopredationorother While itistemptingtothinkthatdeclinesofbird woodland structure. songbirds havedeclinedduetolong-termchangesin emerging evidencethatnumbersofsomewoodland led tothedeclinesofmanyfarmlandsongbirds,and it experimental–thatchangesinfarmingpracticeshave is weak.Rather, thereiscompellingevidence–someof that breedingsongbirdnumbersarelimitedbypredation more vulnerabletopredation.Bycontrast,theevidence of predationonincubatingadults. improve hatchingsuccess,but canalsoincreaselevels by birds.Exclosuresthatprotect individualnestscan mammals andiswhollyineffective atreducingpredation protect relativelysmallareas,canbebreachedby UK atleast,suchfencingisusuallyonlyemployedto levels ofpredationonnestingcolonies.However, inthe Excluding mammalswithelectricfencingcanreduce useful toolforconservationmanagerstoo. and thisformofinterventioncanprovideanadditional where predatorremovalhasincreasedbreedingnumbers, examples, mostcommonlyinvolvingground-nestingbirds, promoted bygamemanagers.Nevertheless,thereare explain whypredatorremovalismorewidelyusedand only thelatterisforconservationmanagers,whichmay numbers. Bothareobjectivesofgamemanagers,while thus morereadilyachievablethanincreasingbreeding years. Producingasurplusforshootingintheautumnis consistent atincreasingbreedingnumbersinsubsequent prey aliveattheendofbreedingseason,itisless generally bykillingthem–oftenincreasesnumbersof the worlddemonstratethatwhileremovalofpredators– Reviews ofpredatorremovalstudiesundertakenaround 9 SUMMARY Many other, non-lethal solutions to reduce predation and Many other, its impacts are available, though their efficacy is poorly known. There is clear scope for more research here. Numbers of many predators of birds have increased in the UK in recent decades. There is growing evidence that breeding populations of some ground-nesting birds are limited by predation. By contrast, there is little evidence that breeding songbird numbers are limited by predation. Post-breeding numbers of ground-nesting birds can be successfully increased by killing their predators, although this less consistently increases their breeding numbers in subsequent years. Diversionary feeding – providing predators with alternative Diversionary feeding – providing predators – might provide food in the hope that they kill less prey trials in the UK a practical non-lethal solution but, so far, partial success. have been few and have met with only Further tests are needed to improve upon these techniques. While conditioned taste aversion has been used to reduce predation on birds’ eggs in the US, no successful field trials have been conducted in the UK. this review shows that: In summary, • • • • • Alternatively, rather than attempting to reduce predation, Alternatively, habitats could be managed to boost productivity or thus compensating for losses to survival of bird prey, predation. While each of these potential solutions seems sensible, disappointingly few have ever been tested. Such testing is needed in order to develop practical and effective solutions. Habitats can be managed to lessen predation by reducing Habitats can be managed to lessen predation vulnerable and predator numbers, by making prey less for losses by allowing them to re-nest thus compensating field to predation. For example, hedgerows, grassland, margins and cereal crops could be managed to make nests and chicks less likely to be found by predators. Habitats rich in bird food could be provided to ensure that chicks do not go hungry and their begging calls do not forced to attract predators, and that adult birds aren’t forage for longer and in more dangerous places than otherwise. Potential predator breeding sites and perching places could be removed, and food supplies and fragmented habitats that sustain populations of generalist predators could be better managed. 10 CRYNODEB amgylcheddol eraill,felnewidiadau yneucynefinoedd, yn disgynunaioherwyddysglyfaethu neuffactorau Tra’i bodyndemtasiwn tybiobodpoblogaethauadar tymor hiryngnghyfansoddiady coetiroedd. cân mewncoetiroeddwedilleihau ynsgilnewidiadau Mae tystiolaethnewyddyndangosfodniferoeddrhaiadar ffermio wediachosiiniferyradarcânarffermyddddirywio. ohoni’n arbrofol–yndangosfodnewidiadaumewnarferion yn brin.Ynhytrach,maetystiolaethgref–rhywfaint yr adarcânsy’nmaguyncaeleucyfynguganysglyfaethu ysglyfaethu. Aryllawarall,mae’rdystiolaethfodniferoedd nythod neueucywionynfwyagorediniwedsgil ysglyfaethu nagrwpiauadareraill,efallaioherwyddfodeu adar helaynfwytebygolofodgyfyngedigsgil rhai adarmagusy’nnythuaryddaear, feladarhirgoesac DU acmewnmannaueraillynawgrymufodpoblogaethau Mae tystiolaethgynyddolynsgilastudiaethauo’rfathy mwyaf argyhoeddiadologyflawnihyn. yr ysglyfaethwyrynamrywio;cynnalarbrofionyw’rdull adar arsafleoedd(neuamseroedd)llemaeniferoedd angen astudiaethausy’ncymharuniferoeddysglyfaeth ymdopi âlefelauucheloysglyfaethu.Ynhytrach,mae eraill, acmaeniferorywogaethauadarwediesblygui o adaragybyddaiwedimarwbethbynnagamresymau oherwydd gallysglyfaethwyrfodynlladdniferoeddtebyg lefelau ucheloysglyfaethuyngyffredinolddigonol, oherwydd ysglyfaethu,ddimmorsyml.Nidywcofnodi gostwng, neufodeuniferoeddwedi’ucadw’nisel Fodd bynnag,nidywdangosbodpoblogaethauadarwedi gostyngiadau hynwedi’uhachosiganysglyfaethu. yr ysglyfaethwyrhynwedigostwng,sy’nawgrymubody gyfnod tebyg,maepoblogaethaucyfranoysglyfaethadar DU neurannauo’rwladllenadydyntynfrodorol.Dros ysglyfaethwyr, maeeraillyncynydduarôleu cyflwynoi’r newidiadau iarferionrheolitirwedicynydduniferoeddrhai blaladdwyr, erledigaethganddynneu’rddau.Efallaifod newydd arôldioddefeffaithniweidiolllygrugan y DUyndegawdaudiwethaf.Maeniferffynnuo’r Mae niferoeddyrysglyfaethwyraradarwedicynydduyn mae ysglyfaethusylweddolarnynnhw. poblogaethau adarleihauneugaeleucadw’nisellle ers miloeddoflynyddoeddhebeudileu.Ond,gall ochr ynâ’rrhanfwyafo’uhysglyfaethadarnaturiol rhywogaeth oadar, acmaeysglyfaethwyrwedibyw Mae ysglyfaethuynelfennaturiolofywydpob Crynodeb cynefinoedd sy’ngyfoethogo ran bwydiadarfelnafydd i ysglyfaethwyrganfodnythod achywion.Gellircreu ymylon caeauachnydaugrawn feleibodynfwyanodd enghraifft, gellidrheoligwrychoedd, glaswelltiroedd, felly adferycolledionagafwyd ynsgilysglyfaethu.Er llai agorediniwed,athrwyganiatáu iddyntail-nythuac leihau niferyrysglyfaethwyr, trwywneudyrysglyfaethyn Gellir rheolicynefinoeddermwynlleihauysglyfaethutrwy ysglyfaethu aradarsy’ndeori. llwyddiant deori,ondgallhefydgynyddulefelau amddiffynfeydd sy’ngwarchodnythodunigolwella aneffeithiol oranlleihauysglyfaethuganadar. Gall ffensys. Gallmamaliaiddorriffensacmae’ngwbl dim ondiamddiffynllecynnaucymharolfachydefnyddir ysglyfaethu arsafleoeddnythuadar. Ond,ynyDUoleiaf, Gall codiffensdrydanirwystromamaliaidostwng reolwyr cadwraeth. ymyrraeth fodynarfdefnyddiolychwanegolhefydi gynyddu’r niferoeddsy’nmagu.Gallydullymao mae caelgwaredaryrysglyfaethwyrwediarwainat ymwneud agadarsy’nnythuaryddaearynbennaflle reolwyr adarhela.Serchhynny, ceirenghreifftiausy’n ysglyfaethwyr ynfwycyffredinaccaeleihybugan Efallai fodhynynegluropamcaelgwaredar yr ailo’rrhainsy’nberthnasolireolwyrcadwraeth. ymhlith amcanionrheolwyradarhela,tramaidimond na chynydduniferoeddsy’nmagu.Mae’rddaubeth er mwyneusaethuynyrhydreffwyllwyddiannus blynyddoedd dilynol.Maecynhyrchuadarychwanegol ran cynydduniferoeddyradarsy’nmagumewn fyw arddiweddytymormagu,mae’nllaicysono lladd –ynamlcynydducyfranyrysglyfaethsy’n cael gwaredarysglyfaethwyr–yngyffredinoltrwyeu ysglyfaethwyr ymmhedwarbanbydyndangostrabod Mae adolygiadauoastudiaethaucaelgwaredar fwyd amgenachyflyruigysylltubwydâblasdrwg. neu eucauallan,rheolicynefinoedd,darparuffynhonnell ymyriadau o’rfatholygucaelgwaredarysglyfaethwyr gall rheolwyrbywydgwylltymyrrydileihaueieffaith.Gall Os credirbodysglyfaethuyncyfynguarniferoeddadar, ddirywio wrthi’wcynefinoeddgaeleulleihau. cael eugostwngganysglyfaethuarôli’wniferoedd poblogaethau’r petrisllwydynfwytebygolofodwedi yw’r rheswmynypendraw. Erenghraifft,mae mewn rhywogaeth,efallaimainewidcynefin gall ymddangosmaiysglyfaethusy’nachosidirywiad mae’r achosionposibhynynamlgysylltiedig.Ery CRYNODEB 11 Niferoedd llawer o ysglyfaethwyr adar wedi cynyddu Niferoedd llawer o ysglyfaethwyr adar wedi yn y DU dros y degawdau diwethaf. Mae tystiolaeth gynyddol yn dangos fod poblogaethau rhai adar sy’n magu ar y ddaear yn cael eu cyfyngu gan ysglyfaethu. Ar y llaw arall, ychydig o dystiolaeth sy’n dangos bod ysglyfaethu wedi cyfyngu ar niferoedd adar cân. Gellir cynyddu niferoedd adar cân sy’n magu’n nad yw llwyddiannus trwy ladd eu hysglyfaethwyr,er hyn yn cynyddu niferoedd yr adar sy’n magu mewn blynyddoedd dilynol, mor gyson. Mae sawl ateb arall heblaw lladd, er mwyn lleihau ysglyfaethu, er na wyddom pa mor effeithiol ydynt. Mae’n amlwg fod angen gwaith ymchwil pellach. I grynhoi, mae’r adolygiad yma’n dangos bod: I grynhoi, mae’r adolygiad yma’n dangos • • • • • n yn denu ysglyfaethwyr ac na µ

Darparu ffynhonnell fwyd amgen – gall rhoi bwyd gwahanol i ysglyfaethwyr yn y gobaith y byddan nhw’n lladd llai o ysglyfaeth ateb y broblem heb orfod lladd, ond hyd yma, prin fu’r profion yn y DU a llwyddiant rhannol a gafwyd. Mae angen cynnal profion pellach i wella’r â bo’r dechneg o gysylltu bwyd technegau yma. Tra blas drwg wedi’i defnyddio i leihau ysglyfaethu wyau adar yn y UDA, nid oes profion maes llwyddiannus wedi eu cynnal yn y DU. Fel arall, yn hytrach na cheisio lleihau ysglyfaethu, gellid rheoli cynefinoedd er mwyn cynhyrchu mwy o ysglyfaeth adar a sicrhau ei fod yn goroesi, gan wneud iawn am y yr atebion bod colledion a gafwyd yn sgil ysglyfaethu. Tra posib yma’n ymddangos yn synhwyrol, mae’n siomedig mai ychydig iawn ohonynt sydd wedi’u profi erioed. Mae angen cynnal profion o’r fath er mwyn cael atebion ymarferol ac effeithiol. cywion yn llwgu, na’u s hir am fydd yr oedolion yn cael eu gorfodi i chwilota’n cael gwared fwyd mewn mannau mwy peryglus. Gellid mannau clwydo, ar safleoedd magu yr ysglyfaethwyr a’u a gellid rheoli cyflenwadau bwyd a chynefinoedd gwasgaredig sy’n cynnal poblogaethau ysglyfaethwyr cyffredinol yn well.

Curlews Mark Hamblin (rspb-images.com) Hamblin Mark 12 3. A review of the effects of predation on bird populations

they add to, rather than replace, others forms of mortality, 3.a. A theoretical background and the population may decline. Such predation losses are termed ‘additive mortality’ by ecologists. Only if predation Birth, death and population regulation losses are additive can a predator limit its prey. In Wytham Bird numbers rise or fall depending on the balance Woods, mortality due to predation would have become between births (eggs laid) and deaths, and whether additive had sparrowhawks killed all the doomed surplus individuals move into or out of the local population. Birds of great tit fledglings, as well as some of those that would die for many reasons, but principally because of lack of otherwise have survived to breed the following year. Had food, exposure to extreme conditions, parasites, disease this occurred, the great tit breeding population would have and injury, or being killed by other animals. The latter form declined, yet it did not. In this instance, mortality was not of mortality, predation, is the subject of this review. additive and sparrowhawks did not limit great tit numbers.

In many bird species, predation is the major cause of Many studies of predation in wild birds have been unable egg and chick losses, and an important component of to determine whether predation mortality is additive or fledgling and adult mortality (Newton 1998). The loss not, and this frequently confounds the interpretation of of a single bird to a predator will immediately reduce these studies. Except in extreme cases, it is not possible A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS A REVIEW OF THE EFFECTS PREDATION that species’ population by one. Common sense would to conclude that predators have driven the decline of a thus seem to dictate that an increase in predation would population of their prey simply by measuring high levels inevitably lead to a reduction in population size over time, of predation. and vice-versa. However, by the end of the breeding season, populations of many bird species have more than The magnitude of predation that can occur before the doubled due to the numbers of young fledged. So, for mortality caused becomes additive varies between a population to remain stable, more than half the birds populations and species, and is influenced by clutch and alive at the end of the breeding season must die before brood size, and also by the season during which predation the beginning of the next, if not from predation then for occurs. Clutches of eggs, if predated, can be replaced other reasons. When birds are killed, the resources that reasonably easily in some bird species thus compensating they would have used – such as food or nest sites – for the losses, whereas predation of adult birds just prior become available for others that might otherwise have to the breeding season is more likely to be additive as died or been unable to breed. Consequently, these they cannot be so easily replaced before nesting begins remaining birds are more likely to survive and breed, (Newton 1998). In addition, short-lived species that and thus may compensate for the loss of those birds produce a large number of young, such as the Wytham killed by predators (Newton 1998; see Figure 1). great tits, are better able to compensate for increased mortality from predation than long-lived species that Compensation like this can buffer populations against high produce few, such as some seabirds. levels of predation. For example, when sparrowhawks were present in Wytham Woods, Oxfordshire, they killed Specialist and generalist predators and 18–34% of fledgling great tits each year. Despite this, predator ‘traps’ numbers of breeding great tits were similar whether If a predator that specialises on a single prey species sparrowhawks were present or not (see Figure 2). The causes its prey population to decline, more of the sparrowhawks were thus killing a similar number of birds predator population will starve and predator numbers will that in their absence would probably have died anyway – fall. The two populations may thus regulate each other, most likely starved – before the next breeding season and even though a specialist predator may reduce prey (Perrins and Geer 1980; McCleery and Perrins 1991). The numbers, it cannot normally drive its prey to extinction. sparrowhawks simply changed the cause of death, from over-winter starvation to predation; they were taking an However, a predator’s numbers need not be regulated by otherwise doomed surplus. its prey. If prey numbers increase, for example due to an abundance of food, predator numbers may increase too, The important concept of additive mortality but only up to a point at which the predator then becomes There are, however, limits to which losses of birds to limited not by its prey, but by something else, such as lack predation can be compensated for in this way. If levels of of space for breeding territories. Numbers of prey, however, predation continue to rise, losses may become so great that could continue to rise until they too become limited, It is entirely possible that just as many birds will 13 B die from starvation as when predators were

absent, and the losses from predation will simply A THEORETICAL BACKGROUND add to those from starvation. The mortality due to predation is thus ‘additive’. If this happens, the D population at the beginning of the next breeding season (E) will be lower than at the beginning of the first (A), and the population has declined. In A C this case, predators are limiting the population of their prey, and the prey population is lower than

Number of birds it would have been had there been no predators. E An alternative possibility, however, is that since predators have killed birds during the breeding season, there will be more food to be shared by those remaining over winter, and so fewer will starve. This increased survival of the Begin End Begin breeding remaining birds allows the population to breeding breeding following year compensate for the losses to predation earlier in the year, so that by the next breeding season the prey population is the same (C) as at the beginning of the first and as if predators had Figure 1 An explanation of how a large number of birds must die (the ‘doomed been absent. Thus, even though predators kill predators might, or might not, limit surplus’) between the end of one breeding birds, a prey population need not be limited by their prey. season and the beginning of the next (C). If its predators and can remain stable over time there are no predators, then the most likely because of compensation. Imagine a population of birds whose numbers cause of death will be starvation. vary throughout an annual cycle, from the This is a very simplistic interpretation of a beginning of one breeding season to the Now imagine that there are predators, and complex set of processes. Clearly, predation, beginning of the next. that they kill birds during the breeding season starvation and other causes of death can occur (dashed lines). The population at the end of throughout the year, compensation can take Imagine, in the first instance, that this the breeding season will thus be smaller (D) other forms (for example re-nesting following population has no predators (solid lines). The than if predators were absent (B). What predation of an earlier nest, rather than number of birds in the population will increase happens to the population between the end improved survival) and prey populations need between the beginning of the breeding season of one breeding season and the beginning of not end up only at points C or E. However, it (point A) and its end (B) as birds reproduce. For the next is crucial to understanding the impact does show the general principles of how the population to remain stable between years, of predation. predators might, or might not, limit their prey.

Sparrowhawks

present disappeared returned Sue Tranter (rspb-images.com)

100

10 Breeding pairs

1 1950 1970 1990 Great tit

Blue tit Great tit

Figure 2 Tit populations at Wytham the early 1970s. When sparrowhawks were However, there was no evidence of this, thus Woods, Oxfordshire. present they killed 18–34% and 18–27% of sparrowhawks did not limit tit populations at fledgling great and blue tits, respectively, each Wytham (McCleery and Perrins 1991; redrawn The population of blue and great tits in Wytham year. If mortality due to sparrowhawks had from Newton 1998; Perrins and Geer 1980; ). Woods, Oxfordshire, has been studied since been additive and had they been limiting tit the 1950s. Sparrowhawks were present in the populations, then we would have expected tit woods until 1960, then disappeared during the populations to rise when sparrowhawks organochlorine pesticide era, and returned in disappeared and fall when they returned. 14

perhaps by their food. Thus, prey populations can have two the decline of the bullfinch in the UK (Newton 2004). ‘equilibrium’ levels; a lower one at which they are regulated Predators can even influence the survival of their prey by predation, and a higher one where they have ‘escaped’ without killing them, as prey may avoid foraging in predator regulation and are now limited by, for example, habitats where they are vulnerable to predation and may their food. This may sound highly theoretical, but has thus be more likely to starve (e.g. Hilton et al. 1999). important practical consequences as it is possible to see how prey whose numbers have declined can become These complex interactions between predation and regulated by their predators when at low densities. habitat are considered in detail later in the report. Conversely, short-term interventions, such as improving food supplies or controlling predators, may take a prey population beyond the point at which it is ‘trapped’ by 3.b. Predators and their populations predator regulation, and allow it to increase to a higher level. Native mammalian predators of birds and their eggs in the Most predators of birds in the UK are generalists, preying UK include foxes, mustelids (such as stoats, weasels and on a variety of vertebrate species, and sometimes on badgers), hedgehogs and rodents. Added to this are other foods, such as invertebrates, plant material and several introduced species, principally domestic and feral A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS A REVIEW OF THE EFFECTS PREDATION refuse. This is important because populations of cats, American mink, brown rats and grey squirrels. generalist predators are less influenced by the abundance Furthermore, some mammals are native to some parts of of any one prey species. Thus, for example, should one the UK but have been introduced to others, for example species of bird prey become rare, predators may simply hedgehogs on the Outer Hebrides. Mammals take eggs switch to alternative food sources allowing the prey and chicks, although predation of incubating female birds population to recover (Newton 1998). Alternatively, and on the ground can also be significant. The main avian more worryingly for the conservation status of particularly predators, all native, are raptors, corvids (particularly rare prey, if that prey is preferred for any reason – for crows, magpies and jays), large gulls and great skuas. example, because it is easy to catch – a predator whose Corvids take mainly eggs or small chicks, raptors prey numbers are being sustained by alternative, more mainly on chicks, juveniles or adults, and gulls and skuas common, prey could force numbers of the rare prey will eat eggs and birds at all stages. down, possibly even to extinction. It has been suggested that recent increases in predator Habitat modification and predation populations are largely responsible for the declines of Levels of predation can change even if predator numbers many bird species. In this section, we consider the first remain constant, particularly if habitats are modified in part of this argument and investigate trends in ways that make prey more vulnerable to predation. For populations of predatory birds and mammals in the UK. example, skylarks are obliged by luxuriant crop growth in autumn-sown cereal crops to nest in the sparser vegetation Predatory birds adjacent to tractor tramlines, and consequently suffer high Population trends in the majority of bird species in the UK predation rates (Donald 2004), probably because predators are well known from long-running bird monitoring schemes. such as foxes hunt along the tramlines. In addition, habitat Populations of buzzards, sparrowhawks, magpies and modification can increase predator numbers. Where forests carrion crows have increased strongly over the last 35 become fragmented by farmland, numbers of generalist years – buzzards dramatically so (over five-fold; Eaton et predators, such as foxes and crows, can increase, leading al. 2007a; see Table 1 and Figures 3 and 4). Not all of to higher levels of predation on species living in forests and these increases have been sustained, and while magpie along their edges (Andrén 1992; Chalfoun et al. 2002; and sparrowhawk populations increased rapidly until 1985 Paton 1994; Thompson et al. 2002). and 1990, respectively, their populations have since levelled off. Over the same 35-year period, the kestrel Furthermore, if habitat deterioration reduces food supplies population declined. for birds, they may be forced to forage in more dangerous places, making them vulnerable to predation (Martin 1992). Looking over the medium-term, using data from the For example, loss of seed food due to changes in Breeding Bird Survey for 1994–2006 (see Table 1), a agricultural practices may have contributed both directly, similar picture emerges. Over the UK as a whole, buzzard through starvation, and indirectly, through predation, to and carrion crow populations have continued to increase; Table 1 Trends in populations 15 Species UK UK England Scotland Wales N. Ireland of widespread predatory birds. 1970–2005 1994–2006 1994–2006 1994–2006 1994–2006 1994–2006 PREDATORS AND THEIR POPULATIONS Buzzard +515% +49% +111% +20% –5% – Long- and medium-term trends in Kestrel –28% –25% –9% –65% –– populations of widespread predatory birds (Eaton et al. 2007a; Raven et al. Sparrowhawk +108% –1% –8% ––– 2007; BTO/JNCC/RSPB). Figures for Magpie +97% –1% –4% +40% –8% +24% increasing and declining species are Jay –9% +1% –10% – +33% – given in black and blue, respectively. Carrion crow +80% +21% +29% –5% +30% – Hooded crow – –28% – –48% – +70% Raven – +57% +175% +49% +13% –

6

5 2.5 4 2

3 1.5

2 1 Population index (1970=1.0)

1

Population index (1970=1.0) 0.5

0 0 1970 1975 1980 1985 1990 1995 2000 2005 1970 1975 1980 1985 1990 1995 2000 2005 Year Year

Sparrowhawk Kestrel Buzzard Carrion Crow Magpie Jay

Figure 3 Population trends of sparrowhawks, kestrels and Figure 4 Population trends of carrion crows, magpies buzzards in the UK, 1970–2004 (data from BTO). and jays in the UK, 1970–2004 (data from BTO).

Table 2 Sizes of breeding Species Population (year) % of potential range occupied8 populations of birds of prey in the UK, and the percentage of Honey buzzard 33–69 pairs (2000)1 probably <5% their potential range occupied. Red kite >1,000 pairs (2006)2 5% White-tailed eagle 36 pairs (2006)2 <5% Sources: Marsh harrier 360 breeding females (2005)3 10% 1Baker et al. 2006; 2RSPB unpublished data; 3Eaton et al. 2006; 4Sim et al. 2007; 5Holling Hen harrier 806 territorial pairs* (2004)4 60% et al. 2007; 6Eaton et al. 2007b; 7Eaton et al. Montagu's harrier 12 pairs (2002)5 probably <5% 2004; 8UK Raptor Working Group (2000), 1 itself taken from Newton 1994. Goshawk 410 pairs (2000) 15% Sparrowhawk 40,100 pairs (2000)1 100% * includes the Isle of Man. Buzzard 31,000–44,000 territorial pairs (2000)1 70% Golden eagle 442 pairs (2003)6 60% Osprey c.200 pairs (2005)3 20% Kestrel 36,800 pairs (2000)1 100% Merlin 1,300 pairs (1994)1 90% Hobby 2,200 pairs (2000)1 probably 70% Peregrine 1,400 pairs* (2002)7 95% 16

sparrowhawk, magpie and jay have remained more or (244,000: Harris et al. 1995) and 1999/2000 (258,000: less stable; while hooded crow and kestrel populations Webbon et al. 2004), although each produced with a have declined. Over the same period, raven numbers different method, suggest relative stability over the last increased strongly (Raven et al. 2007). couple of decades. However, mammal data collected by the Breeding Bird survey indicated a sharp decline in fox Populations of the UK’s rarer predatory birds have mostly numbers of 34% between 2000 and 2005 (Davis et al. increased in recent years. These increases are largely part 2007). Despite these differing conclusions, and evidence of a continued recovery from the deleterious impacts of of a recent downturn, it does seem likely that fox pesticides in the 1960s and 1970s (Newton and Wyllie numbers increased over the 20th century. 1992) and human persecution that led to the loss of five raptor species from the UK just after the turn of the 20th Few data are available on trends in mustelid populations. century (Newton 1979; Thompson et al. 2003). While Game bag records show that following an increase peregrine numbers increased by 10% from 1991 to 2002, from 1961 to the mid 1970s, stoat bags subsequently and from much lower numbers prior to 1991 (Eaton et al. declined, most likely due to a reduction in trapping 2004), and hen harriers increased by nearly 40% between effort associated with a greater emphasis on reared 1988 and 2004 (Sim et al. 2007), golden eagle numbers rather than wild gamebirds (GCT 2004; McDonald and A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS A REVIEW OF THE EFFECTS PREDATION remained fairly stable between 1992 and 2003 (Eaton et Harris 1999). The dramatic decline in weasel bag al. 2007b), limited by persecution (Whitfield et al. 2004). numbers, down to a quarter of 1960 levels, more likely Marsh harrier numbers more than doubled between 1995 reflects a genuine trend (GCT 2004) and may be and 2005 (Eaton et al. 2006), and over the last 25 years associated with reductions in the extent of rough goshawk numbers have increased nearly ten-fold (Ogilvie grassland, an important habitat for voles, a favoured et al. 2004). Despite these recent increases, populations weasel prey. However, reductions in trapping effort of many of the UK’s birds of prey still remain well below cannot be ruled out (McDonald and Harris 1999). the level that the habitat could sustain, with some Numbers of bag records of the introduced American mink species still absent from large areas of the UK (UK Raptor rose dramatically following its first breeding in the UK in Working Group 2000; see Table 2). 1956, but have stabilised since the early 1980s, with a hint of a recent decline. Pine marten (Balharry et al. 1996) Predatory mammals and polecat (Birks and Kitchener 1999) populations have In contrast to birds, monitoring of mammal populations increased and extended their range, although there is in the UK is generally less well developed, and trends debate over the status of pine marten in England and much more poorly known. Wales (MacDonald and Baker 2005), and both species remain relatively scarce and localised. Badger numbers The National Gamebag Census, a collation of increased by an estimated 77% between the mid 1980s gamekeepers’ and farmers’ bag records, has shown a and the late 1990s, though with large regional variations fivefold increase in the numbers of red foxes killed per unit (G Wilson et al. 1997). area since 1961 (GCT 2004), albeit with a levelling off, or even a decline, since 1990. Although changes in killing Game bag records suggest that grey squirrel numbers effort and effectiveness, for example by using spotlights, rose across most of the UK during 1975–2000 (Whitlock can affect bag sizes, the scale of the change in bags along et al. 2003), although recent trends are more stable with with evidence of range expansion (e.g. Reynolds and some range expansion at the northern edge (Battersby Tapper 1993) does suggest a real increase in fox densities et al. 2005). and numbers over the last forty years. This increase may be partly due to large-scale rear and release of gamebirds In summary, while populations of most of the UK’s avian improving the foxes’ food supply (GCT 2004). In addition, predators have increased over the last few decades, reductions in gamekeeper numbers since the end of the recovering from the deleterious impacts of pesticide 19th century, and abandonment of game management in pollution and human persecution, those of a few many areas, may have allowed a recovery of fox (and widespread predators have more recently stabilised other predator) numbers. (magpie and sparrowhawk). Among the predatory mammals, foxes, badgers, polecats, pine martens, and By contrast, estimates of the UK red fox population from non-native American mink and grey squirrels have 1981 (252,000 adults: Macdonald et al. 1981), 1995 increased, while weasel numbers may have declined. EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 17 et al. 1999). 2005; Raven and Noble 2006), those 1994; Parr 1992). 1994). et al. et al. et al. et al. to be the most important nest predators at one study to be the most important crows and lesser black-backed area (Antrim), with hooded at another where foxes were largely gulls most important Islands). Annual breeding success absent (Lough Erne per breeding pair in Antrim and averaged 0.19 young pair on Lough Erne, with 0.38 young per breeding that the breeding success at calculations indicating the rate of Antrim was sufficiently low to account for Ireland as a population decline seen across Northern whole (Grant breeding on a moor in north-east Scotland declined rapidly to extinction during the 1980s. This decline coincided with the cessation of predator control and the planting of conifers close to the breeding area (Harding Golden plover in breeding Evidence that predation may cause declines bird species, numbers has been found for other wading golden plovers too. While the UK breeding population of decades, with has shown mixed trends over the last few evidence of declines in some regions (Gibbons 1993; Sim In 2003, the Northern Ireland Environment and Heritage In 2003, the Northern Ireland Environment to Service initiated a management trial in Antrim crows determine the effectiveness of killing hooded breeding and foxes to promote a recovery in curlew is still at an early success and numbers. Although the trial the impact on stage, crow numbers have declined, while Despite curlew numbers fox abundance is less clear. – from continuing to decline on all monitored sites hatching 72 to 49 pairs between 2002 and 2006 – data), success seems improved (RSPB, unpublished in numbers of but it is too soon to expect an increase breeding pairs. Before these changes in management, numbers of golden plovers varied between 14 and 23 pairs over a ten-year period. During the subsequent decline, nest predation increased and annual breeding success declined, from 0.37 down to 0.02 young per breeding adult. Numbers of common gulls and carrion crows increased rapidly and were the main predators of nests in the first few years after the management changed, with foxes becoming increasingly important (Parr 1993). While the decline could have been explained by increased losses of adult golden plovers over winter, the evidence suggested that the decline was most likely a consequence of high levels of nest predation (Harding 1993; et al. 2002). et al. 1999). Foxes appeared et al. 2000; Henderson et al. 2005), particularly in Wales and Scotland where 2005), particularly in Wales et al. Breeding wading birds Curlew While numbers of breeding curlews in the UK have declined over the last few decades (Gibbons When reading this evidence, it is worth bearing in mind When reading this evidence, it is worth some scientific the manner in which it was collected, as than approaches provide more compelling evidence simply others. For example, as outlined earlier, not on its documenting high levels of predation does are own provide convincing evidence that predators combined with further analyses unless limiting their prey, high showing that the level of predation is sufficiently to have caused a reduction in prey numbers. More are studies that compare numbers convincing, however, of prey at sites (or times) where predator numbers differ; those performed as experiments are generally the most convincing. In this section, we consider the evidence from a range In this section, we consider the evidence field studies on of detailed observational or experimental Each the impacts of predators on wild bird populations. one form or other, of these studies has been published, in this published in the scientific literature; here we review so that the evidence focus on UK studies, evidence. We although in used is directly relevant to UK conservation, elsewhere. a few instances we draw insight from studies in turn, those on breeding These field studies cover, seabirds. wading birds, gamebirds, songbirds and 3.c. Evidence of impacts of predators on 3.c. Evidence of bird populations The trend in stoat numbers is less clear. Changes in Changes is less clear. The trend in stoat numbers such as pig farming, land-management practices, have may sheep husbandry, pheasant rearing and of generalist predators, such as boosted populations foxes and crows. their populations halved between 1994 and 2005 (Raven and Noble 2006), their numbers in Northern Ireland have by 58% between 1987 and fallen even more dramatically, 1999. These declines occurred on wet grasslands, bogs and moorland, as well as intensively managed grassland (Stanbury An RSPB study of curlew breeding success in Northern Ireland found that 82–95% of breeding attempts failed at the nesting stage, with predation accounting for about 90% of nest failures (Grant Sim 18

Lapwing were removed. Overall, no consistent effect of predator Breeding lapwings have undergone widespread and control on the survival of over 3,000 lapwing nests was marked declines in the UK over the last few decades found. However, the impact of control on nest survival (Baillie et al. 2005; Fuller et al. 1995; Gibbons et al. 1993; varied considerably between sites (see Figure 5), and Henderson et al. 2002; O’Brien 2004; Wilson et al. 2001). analyses showed that predator control did result in Changes in farming practices, such as land drainage and significant improvements in nest survival at sites where the switch from spring-sown to autumn-sown cropping, predator densities were high (Bolton et al. 2007). are almost certainly the main causes of these declines (Chamberlain et al. 2000; Sheldon et al. 2005; Shrubb Radio-tracking of nearly 500 chicks on a selection of these 1990; Taylor and Grant 2004; Wilson et al. 2001). sites similarly showed no consistent effect of predator control on chick survival. However, by chance, the sites Due to population and range contraction, lapwings in selected for radio-tracking generally held low densities of some areas may now be more vulnerable to predation foxes and crows, so control measures tended to be than previously. For example, eggs or chicks of lapwings unnecessary. At other sites, where radio-tracking was not breeding as isolated pairs or at low densities are more undertaken and predator densities tended to be higher, likely to suffer predation than those breeding at high the proportion of adults with young late in the season – A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS A REVIEW OF THE EFFECTS PREDATION densities or in large colonies (Berg et al. 1992; Seymour an index of breeding success – was twice as high in years et al. 2003; Stillman et al. 2006), probably because large when predators were controlled (Bolton et al. 2007). concentrations of lapwings can effectively defend their nests against predators. In addition, the loss of Overall, predator control had no impact on lapwing gamekeeping from some upland areas may have led population trends across the 11 sites (Bolton et al. 2007). to increased predation independent of habitat change. There are several explanations for this result. Perhaps the predators that were killed would, if present, merely Very low breeding success, rather than a simple lack have taken a similar number of lapwing eggs and chicks of suitable breeding habitat (such as spring tillage and to those that would ultimately have died for other adjacent grassland), may have been responsible for the reasons, so foxes and crows had no impact on lapwing extinction of lapwing colonies in Hampshire recently, populations (i.e. mortality was not additive). Alternatively, where predation probably accounted for almost 60% lapwing populations might have been increased by killing of nest failures (Milsom 2005). Similarly, records collated these predators, but this effect was hidden by the by the British Trust for Ornithology (BTO) show that nest mobility of adult lapwings: numbers settling to breed at losses to predation were higher during the 1990s than a site will depend not only on breeding performance at in earlier years, even though for most nests the cause that site in previous years, but also on the attractiveness of failure was unknown (Chamberlain and Crick 2003). of nesting conditions on neighbouring sites. Finally, an In a four-year study in northern England, adult and young effect on lapwing populations might have been lapwings were more likely to return to breed in an area demonstrated had a higher proportion of predators been with good numbers of gamekeepers – Teesdale, County killed. However, fox densities following control in this Durham – than an area with fewer keepers – Eden Valley, experiment were similar to those reported in another Cumbria (Thompson et al. 1994). The Teesdale population experiment carried out by the Game Conservancy Trust remained stable over the study period, whereas the Eden (GCT) on Salisbury Plain, where an effect of predator Valley population halved. control on grey partridge breeding numbers was found (Tapper et al. 1996; see p.21). In 1996, the RSPB began an eight-year experiment to investigate the impact of fox and crow control on This study shows how it is possible, by experimental lapwings breeding on its lowland wet grassland nature removal, to demonstrate an impact of predators on some reserves. On each of 11 sites, fox and crow control was measures of breeding success of their prey, but undertaken for four consecutive years, and its impact demonstrating an unequivocal impact on subsequent prey compared with four years without predator control. Adult population size is rarely straightforward. fox and territorial crow numbers were reduced by 40% and 56%, respectively, during years of predator control, but there was no change in total crow numbers as non-breeding individuals moved in when territorial birds EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 19

Hatching success Hatching Mark Bolton Mark Bolton 0 0.3 0.1 0.7 0.4 0.4 0.2 0.6 0.5 2000 control no control Pulborough Brooks (above right), fox and crow Pulborough Brooks (above control had no clear effect on hatching success (a contributor to breeding success), nor on the number of lapwing pairs. If anything, hatching success tended to be lower when foxes and crows were controlled, and the number of breeding pairs tended to decline early on in the period of control, but remained stable thereafter and once control ceased. 1996

0 5

10 15 40 35 30 20 25

Breeding pairs Breeding Fledglings / pr / Fledglings Nigel Butcher Mark Bolton 0.1 0.8 0.7 0.3 0.4 0.9 0 0.2 0.5 0.6 control as Berney Marshes (above left) that suffered as Berney Marshes (above high levels of predation, predator control was effective. Breeding success (orange line) improved over the first period of fox and crow control, deteriorated dramatically when control stopped, and improved once again when control was restarted. The number of pairs of lapwings at Berney followed a similar trajectory (grey line). By contrast, on other sites, such as 2004 Year Year no control 2000 control Lapwings and BERNEY MARSHES, NORFOLK MARSHES, BERNEY PULBOROUGH BROOKS, WEST SUSSEX 1996

0

10 20 30 70 60 50 40 Breeding pairs Breeding Images clockwise from top left: a fox filmed at a lapwing nest on an RSPB reserve; a week-old lapwing chick; RSPB biologists Richard Allcorn and Laura Hurt radio-tracking lapwing chicks; adult lapwing alarm-calling. predator control. Figure 5 Fox and crow control had no consistent effect on hatching or fledging success of lapwings, nor on their subsequent population trends, across eleven RSPB lowland wet grassland there was great nature reserves. However, variation between reserves. On some, such 20 A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS predator control)werehigh(Tharme low whencrowdensities(areasonable indicatorof Furthermore, goldenploverand lapwingdensitieswere the extentofmuirburnwastaken intoaccount. wader andmeadowpipitdensitiesremainedevenwhen 1998; Whittingham1996).However, thedifferencesin vegetation preferredfornestingbythesewaders(Robson muirburn (heatherburning),whichcreatestheshort to otheraspectsofmoorlandmanagement,particularly benefits breedingwaders,thedifferencesmaybedue While thesefindingsmightsuggestthatpredatorcontrol were morelikelytobedue predator controlthanto together, thesefindingssuggestthat thedifferences type ofmoorlandduetoillegalpersecution(Court golden eaglesarealsoknowntobelessabundantonthis abundant ongrousemoors.Henharriers,peregrinesand (meadow pipit,skylark,whinchatandcrow)wereless Figure 6).Bycontrast,fourspeciesofpasserinebirds were takenintoaccount. even whenhabitatandotherdifferencesbetweenthemoorlandtypes 2004; Etheridge grouse moorsthanother(Tharme golden ploversandcurlews(thoughnotsnipe)higheron English andScottishuplands,withdensitiesoflapwings, by theRSPBandGCTacrossextensiveareasof 1997). Suchdifferenceswerefoundinastudyundertaken in prep;HaworthandThompson1990; on moorsmanagedforredgrouseshooting(Ewald waders, theirpopulationswouldbeexpectedtohigher the breedingseasonwerelimitingpopulationsof for thedrivenshootingofredgrouse.Ifpredationduring in crows, onmoorlandisatraditionalpartofmanagement densities bird Intensive controlofsomepredators,principallyfoxesand wading and shooting Sport shooting thanonothermoors(Tharme snipe, werehigheronheathermoorlandmanagedforredgrouse Densities ofbreedinggoldenplovers,curlewsandlapwings,butnot snipe densitiesonheathermoorland. Figure 6 2

the uplands the Pairs per km (+ 1 SE) 0.5 1.0 1.5 3.0 2.0 2.5 3.5 0 Golden plover Golden plover, curlew, lapwingand grouse shooting managed forred heather moorland et al. et 1997; Whitfield Curlew et al. et 2001). Theseresultsheld Lapwing other moors et al. et et al. et et al. et 2001). Taken 2003, 2004). 2001; see Snipe et al. et et al. et et al. et archipelago (Jackson important densitiesofthesewadingbirdsonthe helped maintaintheextraordinarilyhigh,internationally on theseislandsbeforetheintroductionofhedgehog It seemslikelythattherelativelackofgroundpredators predators oftheeggswadingbirds,suchaslapwings. Hedgehogs introducedtotheWestern IslesofScotlandhavebecome breeding black-headedgullsincreased, theavocet reserve, Suffolk(Hill1988),showed thatasnumbersof A long-termstudyofavocetsat RSPB’s HavergateIsland Avocet declines (JacksonandGreen2000;Jackson hedgehogs hasbeensufficientlyhightoaccountfortheir spread, andpredationoftheneststhesebirdsby those partsoftheislandgrouptowhichhedgehogshave declined markedly. Thesedeclineshavebeengreateston particularly dunlins,redshanks,snipeandlapwings,have that nestontheislands’coastalgrasslands(machair), introduction there,populationsofbreedingwadingbirds Hebrides whereitwasintroducedin1974.Sinceits it isaconservationpriority, itisnotnativetotheOuter the on While thehedgehogisnativetomuchofUK,where birds wading and Hedgehogs (Fletcher be drawnbeforecompletionofthestudyin2008 in subsequentyearsarelessclearandconclusionscannot wader breedingsuccess,buteffectsondensity Initial findingssuggestthatpredatorcontrolimproves corvids andmustelidsonmoorlandinnorthernEngland. is undertakinganeight-yearexperimentcontrollingfoxes, densities areduetopredatorcontrolspecifically, theGCT robust assessmentofwhetherdifferencesinwader vegetation management.Despitethis,toprovideamore Outer Hebrides Outer et al. et 2005; Fletcher2006). et al. et 2004). et al. et

2004). Digger Jackson Digger EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 21 1996; see Figure 7). 1996.) et al. et al. The Salisbury Plain The Salisbury 1996). Predator control period et al. Changes in partridge populations at two Changes in partridge Plain during years with sites on Salisbury without predator control. (shaded) and each stock are given for Spring and autumn (Redrawn from Tapper year. Figure 7 Figure experiment. In 1984, the GCT started a six-year experiment looking In 1984, the GCT started a six-year experiment testing at the second of these causes, specifically partridge whether control of predators could increase on two large stocks. The experiment was conducted partridge areas of Salisbury Plain with similar initial were densities. Foxes, corvids, stoats and rats with controlled in one area in the first three years, second. treatments switched between plots in the were all Numbers of foxes, carrion crows and magpies period markedly reduced during the partridge nesting (Tapper This study showed clearly that mortality due to predation was, at least in part, additional to all other forms of rather than simply replacing it, and that predation mortality, limited the grey partridge population on this site. By the late 1990s, the GCT advised that a combination of in-field management techniques, particularly set-aside strips, conservation headlands and beetle-banks, could provide the nesting and brood-rearing habitats required to stabilise the grey partridge population, though predator control could help speed its response to habitat management. More partridge pairs bred successfully when these predators were controlled, and they produced larger after three years, autumn broods. Consequently, populations were 3.5 times higher with predator control as well as being sufficient to than without. Moreover, sustain some shooting, the population level was maintained until the following spring, and after three years, spring densities were 2.6 times higher with predator control than without (Tapper Year Year 2000a; Potts 1980, 1986). First, and et al. on the farmland study area dropped steeply from 1984 1985 1986 1987 1988 1989 1990 1991 1984 1985 1986 1987 1988 1989 1990 1991 SITE 2 SITE 1 2

0 0

50 50 450 400 450 400 350 350 300 250 300 250 200 200 150 150 100 100

Number of partridges of Number Number of partridges of Number invertebrates important in chick diets most importantly, have declined due to the use of herbicides, leading to reduced chick survival. Second, brood production has declined due to predation by foxes and corvids. Third, nesting cover has been lost as field boundaries have been It was also removed to improve farming efficiency. recognised that shooting losses could be contributory where grey partridge densities were low. Grey partridgeGrey partridges on the South Downs, Sussex, have been studied by the GCT since 1968 (Aebischer and Ewald 2004; Potts 1980, 1986). Grey partridges have declined dramatically across the UK, and the population on the South Downs has suffered a similar fate. Here, densities per km Gamebirds population declined as gulls took their eggs and young. population declined as gulls took their eggs and numbers recovered breeding success Avocet which following the onset of gull control in 1964, population. substantially reduced the size of the gull avocet breeding success after 1971, However, to predation by deteriorated once more, apparently due at a second kestrels. Gull control was initiated in 1966 Minsmere reserve, Suffolk, at the RSPB’s avocet colony, These but had no impact on avocet breeding success. that examples illustrate the rather complex interactions and the can occur among various predator species site to another, difficulty of generalisation from one study even for the same species of predator and prey. about 20 pairs, stabilised at about 4.5 in the early 1990s, and fell again to under two pairs in the late 1990s. Three plausible causes for the decline were identified (Aebischer 22 A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS Some eggswerecarried2kmawayfrom theclutchsite. predated gaveameasureofpredator activityinAbernethyForest. predated byacrow. Therateatwhich theseclutcheswere A predatedhen’s egg Productivity Spring density (pairs / km2) 10 30 15 25 20 0.5 5 0 3.5 2.5 1.5 3 0 1 2 9919 9319 971999 1997 1995 1993 1991 1989 pressure, intensive High shooting farming orpo rdto Raptorpredationpresent No raptorpredation A hen’s eggfromanartificialclutch Management scenario pressure, intensive Moderate shooting farming Year Moderate shooting pressure, habitat

management Ron Summers Ron A malecapercailliedisplaying. has relativelylittleimpact. by habitatmanagement,raptorpredation densities. Wherepopulationsareboosted has arelativelylargeimpactonspring At lowpartridgedensities,raptorpredation nest coverandlegalcontrolofpredators. Habitat managementincludesprovisionof (from datainWatson 2004a). different managementscenarios of greypartridgeinSussexunder Figure 8 and elsewhereinScotland. productivity atAbernethy Figure 9 (Data fromSummers confidence limitsforthereferencesites. elsewhere. Theverticallinesare95% controlled there,andatreferencesites predators were(shaded)andnot female) atAbernethyduringyearswhen Productivity ofcapercaillie(chicksper breh Otherreferencesites Abernethy Predator controlperiod Predicted springdensities Capercaillie et al. et

2004b) Philip Newman (rspb-images.com) Newman Philip EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 23 in prep), it 2004b; see Figure 9). et al. et al. success in this study. The study recommended that The study recommended success in this study. forests should favour bilberry and management of these it also and crows. However, include control of foxes or removing fences, thus suggested that marking from fence collisions, could reducing adult mortality of chicks and eggs to predation. compensate for losses Abernethy In the late 1980s, the RSPB acquired the production in Forest reserve in Scotland. Capercaillie chick it was thought possible that and the forest was poor, might be the predation on capercaillie nests by crows found. cause, because crow-predated eggs were if predation alone was it was not known However, sufficient to cause the poor productivity. the effect of crows on capercaillie investigate To out a study from 1992 to 1996, RSPB carried productivity, during in which crows were killed. Capercaillie productivity there was no this period was compared to years when also with several control (1989–1991 and 1997–1999), and changes in levels other sites at which there were no abrupt are hard to of predator control. Because capercaillie nests the rate of find, predation pressure was estimated from hens’ eggs. predation on artificial nests containing domestic part of the The results showed that during the latter at predator control period (1994–1996), productivity productivity Abernethy improved and was greater than Abernethy was elsewhere; at other times, productivity at lower than elsewhere (Summers On the face of it, then, predator control did increase rainfall was lower However, capercaillie productivity. during the years of predator control than at other times, confounding these results somewhat. Nevertheless, analysis showed that capercaillie productivity was high High when both predation and rainfall (in June) were low. June rainfall probably caused chicks to become wet and die from hypothermia. It therefore seems likely that crow but only in control can improve capercaillie productivity, Although fox control was years when June is dry. attempted, there was no detectable change in fox numbers, and no association between capercaillie productivity and fox abundance, based on counts of droppings. Based on these results, and knowing that numbers of full-grown capercaillie are determined by breeding productivity at Abernethy (Summers seems likely that crow predation there can limit capercaillie numbers, at least in years when June is dry. Because of this, the RSPB has reinstated crow control at Abernethy to benefit the population of capercaillie. et al. 2001). et al. 1996; Summers et al. 2000, 2001). would be little affected by raptor 2 2004), and concluded that et al. et al. 2007) concluded that shooting pressure was et al. 2004a). The main predators were carrion crows, foxes and, to a lesser extent, raptors, although the effects of each could not be readily distinguished. Although pine martens are known predators of capercaillie eggs and chicks, they had no discernable effect on breeding Notwithstanding a very recent change in its fortunes, by the mid 1990s, the capercaillie had become one of the fastest declining species in the UK (Wilkinson Capercaillie However, the more recent decline on the Sussex study site the more recent decline However, not associated with changes in chick in the late 1990s was but instead with increased survival or brood production, analyses losses of adults. While preliminary over-winter had declined most in areas where showed that partridges had increased (Aebischer raptors, especially buzzards, 2004a; study (Watson 2000), a more comprehensive Watson predation (Watson 2004a and b). However, raptor predation 2004a and b). However, predation (Watson already reduced could accelerate the decline of populations for example, farming practices. to this low density by, management Populations boosted by sympathetic habitat little affected by to increase breeding productivity would be typical levels of raptor predation (Figure 8). Using data on partridge survival rates from radio tracking Using data on partridge survival rates from study sites in and estimates of raptor density from 20 populations England, it has been calculated that partridge above five pairs per km largely responsible for this recent decline, not raptor largely responsible for this recent decline, pressure was predation. In two areas, where shooting stabilised relaxed or removed, grey partridge numbers The initial findings even in the presence of raptor predation. raptor were explained when it was discovered that pressure. densities were greatest in areas of high shooting (Smart In addition, buzzards rarely prey on grey partridges 2004a and b). 2002; Swann and Etheridge 1995; Watson An 11-year study (1991–2001) across 14 forest areas in Scotland investigated the causes of this poor breeding success (Baines capercaillie reared more young in forests with more bilberry bushes and fewer predators. Bilberry provides while its leaves and berries, and the good cover, invertebrates it supports, are an important component of capercaillie chick diet (Picozzi Possible causes of this decline were low breeding success associated with a changing pattern of spring temperatures and reduced chick survival in wet summers, aggravated by fully grown birds colliding with deer fences (Moss 1986; Moss 24 A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS Lindley (Bowker juveniles survivedbetweenSeptember andFebruary In astudyinWales, 66%ofadultsbutonly18% of grouse populationsintheUKare limitedbypredation. several studieshaveinvestigated whetherornotblack Given thatpredationratesarequitehighinblackgrouse, birds (JohnstoneandLindley2003;seeFigure10). about 60%ofdeathsattributabletopredation,mostlyby chicks diedbetweenhatchingand50daysofage,with between 1998and2001showedthatthree-quartersof Storass andWegge 1987).AstudyinWales bytheRSPB main causeofnestfailure(BrittasandWillebrand1991; 1986; Willebrand1988).Predationalsotendstobethe 1984; CaizerguesandEllison1997;PicozziHepburn the maincauseofdeathamongstblackgrouse(Angelstam have foundpredation,largelybyraptorsandfoxes,tobe studies intheScottishHighlandsandcontinentalEurope for youngbirds(Warren andBaines2002).Similarly, raptors andstoatsinequalnumbers,mortalityhighest 1998–2000, withmostdeathsattributedtopredationby that 37%ofblackgrousediedduringautumnandwinter failure. AstudybytheGCTinnorthernEnglandshowed predation isamajorcauseofmortalityandbreeding Black grousefallpreytoavarietyofpredators,and populations inEnglandandWales (Calladine management hasledtotherecoveryofblackgrouse Warren andBaines2004).Appropriateconservation fragmentation ofmoorlandfringehabitats(Baines1994; northern Englandithasbeenattributedtothelossand disposed ofratherthanbeingleftforscavengers. unpublished data).AtAbernethy, theseremainsarenow winter foodsourceforcrowsandfoxes(RSPB, Discarded visceraofculledreddeerareasubstantial (Andrén 1992;Kurki by reducingthefragmentednatureofpinewoodlands capercaillie andtodetergeneralistpredators,forexample (Summers a moresustainableformoflong-termmanagement Ultimately, however, habitatmanagementislikelytobe conifer plantations(Pearce-Higgins recent declinehasbeenlargelyduetothematurationof degradation (Warren andBaines2004).InPerthshire,its largely aconsequenceofhabitatloss,fragmentationand since thelate19thcentury. Ultimately, this decline is The blackgrousehasdeclinedacrossmuchofEurope grouse Black et al. et et al. al. et et al. et 2003). 2007). Althoughthislowsurvival, 2004b), bothtoprovidefortheneedsof et al. et 1998; Summers et al. et 2007), whilein et al. et et al. et 2004b). 2002; on siteswhereitwasnot(Calladine grazing wasreduced,yetdeclinedbynearly2%peryear increased bynearly5%peryearatsitesonwhichsheep subsequent experiment,maleblackgrousenumbers to provideshelterfrompredators(Figure11).Ina and theseplaceshadmorechickfoodtallvegetation higher onmoorswithlight,ratherthanheavy, grazing, and thosewithout(Baines1996).However, bothwere (who undertakemanagementincludingpredatorcontrol) 20 moorsdidnotdifferbetweenthosewithgamekeepers grouse densitiesandbreedingsuccessmeasuredacross In astudybyGCTinnorthernEnglandandScotland,black grouse numberstoo,butonlyindryyears. prep), crowcontrolherewouldbelikelytoimproveblack capercaillie comparedwiththose onasecond.The breeding successandtrendsof blackgrouseand killed ononeislandoverafive-year period,andthe black grousenumbershadrisenby85%(Lindley shooting management.Bytheendofproject,male control tookplaceonsomeofthesesitesaspart burning onadjacentdryheath.Fox,crowandstoat management includedconiferthinning,andmowing and bilberry, importantchickfeedinghabitats.This grouse sitestostimulatetheregenerationofheather habitat managementwasundertakenonseveralblack As partofarecoveryprojectinWales runbytheRSPB, productivity (GrantandDawson2005;Summers grouse numbersatAbernethyarelargelydeterminedby only inyearswhenrainfallwaslowJune.Becauseblack crow controldidimproveblackgrouseproductivity, but RSPB’s AbernethyForestreserve(seeundercapercaillie), Capercaillie and black grouse on Baltic islands Baltic on (Marcström grouse black In anexperimentundertakenon twoislandsintheBaltic and Capercaillie more limitedbytheavailabilityofsuitablehabitat. circumstances theymaybe,whileinothersseem are limitedbypredationisthusequivocal;insome The evidencethatblackgrousepopulationsintheUK to thisincreaseinblackgrousenumbers. contributions ofhabitatmanagementandpredatorcontrol 2003). Analysesareunderwaytoclarifytherelative In anexperimentalstudy(Summers dramatically overthelongerterm,from1994to2007. 2000 and2003,numbersneverthelessincreased a short-termdeclineinblackgrousenumbersbetween reportedly duetopredation,mayhavecontributed et al. et 1988), foxesandpinemartens were et al. et et al. et 2004b) at 2002). et al. et et al. et in EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 25 Game keeper Present Absent Black grouse breeding success on moors Light grazing Heavy grazing Mowing dry heather

1 0 3 2 In N England and Scotland, there was no difference in black grouse In N England and Scotland, there was no difference game keeping. breeding success between moors with and without breeding success was higher on moors with light grazing, However, provide shelter as these had more chick food and tall vegetation to from predators. (Adapted from Baines 1996). Figure 11 with and without game keeping. Breeding success (young per female) ±1SE female) per (young success Breeding Avian predator 24

Survived 24 Pat Lindley Pat moorland creates good habitat for breeding black grouse. Moorland management Mammalian predator 3 The fates of 75 black grouse chicks from predator 7 Unidentified

26 Andy Hay (rspb-images.com)) Hay Andy A black grouse chick being measured to determine its age. 41 broods radio-tracked by the RSPB in Wales, 1998–2001 (Johnstone and Lindley 2003). Values given are percentages. Values Figure 10 or starved) 16 Other (chilled Fate unknown 26

treatments were then switched for a further four years. moors (Tharme et al. 2001). This difference remained, When these predators were killed, the average brood size though at a reduced level, after taking account of of capercaillie and black grouse combined was 5.5 and differences in habitat between the two types of moor. 77% of females had chicks. By contrast, when foxes and Gamekeepers – whose densities were three times higher pine martens were not killed, average brood size was 3.3 on grouse moors – burn heather as well as legally killing and 59% of females had chicks. Numbers of breeding foxes, crows and mustelids; both forms of management grouse increased after two years of predator removal. were shown to independently enhance red grouse density. Thus, predation was limiting population size and predator Taken together, these more recent studies suggest that control improved breeding success and population sizes generalist predators can limit breeding red grouse densities. of both grouse species. Historically, predator control was routinely extended to This otherwise straightforward story was slightly raptors, and this drastically reduced the range and complicated by the presence of alternative prey for foxes abundance of many species (Newton 1979). Although and martens. When voles were abundant, predation on now illegal, the killing of raptors continues (UK Raptor grouse was reduced and their brood sizes and Working Group 2000) and limits, in particular, the range percentages of females with chicks were higher. This and abundance of three species that are in the greatest A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS A REVIEW OF THE EFFECTS PREDATION suggests that predator control is less useful when vole perceived conflict with grouse management: hen harrier, numbers are high, usually one year in three or four. peregrine and golden eagle (Etheridge et al. 1997; Whitfield et al. 2003, 2004). Red grouse Over large upland areas of the UK, red grouse populations The 1992–97 Joint Raptor Study (JRS) sought to measure are managed at high densities to sustain driven shooting whether predation by raptors could limit red grouse of grouse on private estates. This management includes numbers at a level substantially lower than would occur legal control of generalist predators of grouse such as in the absence of raptors (Redpath and Thirgood 1997). foxes, corvids and mustelids (Thirgood et al. 2000b). The study took place at Langholm, in Dumfries-shire, While this form of management is widespread, and five other moors across Scotland. Raptors were surprisingly few studies have attempted to determine protected throughout the study and, at Langholm, hen the impact of predation on red grouse populations harriers increased from two to 20 breeding females and (Redpath and Thirgood 1997). peregrines from three to six pairs with protection.

Early studies of high density grouse populations found that The JRS found that hen harrier numbers were determined predators, principally foxes and peregrines, concentrated on not by red grouse, but rather by numbers of alternative non-territorial grouse in winter (Jenkins et al. 1967; Watson small prey, particularly meadow pipits. Where meadow 1985) with most territorial grouse surviving, suggesting that pipit density was high, so was that of hen harriers. In predation was unlikely to limit red grouse breeding addition, fluctuations in harrier numbers between years populations. By contrast, a study of a low-density population paralleled variations in the abundance of small mammals found that over-winter survival rates of territorial and non- (Redpath and Thirgood 1999). Both of these prey species territorial grouse were similar, and suggested that at least were favoured by heavy sheep grazing, which converted part of the winter predation was additive, thus reducing the many areas of heather to grass. density of breeding grouse in the spring (Hudson 1992). Two studies have investigated whether red grouse densities Overall, spring raptor predation at Langholm removed are higher where generalist predator numbers are lower. 30% of breeding grouse and, by the end of the study, The first, undertaken by GCT, looked only at the numbers of hen harriers were removing 45% of grouse chicks grouse shot – a measure of post-breeding densities – and annually. These losses were probably additional to other showed that bags were larger on moors with the highest forms of mortality, and together reduced the post- densities of gamekeepers, probably because these moors breeding grouse stock by half. Over winter, raptors killed had the highest levels of fox and crow control (Hudson 30% of grouse, though it was unknown to what extent 1992). The second study, undertaken by RSPB and GCT this mortality was additive. across extensive areas of the English and Scottish uplands, found that breeding red grouse densities were twice as A population model predicted that, in the absence of high on moors managed for grouse shooting as on other raptors, grouse numbers would increase over two years EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 27 et al. 2000a). et al. However, driven grouse management was not in place at driven grouse However, density at a four of these, and raptors remained at low these reasons, fifth (Redpath and Thirgood 1997). For all of the Langholm we do not know the general applicability we do know that meadow pipit findings. However, densities at Langholm are probably representative of grouse moors across upland areas of the UK (Smith 2001), and thus that other moors could potentially attract nesting hen harriers at densities comparable to those at Langholm during the JRS. The JRS was not a controlled experiment and cannot establish cause and effect. Nonetheless, its results imply that while raptors were not responsible for long-term declines in red grouse bags, predation can limit grouse populations at low density and reduce shooting bags to the point at which driven grouse shooting becomes untenable. This is most likely where raptors breed at high density because of the abundance of alternative prey (the Langholm experience) and where grouse either due to low points in cyclic populations are low, population change, or poor management over the longer term (Thirgood 2000a). These were et al. 1995), as well as by increased et al. 2000c). The impact of raptor predation et al.

The short-term limitation of red grouse numbers by raptors does not explain long-term grouse declines recorded at Langholm and more widely across upland areas of the UK (Thirgood was sufficient to dampen the usual cyclical changes in was sufficient to dampen the usual cyclical increases from grouse populations, and prevented these occurring, whilst numbers did recover from cyclical lows on neighbouring moors where raptors were not protected (Redpath and Thirgood 1997). by 1.9 times and post-breeding numbers by 3.9 times by 1.9 times and post-breeding numbers (Thirgood The densities of hen harriers observed at Langholm in the latter years of the JRS are amongst the highest recorded anywhere on the Scottish mainland (UK Raptor Working Group 2000). The JRS included five other moors where raptor populations were protected, and red grouse populations there did not respond as they did at Langholm, nor did raptor numbers increase as much. populations of foxes and crows, and reductions in heather burning following the long-term decline in gamekeeper numbers (Hester and Sydes 1992; Hudson 1992). associated with the deterioration of heather moorland due to afforestation and increased grazing by sheep and deer (Thompson

Red grouse Red Mark Hamblin (rspb-images.com) Hamblin Mark 28 A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS Fuller 1994; Siriwardena intensive agriculture(Donald declined mostinthoseEuropeancountrieswiththe and technologicaldevelopment.Tellingly, farmlandbirds dramatically increasedasaresultofproductionsubsidies Over thesameperiod,agriculturalintensification confirmed byexperiment. least 15ofthesespecies,thecausetheirdeclineswas summer foodoracombinationofthesefactors.Forat affected bylossofnestsites,lackwinterfood, 2004) concludedthat27outof30farmlandspecieswere work accumulatedoverthelastdecadeorso(Newton impacts ofagriculturalintensificationonbirds. environmental changes,startingwiththewell-researched but setsthisagainstabackdropofotherwidespread songbird populations,principallyinfarmlandandwoodland, This sectionconsiderstheimpactsofpredatorson Benton also declined(Donald1998;SothertonandSelf1999; invertebrates, onwhichbirdsdependforfood,have have beenlessaffected(Fuller elsewhere, too),whilespecieslivinginotherhabitats more thangeneralists(thosethatoccuronfarmland,but and Noble2006).Specialistsoffarmlandhavedeclined 1975–85) andsomespeciescontinuetodecline(Raven occurred mostrapidlyduring1970–90(especially Potts 1991;RobinsonandSutherland2002;Stoate many farmlandspecies(Aebischer agricultural changeisthemajorcauseofdeclines ecological studieshaveprovidedcompellingevidencethat increases innumbersofpredators, suchasmagpiesand predation Various attemptshavebeen madetotestwhether magpie and sparrowhawk Has 2001; Vickery al. et 2005; Siriwardena and numbersintheUKEurope(Donald mainly songbirds,havedeclineddramaticallyinrange intensification Many speciesofbirdassociatedwithfarmlandhabitats, agricultural of Impacts declined ineachofthesehabitats(Gregory areas. Numbersofsomespeciessongbirdhave important populationsinotherhabitats,includingurban most numerousinfarmlandandwoodland,withsome Songbirds arefoundinawidevarietyofhabitats,but Songbirds influenced changes in songbird numbers? songbird in changes influenced 2001; Benton et al. et et al. et 1995; Marchant 2002). et al. et et al. et al. et 2004). Areviewofthelargebody et al. et 1998). Farmlandplantsand 1998). IntheUK,thesedeclines 2002; Chamberlain et al. et et al. et et al. et 1990; Tucker andHeath 2001). Intensive et al. et 1995; Shultz 2000b; Anderson et al. et et al. et et al. et 2002). 2001; et al. et 2000; et al. et (Thomson could havecausedthesesongbirdpopulationdeclines it isveryunlikelythatsparrowhawksormagpies than expectedbychancealone(Figure12).Thus, present thanwhenabsent.Thisnumberisfewer songbird speciesdeclinemorewhenapredatorwas made (23songbirdspecies,twopredators)dida by magpies.Inonlytwooutofthe46comparisons to sparrowhawks,orwhoseeggschicksaretaken The studyconsidered23songbirdspeciesthatfallprey less) whenpredatorswerepresentandviceversa. we mightexpectsongbirdnumberstofall(orincrease If thesepredatorswereaffectingsongbirdpopulations, influenced changesinsongbirdnumbersatthosesites. presence orabsenceofsparrowhawksandmagpies as songbirds,itwaspossibletotestwhetherthe Because theCBCrecordedavianpredatorsaswell conducted surveysofbirdpopulationsonthesesites. Birds Census(CBC)inwhichvolunteerbirdwatchers 1960s until2000,theBTOorganisedCommon songbirds inthiswood. sparrowhawks hadnotreduced densitiesofbreeding (Newton following re-colonisationofthe woodbysparrowhawks (song thrush)showedanysigns ofdeclineinnumbers over thecourseofstudy, butonlyone ofthese Nine outof13songbirdspeciesincreasedinnumbers disappeared, thendeclinewhentheyre-colonised. songbird numberstoincreaseassparrowhawks were affectingsongbirdpopulations,wemightexpect re-colonising theareain1970s.Ifsparrowhawks organochlorine pesticideera,beforerecoveringand (1946–59), thendisappearedfor13yearsduringthe in thisareaduringtheearlypartofstudy between 1949and1979.Sparrowhawkswerepresent Common, Surrey, weresurveyed eachsummer Songbirds breedinginanoakwoodonBookham depress breedingdensitiesoftheirsongbirdprey. songbirds general conclusionthatsparrowhawkstendnotto on predation Two localstudiesofwoodlandbirdsconfirm the Sparrowhawk woodland sites(Thomson over 30yearsonnearly300lowlandfarmlandand analyses ofsongbirdpopulationchangesrecorded One ofthemostpowerful,UK-widestudiesinvolved local andnationalscales. declines. Thesetestshavebeenundertakenatboth sparrowhawks, havebeenamajorcauseofsongbird et al. et et al. et 1997). Thestudyconcludedthat 1998). et al. et 1998). Fromtheearly EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 29 2000). et al. 1991). In addition, et al. 1998). et al. The percentages of Numbers of Numbers Magpie Sparrowhawk Percentage of boxes occupied Percentage of nests successful Figure 13 tit nestboxes occupied and nests that were successful at increasing distances from sparrowhawk nests in Oxfordshire Wytham Woods, (data from Geer 1978). songbird species whose songbird species increased or populations on CBC plots decreased or when magpies were present sparrowhawks (Thomson The species are grouped into those The species are grouped (very whose trends were significant The few) or non-significant (most). was number of significant trends alone. fewer than expected by chance Figure 12 Figure 20-year period (1966–86) when magpie numbers were 20-year period (1966–86) when magpie increasing rapidly (Gooch However, there was no evidence that high failure rates However, were actually caused by high corvid numbers, not least because the predator species responsible for nest failures were unknown. Magpie densities in urban parkland in Manchester in the late 1980s were higher than had previously been recorded in other urban areas, and during the same period fewer than 5% of the blackbird nests in the parkland produced fledged young (Groom 1993). Although the cause of most nest failures was unknown, predation was the most important cause where it was known, and most of this was attributed to magpies. Despite the dramatically high level of nest failure, blackbird populations remained stable songbird nests in regions with high magpie densities did not suffer higher failure rates than elsewhere. by contrast, showed that failure rates The second study, of blackbird and song thrush nests during incubation, but not chick rearing, were higher in areas in which magpies and jays were more widespread (Paradis 180–240 sig. increase 120–180 non-sig. increase 60–120 non-sig. decrease Distance from sparrowhawk nest (m) <60 sig. decrease

6 8 0 0 2 4 5

10 40 25 20 15 45 50 35 30 14 12 10

Percentage Number of species of Number Magpie predation studies investigated whether songbird nests were Two on songbirdsmore likely to fail when (or where) densities of magpies and other corvids were high. Both studies were based on collection of nest record cards, covering many the BTO’s thousands of nests over many years from across the UK. The first study found no evidence that nest failures of 15 potentially vulnerable songbird species increased over a A famous long-term study of great and blue tits in Wytham A famous long-term study of great and blue Oxfordshire, showed similar results and provided Woods, further insights into the effects of sparrowhawks on their The presence of a successful sparrowhawk nest prey. markedly reduced both nestbox occupancy and the nesting success of tits within 60 m of a sparrowhawk’s nest (Geer 1978), probably because sparrowhawks took most tits were unaffected parent tits (Figure 13). However, because sparrowhawk nests were typically 800 m apart. Despite sparrowhawks taking up to a third (18–34%) of all fluctuations in tit recently fledged tits during summer, populations were unrelated to changes in sparrowhawk numbers gradually numbers (Perrins and Geer 1980). Tit increased over a forty-year period as the wood matured. 30 Grey squirrel Roger Wilmshurst (rspb-images.com) A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS A REVIEW OF THE EFFECTS PREDATION

during the study, though low compared with other urban artificial feeding of birds in the winter (Stoate 2005), areas, and it was likely that the blackbird population was the introduction of which has made it more difficult to only maintained by immigration from elsewhere. tease apart the effects of predator control and habitat improvements. Although the Loddington study will A study at Loddington in Leicestershire, run by the GCT, continue for several more years before further investigated the effects of removing magpies and crows conclusions can be drawn, for the spotted flycatcher, on songbird nesting success and population levels (Stoate at least, predator control may have been beneficial. and Szczur 2001). Game management began in 1993 and Flycatcher numbers increased in woodland, though not involved a range of habitat improvements and predator in gardens, between 1992 and 2001 – the period of control. Predation by corvids was the main cause of nest predator control – and declined after 2002 once predator failure, and nesting success of four out of six open-nesting control stopped. It seems that predation reduced songbird species was higher in areas with fewer corvids flycatcher breeding success at Loddington, perhaps (Stoate and Szczur 2001). In addition, numbers of five out accounting for fewer breeding birds in subsequent of six songbird species increased following the start of years (Stoate and Szczur 2006a, b). The identity of the game management. Thus, while corvids probably did predators was unknown. influence songbird nesting success, it is unclear whether predator removal or habitat improvements (or both) were Predation on woodland songbirds by squirrels responsible for the songbird population increases. and woodpeckers Many species of woodland birds have declined over the In an attempt to disentangle these possibilities, predator last 30 years (Amar et al. 2006; Fuller et al. 2005; Hewson control ceased at Loddington in 2002. While songbird et al. in press). While several hypotheses have been numbers did decline in 2002 and 2003, they subsequently proposed to account for these declines, the Repeat increased in 2004 and 2005, leading to the conclusion Woodland Birds Survey (RWBS) – which investigated that ‘...the benefits of predator control for songbirds are changes in bird numbers over 20 years on more than 400 not as clear as they are for game’ (Stoate 2006). The woodland sites in the UK – concluded that the most likely increase in numbers may have resulted from improved cause was changes in woodland structure (Amar et al. EVIDENCE OF IMPACTS OF PREDATORS ON BIRD POPULATIONS 31 et al. 2002; 2003) et al. et al. 2004). In an et al. 2006). 2005). A recent study, however, has shown however, 2005). A recent study, Grey squirrel drey densities. squirrel drey Grey et al. et al. Decline Stable or increase Decline Stable or Grey squirrel drey densities were higher in drey densities were higher Grey squirrel and numbers of hawfinches woodlands where than in woodpeckers declined lesser spotted increased they remained stable or those where mid-1980s and 2003–04. (Data between the from Amar Figure 14 Figure 2002). However, there have also been spectacular 2002). However, declines (>90%) in several UK cities (Crick Cats and house cats in the There are an estimated nine million domestic sparrows their possible UK, and there is increasing concern about all cats impact on native wildlife. By no means are by relatively hunters, but extrapolations from prey returns small samples of cats (May 1988; Woods indicate that, nationally, they kill millions of birds each they kill indicate that, nationally, whose numbers have halved The house sparrow, year. the species in the UK since the mid 1970s, is one of house sparrows most frequently killed by cats. However, rates, and are short-lived and have high reproductive on sparrows whether cats impose an additional mortality would have or simply kill similar numbers to those that or weak died anyway – for instance by taking young unclear. individuals (Møller and Erritzøe 2000) – is killed by Populations of some other birds commonly (Raven cats, for example greenfinches, have increased and Noble 2006). Declines of house sparrows in the countryside have been attributed to agricultural intensification (Hole Summers-Smith 1999) and a high proportion of their population occurs in gardens (Bland that nestling starvation – sometimes of whole broods – has played a major role in reducing house sparrow 2005). A large- breeding success in Leicester (Vincent scale experimental test of whether lack of invertebrate food for chicks limits house sparrow populations is currently being run by the RSPB in London. Whether cat predation has contributed to house sparrow Nevertheless, it is prudent to declines remains unclear. adopt a precautionary approach to the impact of this and to design and adopt methods non-native predator, that reduce the numbers of sparrows, and other animals, killed by cats. area of Bristol, cats killed at least 45% of the estimated post-breeding population of house sparrows annually (Baker et al. 2005), whose et al. Lesser spotted woodpecker 2004) found only et al. 2006). Despite this, there may be 2005). However, willow tits did not 2005). However, et al. Hawfinch et al. 2005). et al. 2005). Overall, though, population changes of the 1 0 2 3

et al. 2006; see Figure 14). While squirrels may thus be implicated in their declines, this study by no means it is possible proves that they were responsible. However, that canopy-nesting species, such as hawfinch, are particularly vulnerable to predation by grey squirrels (Fuller 2006). These changes were themselves probably a 2006). These changes were themselves stands, a consequence of the ageing of woodland increased reduction in active management and, possibly, of under-storey deer browsing and consequent reduction vegetation (Amar a few species for which predation from increased a few species for which predation from spotted numbers of nest predators – such as great – is responsible woodpeckers or introduced grey squirrels (Fuller One of the UK’s most rapidly declining woodland species, One of the UK’s the willow tit, nests in soft dead wood, and their nests are thought to be particularly vulnerable to predation by great spotted woodpeckers (Fuller great majority (33 out of 35) of woodland bird species were apparently unaffected by squirrels. decline any more rapidly on woodland CBC plots where great spotted woodpeckers increased most (Siriwardena 2005), and while they did on farmland, this is a relatively unimportant habitat for willow tits in the UK. A similar analysis investigated marsh tit trends and found no relationship with great spotted woodpecker numbers on CBC plots (Siriwardena 2006). anecdotal evidence to suggest that they were major anecdotal evidence to suggest that they that changes predators of birds. While the RWBS showed cause of in woodland structure was the most likely populations population declines among woodland birds, and lesser of two out of 35 species studied, hawfinch declined more at sites with spotted woodpecker, (Amar relatively high densities of squirrel dreys A review of the impacts of the introduced grey squirrel A review of the impacts of the introduced on birds in the UK (Hewson numbers have increased dramatically over the last 30 years (Baillie Squirrel dreys / ha (±1SE) ha / dreys Squirrel 32 A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS success wasunrelatedtomink presenceorabsence mink remained(Figure15).Despite this,overallbreeding Western Isles,wasbetterthanonnearbyLewis where eradication, thesurvivaloftern nestsontheUists,in ultimately abandoned(Craik1997). Followingmink mink controlcontinuedtosuffer breedingfailureandwere birds fromelsewhere,whereasislandsnotsubjectedto was restoredandcoloniesgrewduetoimmigrationof controlled onasampleofislands,breedingproductivity the Scottishwestcoast(Craik1995).Whenminkwere almost zerowhenintroducedAmericanminkcolonised small gullsandternsonsea-lochisletswasreducedto impact onseabirdpopulations.Thebreedingsuccessof Rats arenottheonlyintroducedpredatorhavingan the firsttimein50years2002(Zonfrillo2002). early 1990sratswereeradicated,andpuffinsbredfor numbering severaltensofthousandsbirds.Duringthe 19th centurycausedtheextinctionofapuffincolony island intheFirthofClyde,introductionrats islands (Ratcliffe2004a).OnAilsaCraig,forexample,an eradication programmeshavebeenimplementedonUK (de Leon that stormpetrelsarelargelylimitedtorat-freeislands only asingleislanddidtheyco-occur, stronglysuggesting archipelagos, whilebrownratswerepresenton29; found tobreedon42outof142islandssurveyedinthe one studyinOrkneyandShetland,stormpetrelswere the ratswereremoved(Ratcliffe2004a).Forexample,in disappeared andstayedaway, onlyre-colonisingwhen When ratshavecolonisedislands,seabirds for nestingseabirds,particularlyburrow-nestingpetrels. probably themainpredatorstorenderislandsunsuitable operate intheUK.Here,introducedbrownratsare predators comefromoverseas,suchprocessesalso Although thebestexamplesoflimitationseabirdsby predator-free nestsites(BirkheadandFurness1985). to arichfoodsupply),orbycreatingcompetitionfor in areasthatwouldotherwisebesuitable(e.g.adjacent populations –simplybypreventingthemfrombreeding thus limitseabirdpopulations–andindeedotherbird on predator-free islandsorcliffledges.Predatorscan predators. Forthisreason,seabirdsoftenfavournesting in largecoloniesthatcanbeextremelyattractiveto chicks oradultbirdstopredation.Inaddition,theybreed are poorlyadaptedtocompensateforlossesofeggs, Rothery 1991;Lack1968;Weimerskirch 2002),andso several yearsoldandlaysmallclutches(Croxall Seabirds tendtoliveforalongtime,donotbreeduntil Seabirds et al. et 2006). Sofar, aroundadozenrat and otherspeciesincreased(Nordström populations ofarangeseabirds(skuas,ternsandauks) were removedfromsmallislandsintheBaltic, supply isfavourable.WhereintroducedAmericanmink promotes improvedproductivityinyearswherefood is requiredheretoassesswhetherminkeradication due tostarvationanyway. Furthermonitoringofterns attempts thatfailedduetominkweredestinedforfailure was poorthroughouttheWestern Isles,sothatnesting was becauseduringthestudyyears,foodavailability contributing todeclinesofkittiwakes(Heubeck switched increasinglytoseabirdprey, probably quantity ofdeadfishdiscardedfromtrawlers,greatskuas reductions intheavailabilityofsandeelsand species. Whenchangesinfisherypracticesledto and maybepartlyresponsiblefordeclinesofsomeprey Great skuasareefficientpredatorsofotherseabirds, further testsareunderway. has sofarprovedinconclusive(SmartandRatcliffe2000), diversionary feedingtominimisetheimpactofkestrels by kestrelsisalsohighatafewcolonies,andwhile predation inasinglenight(Pickerell2004).Predation defences canoccur, sometimesresultinginserious predation atmostcolonies,butbreachesofsuch 2004b). Electricfencinghasbeenusedtoreducefox tides, humandisturbanceandfoxpredation(Ratcliffe nesting onbeacheswheretheyarevulnerabletohigh and thisisatleastpartlyexplainedbytheirhabitof Little ternshavedeclinedby39%intheUKsince1971, removal (Nordström2003). species onthisarchipelagoincreasedfollowingmink Overall, populationsof14out22seabirdsandother unpublished data). suggest arapiddeclineofLeach’s petrelsthere(JNCC, mainly onseabirds,especiallyLeach’s petrels(Phillips rapidly growinggreatskuapopulationonStKildapreys 1997; OroandFurness2002)Arcticskuas.The (Clode andMcDonald2002;Ratcliffe al. 1999), andrecentsurveysonthemaincolonyofDun et al. et et al. et 2005). This 2003). et al. et et SUMMARY OF THE IMPACTS OF PREDATION ON BIRD POPULATIONS 33 2005). et al. 3.d. Summary of the impacts of 3.d. Summary populations predation on bird included in this review that A summary of the studies of predation on bird populations in investigated the effects each prey species studied 3. For the UK is given in Table multiple species), the table outlines (some studies covered not there was the predator species involved, whether or species’ breeding evidence that predation limited the prey broadly population and the type of evidence obtained, of these studies classified into ‘fair’, ‘good’ or ‘best’. None prey; even those simply measured levels of predation on with the lowest type of evidence (fair) determined was high enough whether the level of predation measured to have caused the observed declines. Nevertheless, (or where) studies that compared prey numbers when or varied widely predators were present and then absent, Within these in abundance, provided better evidence. predator studies, those that were undertaken as formal evidence, with removal experiments provided the best predator numbers those that relied on ‘natural’ variation in yielding good evidence. a comprehensive While this table does not attempt to be give a useful summary of all relevant studies, it does it shows the growing evidence In particular, overview. from the UK that populations of some ground-nesting golden birds, such as breeding waders (e.g. curlews, (grey plovers, lapwings, avocets) and gamebirds partridges, capercaillie, black grouse, red grouse), are more likely to be limited by predation than other groups; in the table, most studies of these species fall in the left hand ‘evidence’ columns. The reason for this is currently but may be that their nests or young are more unclear, vulnerable to predation. Similar results have recently been found in Germany (Langgemach and Bellebaum 2005) and the Netherlands (Teunissen In addition, the table shows that the evidence that songbird numbers are limited by predation is weak, with most studies falling in the right hand ‘no evidence’ a classic study of the impact of columns. In particular, sparrowhawks and magpies on nearly two dozen species of songbird over three decades on several hundred sites found no evidence that either predator influenced trends there is compelling evidence in songbird numbers. Rather, that changes in farming practices have led to the declines of many farmland songbirds, and emerging evidence, particularly from the RWBS, that numbers of some woodland songbirds have declined due to long-term changes in woodland structure. 2005). et al. Survival of tern nests in the Western Isles of Survival of tern nests in the Western Scotland in 2004–05 (Ratcliffe Symbol size is related to colony size, with the proportion that hatched in purple, and proportion that failed in orange. Following mink eradication on the Uists (bottom left island group) the survival of tern nests there was better than on nearby Lewis (top right) where mink remained. Figure 15 34 A REVIEW OF THE EFFECTS OF PREDATION ON BIRD POPULATIONS omntr mrcnmn 18 15 18 18 15 Grey squirrel Greysquirrel American mink American mink Green woodpecker Hedgehog Woodpigeon 1 Little tern Americanmink Common tern Noevidence Kittiwake Hedgehog Common gull Black-headed gull Redshank Curlew Evidence Snipe Dunlin Lapwing Golden plover Avocet Brownrat Grey partridge Capercaillie Black grouse Predatorspeciesstudied Red grouse Kestrel Storm petrel Prey species Good reduced thepreypopulation. that thisisunsustainablyhighorlikelytohave Fair Type ofevidence numerals inbold. of introducedpredatorsarealsogivenas in allstudiesreferredto.Theindividual these predatorswereintroducedontoislands studied; withtheexceptionofgreysquirrels, not nativeandwereintroducedtothearea Predators whosenamesaregiveninbold the backofthisreport. complete referenceforeachstudyisgivenat authors anddatearegivenbelow. The columns referstoanindividualstudy, whose Each numeralintheevidence/no limited preynumbers No evidence prey numbers Evidence Key: are excluded. success onlyandthosefromoutsidetheUK Those thatlookedatmeasuresofbreeding densities andtrendsintheUKareincluded. at impactsofpredatorsonpreynumbers, of evidenceobtained.Onlystudiesthatlooked predation limitedpreynumbers,andthetype whether therewasanyevidencethat given: thestudiesthatinvestigatedpredation, For eachpreyspeciesthefollowingare in thisreview a summaryofevidencepresented on birdpopulationsintheUK; Table 3 High levelofpredationandfurtherevidence Comparison ofpreytrendsornumbers Impacts ofpredators Evidence thatpredationlimited No evidencethatpredation o 21 19,20 12 Fox Great skua 4,13 2 Hooded/carrion crow, Carrion crow, fox, 8 30 Carrion crow, fox, Carrion crow, commongull,fox 4,5 Black-headed gull 3 Carrion crow, magpie,buzzard, Carrion andhoodedcrow, raptors,fox Goshawk, carrioncrow, fox Carrion crow, fox,henharrier, peregrine Goshawk prohw,fx10 sparrowhawk, fox esrbakbce ul o 74 17 lesser black-backedgull,fox 16 11 13 12 30 29 28 27 26 25 23 14 19 18 8 7 6 5 9 2 10 Best in abundance*. predators werepresentorabsent,varied in places(orattimes)wherewhen) 15 24 22 21 20 17 4 3 1 reference, thus: Each numberreferstoaparticular with thosewhere(orwhen)theywerenot. which predatorswereexperimentallyremoved, of preynumbersinareas(oryears)from eghg15 hedgehog hedgehog Grant andDawson2005;Summers Summers Baines Baines 1996 Thirgood Summers Petty Tharme Hudson 1992 De Leon Jackson andGreen2000; Bolton Tapper Harding Hill 1988 Bowker Newton 1998 Siriwardena 2005 Siriwardena 2006 Stoate andSzczur2006a,b Groom 1993 Thomson Thompson Heubeck Craik 1997 Watson 2004a,Watson Newton Amar Ratcliffe 2004b Oro andFurness2002 Grant Formal experiments;i.e.comparison et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et et al. et 2003 et al. et 2006 1999 2004 2007 2001 1996 2006 2007 2000c 1994; Parr1992 1997 in prep 2004; Summers 1997 1998 1994 et al. et 2007 et al. et arGo etFi odBest Good Fair Best Good Fair et al. et et al. et h in prep 2004 2004; 4, 14 a e b , 15 a b c the previousyear. year areoftenstronglylinkedtoproductivity capercaillie andblackgrousenumbersinone numbers weremeasured,butitisknownthat d low densities. populations, butonlywhentheywereat g several alternativecauses. predation, butcouldnotbeseparatedfrom f e grey partridgesonthisSussexstudysite. was responsiblefortherecentdeclineof h at another(Minsmere). areas withdifferingpredatornumbers. not. Comparisonsofpreynumbersacross introduced, withthoseontowhichtheyhave areas ontowhichpredatorshavebeen when absent.Comparisonofpreytrendsin when predatorswerepresentwiththose were not.Comparisonofpreytrendsattimes management withthoseonlandwherethey predators werecontrolledforgame Comparison ofpreynumbersonlandwhere *Examples are: s m was unknown. not gardens;theidentityofpredator may limitthisspecies,butonlyinwoodland, increased dramaticallyoverthelonger-term. black grousenumbers,thoughthepopulation have contributedtoashort-termdeclinein Possible evidenceofimpactsparrowhawk Sparrowhawk analysisfromreference23. Breeding successratherthanbreeding Raptor predationlimitedgreypartridge Shooting pressure,notraptorpredation, At onesite(HavergateIsland)only, butnot Results fromthisstudysuggestpredation Higher returnratesinwell-keeperedarea. Apparently highlevelsofpredationmay Magpie analysisfromreference23. 110 11 9 6 a a d 22 22 4 7 c 12 d 16 SUMMARY OF THE IMPACTS OF PREDATION ON BIRD POPULATIONS 35 , , , , , , , , , , , , , , m m m m m m m m s s s s s s s s s s s s m s , s f m m m m , 24 , 24, , 24 , 24 , 24 , 24 , 24 , 24, , 24 , 23 , 23 , 27 , 28 , 24 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 , 23 m 22 , 23 , 23 m m m m m , 23 24 m m s s m 22 24 22 22 22 22 22 22 29 29 22 22 22 22 s s s 22 s s s 23 23 23 23 22 22 23 23 23 23 23 22 23 23 23 23 f g m , ?24 26 s 23 23 25 Fair Good Best Fair Good Best grey squirrelgrey squirrel 22 22 grey squirrel 22 grey squirrel 22 grey squirrel 22 grey squirrel 22 grey squirrel 22 grey squirrel 22 grey squirrelgrey squirrel 22 22 grey squirrel grey squirrelgrey squirrel grey squirrel grey squirrelgrey squirrel 22 22 22 grey squirrel 22 grey squirrel Sparrowhawk, magpie, Sparrowhawk, magpie, Sparrowhawk, magpie, Sparrowhawk, magpie, Sparrowhawk, magpie, Sparrowhawk, magpieSparrowhawk, magpieSparrowhawk, magpieSparrowhawk, magpieSparrowhawk, magpie 23 23 23 23 23 Sparrowhawk, magpie, Sparrowhawk, magpieSparrowhawk, magpieSparrowhawk, magpie, 23 23 Unknown, jay, Great spotted woodpecker, jay, Great spotted woodpecker, Carrion crow, sparrowhawk, magpie, fox sparrowhawk, Carrion crow, Sparrowhawk, magpie, Sparrowhawk, magpie, Sparrowhawk, magpie, Sparrowhawk, magpie, Sparrowhawk, magpie, 23 4, Sparrowhawk, Sparrowhawk, magpie, Grey squirrel Grey squirrel magpieSparrowhawk, 22 23 Sparrowhawk, magpie, Blue tit Great tit Dunnock Coal tit Willow warbler Bullfinch HawfinchYellowhammer Reed bunting Grey squirrel 22 Linnet Lesser redpoll squirrel Grey Greenfinch Goldfinch Siskin Grey squirrel Tree sparrow Tree Chaffinch Jackdaw Grey squirrel Nuthatch JayStarling Grey squirrel Marsh tit Willow tit GoldcrestSpotted flycatcher Pied flycatcherLong-tailed tit Grey squirrel Grey squirrel Grey squirrel Blackcap warblerWood Grey squirrel Mistle thrush Garden warbler Grey squirrel Meadow pipit Blackbird Song thrush Chiffchaff Robin Redstart Grey squirrel Wren Tree pipitTree squirrel Grey Great spotted woodpecker Great spotted woodpecker Lesser spotted Skylark Prey species Predator species studied Evidence No evidence 36 MITIGATING THE IMPACTS OF PREDATORS increased post-breedingpopulation size(‘autumn improved nestsurvivalofprey in 23of27studies(85%), studies andfoundthatpredator removalresultedin The firstreview(Newton1993, 1998)considered30 limited bypredation. been undertakenonpopulationsofspeciesthoughttobe however, thatsuchexperimentsaremorelikelytohave looked attheirsuccess.Itisworthbearinginmind, and herewesummarisetheresultsoftworeviewsthat However, manymorehavebeenundertakenelsewhere, avocets, capercaillie,blackgrouseandgreypartridges. have alreadybeenmentioned,forexampleonlapwings, Several examplesofsuchexperimentsonbirdsintheUK translocated elsewhere(e.g.Molony killing, thoughlivepredatorscanberemovedand predator removalisachievedbyshootingortrappingand years withandwithoutpredatorremoval.Generally, which thesameparcelsoflandhaveperiodsseveral Some ofthemostinformativestudiesinvolveaswitch,in others, andtheimpactonpreymeasured(Newton1998). predators removedinsomeareasoryears,butnot these studiesshouldbedoneasformalexperiments,with predator removalisausefulmanagementtool.Ideally, populations increase.Suchstudieswillalsoshowwhether the predators,orreducetheirnumbers,andseeifprey a predatorislimitingpopulationofitspreytoremove One ofthemostconvincingwaystodeterminewhether 4.a. Predatorremoval warn preyofapproachingpredators. feeding, conditionedtasteaversionandmethodsthat management ofhabitattoreducepredation,diversionary (most commonlybykillingthem),exclusionofpredators, approaches areconsideredhere;removalofpredators Interventions couldbeofvarioussorts,andseveral surplus ofbirdsintheautumnandwinterforshooting. sufficient youngareproducedeachyeartoprovidea declining. GameManagersinadditionneedtoensurethat the breedingpopulationincreased,oratleaststopped or haddeclined,andwouldseektoensurethatthesizeof Manager wouldinterveneifthepreypopulationwererare of GameandConservationManagersdiffer. AConservation species theyareconcernedabout.However, theobjectives intervene tominimisetheimpactofpredatorsonprey then GameorConservationManagersmaywishto success, survivalofyoungandadultsorbreedingnumbers, If predationisthoughttobereducingnestsurvival,breeding of predators 4. Mitigatingtheimpacts et al. et 2006). high (Marcström capercaillie andblackgrousewhen volenumberswere predators fromthestudyareas; othersmayhavemoved Second, controleffortsmaynot haveremovedall when Junewaswet(Summers earlier; predatorcontrolwasineffectiveforcapercaillie numbers. Severalexamplesofthishavebeengiven predators wouldnotleadtoincreasesinpreybreeding been limitingthepopulationsofpreystudiedandsokilling breeding populations.First,predationmightnothave experiments mightnotalwaysincreasesubsequentprey There arearangeofreasonswhypredatorcontrol others showednoeffectorevendecreases. breeding populationsfollowingpredatorremoval,whereas statistical significance.Somestudiesshowedincreased larger followingpredatorcontrol,thiseffectfelljustshortof consistent. Whiletherewasatendencyforthemtobe the breedingpopulationfollowingyearwereless survival andautumndensities,theeffectsonsizeof while predatorcontrolresultedinmarkedincreasesnest many ofthosefromthefirstreview. Itconcludedthat, 20 individualpredatorcontrolexperiments,incorporating increase breedingbirdpopulations.Thisreviewexamined and foundthatreducingpredatornumbersdidnotreliably whether nestsurvivalchangedornot,butbyhowmuch) the magnitudeofeffectpredatorcontrol(i.e.notjust 1997), amoreformalstatisticalanalysis,tookaccountof By contrast,thesecondreview(CôtéandSutherland have attemptedtoremovepredatorsofsongbirds. food availabilitywaslow(Ratcliffe (Nordström 2003;Valkama experiments ledtoincreasesinbreedingnumbers other species.Amonggamebirds,onlyfouroutof10 birds maybemorevulnerabletopredationthanthoseof sites. Alternatively, theprecocialyoungofground-nesting vulnerable topredationthanotherbirdsthatnestinsafer specifically gamebirdsorwaterfowl,whichmaybemore Most ofthespeciesstudiedwereground-nesters, and oncepredatorswereremoved,preypopulationsrose. prey populationsconcernedhadbeenlimitedbypredation, Thus, itappearsthatinmorethanhalfofallstudies,the population sizesof84%,70%and61%,respectively). post-breeding populationandsubsequentbreeding very similarresults(improvementsinnestsurvival, considered eightmorestudies,andfound,unsurprisingly, A recentupdateofthisreview(Nordström2003) subsequent breedingnumbersin10of17studies(59%). densities’) in12of17studies(71%),andincreased et al. et 1988) andforternswhenchick et al. et 2005). Fewexperiments et al. et et al. et 2004), for 2005).

PREDATOR REMOVAL 37

Gamekeeper Peter Cairns (rspb-images.com) Cairns Peter 38 MITIGATING THE IMPACTS OF PREDATORS

naturally re-colonisedbypredators andonwhichthereare (Newton 1993,1998).Onislands, whicharelesslikelytobe rates andpopulationlevelsreverted totheirformerlevels control stopped,predatorssoon movedbackandpredation been stableorrising.Inmostof theseexperiments,when while ittendedtoincreasepopulations thathadformerly four formerlydecliningpopulationsincludedinthestudy, predator removalfailedtostemthedeclineofthree breeding population.Interestingly, inthesecondreview, whose goalistomaintainandevenincreasethesizeof However, itislesseffectiveforConservationManagers, production ofyoungforshootingintheautumnandwinter. effective toolforGameManagers,asitoftenincreasesthe Both reviewssuggestthatpredatorcontrolmaybean increases inadultnumbers. productivity duetopredatorcontrolwillresultinlocal likely thateffectsoflocalimprovementinbreeding far. Ifbirdsonlydisperseashortdistance,thenitismore 1996) tendtoinvolvepreyspeciesthatdonotdisperse of successfulexperimentsthiskind(e.g.Tapper the impactonsubsequentpreynumbers.Theexamples remove predatorsexperimentallyandhopetodetermine and theirpredatorsaresowide-ranging,itisdifficultto their ownpredatorswereremoved.Finally, becausebirds populations ofsmallerpredatorsmayhaveincreasedas in tofillthehabitatleftbythosethatwereremoved,or Nest survival rate (%) Daily failure rate 10 20 30 40 50 1 2 3 4 0 0 o ntaln Ontramline Not ontramline ie1Site2 Site 1 Position ofnest 0 et al. et habitat-mediated measuresalone(e.g.Gilbert and recoveryofpreypopulationscanbeachievedthrough substitute forprovisionofgoodhabitatandfoodsupplies, Lethal controlofpredatorsshouldnotbeseenasa tool forConservationManagers. where predatorcontrolcanprovideavaluableadditional are cases,particularlyforsomeground-nestingbirds, O’Brien (Reproduced fromDonald2004). crops sufferincreasednestpredation. tramlines inrapidlygrowingcereal Figure 17 sites. (DatafromJackson2001). bars) plotsforeachoftwostudy (orange bars)andunfenced(grey Western Isles,Scotland,infenced introduced hedgehogsonthe wader nests(%perday)to Figure 16 2001; Patterson1967;Smart2004). InastudybytheRSPB Fences canbeeffectiveinreducing predation(Jackson at theRSPB’s Minsmerenaturereserve,Suffolk. on mainlandbeaches(Smart2004) andtheavocetcolony has beenusedtoexcludemammals fromterncolonies electrified (MosebyandRead2006).IntheUK,fencing New Zealand.Fencingdesignsvary, butarecommonly exclusion projectshavebeenimplementedinAustraliaand of landwithhighdensitiesbirds,muchlarger-scale prey. Whilefencingisgenerallyusedtoprotectsmallareas rate atwhichmammalianpredatorsencountertheirbird Physical barriers,suchasfences,cangreatlyreducethe 4.b. Predatorexclusion to belongerlasting(Myers fewer predatorspeciesanyway, theeffectsofcontrol tend et al. et Skylarks forcedtoneston Daily failureratesof 2006; Peach et al. et et al. et 2000; Nordström2003). 2001). However, there et al. et 2007; PREDATOR EXCLUSION AND HABITAT MANAGEMENT 39 2006). 2006). et al. et al. 1997; see Figure 17). This happens less often in Sonic deterrents can also be used to reduce predator Sonic deterrents can in an experiment undertaken by incursions. For example, units, which emit a high-pitched the RSPB, ‘Catwatch’ sensor is activated, reduced the sound when a movement of cat intrusions into gardens number and duration (Nelson more open, later developing, spring-sown cereal crops, which have now been mostly replaced by winter-sown varieties. Lapwings may now nest at lower densities following their population and range contraction due to changes in farming practices. Lapwing nests are much more likely to be predated where nests are at a low probably because there are fewer adults to density, communally defend their nests against predators (Stillman et al. 4.c. Habitat management to reduce 4.c. Habitat management to predation and its impacts of bird While it may be tempting to think that declines to other populations are due either to predation or to their habitat, environmental causes, such as changes potential causes this view is too simplistic as these two cause of are often interlinked. Even if the immediate predation, a a species’ decline in a particular area is may have, for broader habitat or environmental change or prey example, led to increased predator numbers predation can In such circumstances, vulnerability. decline, become the proximate cause of a species’ cause. In while habitat change may be the ultimate (Evans this section, we discuss these inter-linkages and 2004; Martin 1992; Newton 1998; Whittingham habitats could Evans 2004), and consider ways in which to reduce be managed, or land-management changed, declining, aid the impacts of predation on birds and, if their recovery. First, habitat change may alter the vulnerability of birds to predation. For example, because of the switch from of cereals over the last few spring to winter-sowing decades, skylarks are unable to nest successfully later in crop and so are the season in the dense, winter-sown forced to nest close to the more open tractor tramlines where they are often predated (Donald 1999; JD Wilson In principle, birds and their nests could be made less vulnerable to predation by managing the habitat, to ensure that they are better hidden from predators or are in locations less favoured by predators. For a range 2007). et al. 2007). This increased et al. However, fences can sometimes be breached. On the fences can sometimes be However, re-entered the Outer Hebrides, hedgehogs occasionally under the fenced plots through burrows dug by rabbits by a fox can fence. In tern colonies, even a single intrusion 2004). cause severe losses of eggs and chicks (Smart a fenced Furthermore, the pacing of predators around being disturbed perimeter at night can result in parent birds or chicks to from their nests with resultant loss of eggs at exposure. Fences are, of course, wholly ineffective partially replace reducing predation by birds, and this may fox predation that by the excluded mammals. For example, little tern colony has been reduced by at the Great Yarmouth but subsequent fences and nocturnal patrols by wardens, of chicks in kestrel predation has caused heavy losses may also some years (Smart and Ratcliffe 2000). Fences away from delay or hinder chicks that may want to move their nests unless carefully managed (Isaksson on the Outer Hebrides, for example, when hedgehogs on the Outer Hebrides, to the islands) were removed from (which are not native hatching success of breeding dunlins two fenced plots, the compared to that in two nearby and lapwings doubled (Jackson 2001; see Figure 16). areas with no fences Predator-proof nestboxes can also be designed to Predator-proof prevent predation on hole-nesting species. Such boxes stopped weasel predation on tit nests in Wytham Woods, although this had little effect on tit population size (McCleery and Perrins 1991). predation on incubating adults was probably because redshanks tended to sit tight until predators were close and only then flushed. When flushed, they tended to by, fly to the top of the cage and may not have been able to escape the predator fast enough. This study suggests that nest protectors should only be used on shorebirds when predators are still far away. that flush early, An alternative to fencing entire colonies is to protect An alternative to fencing entire colonies to use nest individual nests instead. One approach is placed over exclosures. These are protective cages to the nests that allow incubating adults free access to, yet hinder nest and let chicks leave when they need predators from reaching it. Although such nest exclosures have been widely used, their efficacy has rarely been tested and where it has been, results have been mixed (Johnson and Oring 2002; Mabeé and Estelle 2000). In a recent study in Sweden, protected lapwing and redshank nests had higher hatching success than unprotected nests, but incubating adult redshanks – although not lapwings – were more likely to be predated from protected nests (Isaksson 40 1 Figure 18 Removal of unmanaged rank vegetation from farmland 0.99 reduces the amount of nest cover.

0.98 Reed bunting nests with less cover (lower category) suffered higher predation rates at 0.97 the egg stage. (Reproduced from Brickle and Peach 2004). 0.96

0.95 Daily survival rate (±1SE) 0.94

0.93 1234 Nest cover category MITIGATING THE IMPACTS OF PREDATORS THE IMPACTS MITIGATING 80 Figure 19 Cover strips, designed to reduce predation on grey partridges 70 actually increased their over-winter mortality at three out of four study 60 sites in France. (Data adapted from 50 Bro et al. 2004).

40 Cover strips Control 30 20

Apparent overwinter mortality (%) 10 0 1 234 Study site

of species, predation on nests is lower where there is lapwing nests close to field boundaries are more likely more nesting cover (Gregg et al. 1994; Guyn and Clark to suffer predation, probably because predators use 1997; Jiménez and Conover 2001). In the UK, for these features to navigate or hunt (Sharpe 2006; Sheldon example, hedgerows provide nesting cover for grey 2002; Stillman et al. 2006). partridges, and where these are more plentiful, nest predation is lower, particularly where there is a lot of Such interactions can be yet more subtle. Where dead grass in which to hide nests at the foot of the intensive land management favours high livestock hedge (Potts 1980; Rands 1988). In Germany, dummy densities, this can both increase losses of nests due to songbird nests in wide hedges were less often predated trampling (Beintema and Muskens 1987) and increase the than those in smaller hedges (Barkow 2005). Lapwing chance that the nest will be predated as incubating birds nests may be less well concealed and thus more are continually flushed off the nest (Hart et al. 2002). Any vulnerable to predation on the uniform short grass swards change in habitat that reduces food supplies for young associated with intensive grassland management for birds may increase predation as hungry songbird nestlings livestock (Baines 1990). The general tidying up of are more vocal and may be more easily found by farmland and consequent loss of rank unmanaged predators (Brickle et al. 2000; Evans et al. 1997). vegetation may have reduced the availability of safe Alternatively, reduced food supplies may force birds to nesting sites for reed buntings, as their nests are more forage for longer, or in more dangerous places, making often predated where there is little cover (Brickle and them more likely to be killed by a predator (Hilton et al. Peach 2004; see Figure 18). 1999; Martin 1992; Quinn and Cresswell 2004; Rands 1986). For example, bullfinches are more vulnerable to The vulnerability of birds and their nests to predation can predation when food supplies decline in late winter also be influenced by proximity to particular habitat forcing them to feed farther from cover (Marquiss 2007). features. For example, redshanks feeding close to cover Loss of seed food due to changes in agricultural practices are more likely to be killed by sparrowhawks who use could have contributed both directly, through starvation, cover to hide their approach from the redshank’s view and indirectly, through predation, to the bullfinch’s decline (Hilton et al. 1999; Quinn and Cresswell 2004). Similarly, in the UK (Newton 2004). HABITAT MANAGEMENT TO REDUCE PREDATION AND ITS IMPACTS 41 et al. 2004; see Figure 19). et al. 2003). In the UK, work is planned to test et al. simple management solutions that can reduce predation on lapwing nests by persuading them to nest at high densities and away from field boundaries, both of which independently improve nest survival (Stillmann the fox (GCT 2004), and may have increased predation the fox (GCT 2004), range of bird prey. pressure on a wide studies arise some potential solutions From each of these to reduce predation on birds. that could be implemented and cereal crops could be Hedgerows, grassland retained to make nests and managed, and rank vegetation Bird-food rich chicks less likely to be found by predators. starving habitats could be provided to ensure highly-vocal birds aren’t chicks do not attract predators, and that dangerous places. forced to forage for longer and in more for predators Potential breeding sites and perching places that are could be removed in some open habitats Woodland important for threatened bird prey species. levels of fragments could be joined together to reduce edges than in predation that can be higher near forest populations of their interiors. Food supplies that maintain More broadly, generalist predators could be controlled. boost productivity habitats could be managed in ways that chance of or enhance survival, thus improving a species’ While some of compensating for any losses to predation. providing food these solutions could be quick fixes (e.g. longer-term rich habitats), others clearly require a much investment (e.g. reducing forest fragmentation). these potential solutions remain many of Unfortunately, studies in the untested. For example, although several reduces US have shown that cover around nests few studies predation (Jiménez and Conover 2001), in one However, have experimentally manipulated cover. study in the North American Prairies, the conversion of cropland to grassland contributed to recovery in wildfowl populations as predation on nests fell due to improved nesting cover and avoidance of the converted areas by some mammal predators (Greenwood and Sovada 1996; Phillips 2006). Such rigorous testing is needed in order to develop practical solutions that work. An experiment in France, for example, designed to reduce grey partridge predation by raptors and mammals by planting crops to provide backfired when it apparently increased over-winter cover, mortality of partridges (Bro This was probably because these cover crops did not provide an effective refuge against predators for a species that relies on vigilance and crypsis as its main anti-predator strategies, rather than concealment in dense vegetation. et et al. et al. 1995) et al. et al. 2002). More et al. 2002; Hoover et al. 1999), while drying out of wet 1998), and that predation is 2000a; Thompson et al. et al. et al. 1996; Kurki and Lindén 1995; Kurki 2004) suggested that marking or removing 2004; Chalfoun et al. et al. et al. 1997; Newton 1998; Thompson al. more common in such landscapes near forest edges, or in small forest patches, than in the interior of large forests (Andrén 1992; Andrén and Anglestam 1988; Baines 1995; Huhta 2004). Several studies have shown that numbers of generalist predators are higher in fragmented landscapes (Andrén 1992; Kurki 2004), probably because food was less available in the 2004), probably because food was less season arable site. In each of these cases, the breeding to habitat had been curtailed due, in one way or another, these species change. Such changes may have made Any less able to compensate for losses to predation. that can boost approach – not simply predator control – improve a overall productivity or enhance survival would losses to species’ chance of compensating for any in Scotland predation. For example, a study of capercaillie (Baines Second, some habitat and land-use changes may Second, some habitat ability to compensate for losses constrain a bird species’ if deteriorating habitat quality to predation. For example, breeding season, birds that would were to shorten the to high levels of predation otherwise be well adapted to re-nest if a previous nest is lost to may have less time from spring to autumn sowing of predators. The switch breeding cereals has tended to curtail the skylark’s season (Chamberlain Finally, changes to habitat or land management can changes to habitat or land Finally, increase predator numbers, thus potentially increasing predation on birds. Several examples of this have already been outlined. The deteriorating quality of heather moorland (Thirgood fences, thus reducing adult mortality from fence fences, thus reducing adult mortality from for losses collisions, would help this species compensate of chicks and eggs to predation. grasslands, exacerbated by drainage, makes them less grasslands, exacerbated by drainage, makes (Green suitable for breeding snipe later in the season arable area of Essex, song in an 1988). In one study, season than thrushes stopped breeding earlier in the (Peach those in an area of mixed farming in Sussex broadly, the growing emphasis on large-scale rear and broadly, release of gamebirds for sport shooting has probably improved the food supply of generalist predators, such as favours hen harriers by increasing numbers of their and thus predation pressure preferred staple small prey, on red grouse. The fragmentation of native pinewoods may have increased the numbers of crows and foxes that would normally live at fairly low densities in pinewoods, increasing predation on capercaillie nests (Summers 42 MITIGATING THE IMPACTS OF PREDATORS

Little tern Chris Gomersall (rspb-images.com) 2000). Whilediversionaryfeeding mayhavereducedthe proved inconclusive(Smart2004; SmartandRatcliffe predating littleternsnestingon abeachatGreatYarmouth, Another studyofdiversionaryfeeding intheUK,ofkestrels predators tookthegrousechicks (Redpath chick mortalityremainedhigh,perhapsbecauseother summer (Redpath food (deadratsandcockerelchicks)inthespring the nestlingperiodwhenprovidedwithsupplementary delivered 86%fewergrousechickstotheirnestsduring chicks –wascarriedoutatLangholmMoor. Henharriers attempt toreducehenharrierpredationonredgrouse The best-knowndiversionaryfeedingtrialintheUK–an (Graham cases whereithasbeenproventoworksuccessfully proposed asamethodtoreducepredation,therearefew however, isnotsostraightforward.Althoughcommonly prey willbekilled.Thepracticeofdiversionaryfeeding, are providedwithalternativefoodinthehopethatless The principleofdiversionaryfeedingissimple;predators 4.d. Diversionaryfeeding et al. et 2005; JiménezandConover2001). et al. et 2001). However, despitethis, et al. et 2001). of timetheyspendfeedingin dangerous locations. vulnerability ofprey, forexamplebyreducing theamount Cresswell 2004)ifthiscouldbe showntoreducethe food tothepreyratherthan predator(Quinnand Intriguingly, theremaybemeritinproviding supplementary grouse chicks. tern chicksarealreadyunderway, andareplannedforred tests ofdiversionaryfeedingtoreducepredationlittle and kestrelswiththeconservationoflittleterns.Further harriers toco-existwithdrivengrousemoormanagement, developments couldprovidevaluabletoolstoallowhen diversionary feedingisstillatanearlystage,andfurther Notwithstanding thesecomplications,researchinto a fewweeksatcriticaltimeinthebreedingseason. increased iffoodisprovidedyearround,ratherthanfor than solveit(ReynoldsandTapper 1996).Thisriskis feeding; thiscouldservetoworsentheproblemrather success ordensitymightbeimprovedbysupplementary More broadly, thereisariskthatpredatorbreeding kestrels madeitdifficulttodrawfirmconclusions. weather andavailabilityofalternativeprey(voles)for level ofkestrelpredationinsomeyears,variabilitythe DIVERSIONARY FEEDING AND CONDITIONED TASTE AVERSION 43 2003b). In addition, et al. individuals of the same predator species in their individuals of the same predator species response (Anon 2001; Massei it can be difficult to exclude non-target mammals, because accurate dosing is problematic as individual predators vary in body size (Reynolds 2000), and this method is only really practical where predator numbers are low as each individual predator has to be conditioned. trials designed to reduce predation have been No CTA successfully conducted in the wild in the UK, but experience from the US suggests that trials for some specific UK circumstances could be merited. Some chemicals could deter predators simply by their smell – an innate aversion rather than a learnt one. While a number of products with particularly strong smells, such as creosote or lion dung, are promoted as alternative methods to deter ground predators, the efficacy of these products in reducing predation has not yet been tested experimentally. et al. 1995). et al. 1983) and mammals (Conover 1990) in the et al. 2003a), and there can be marked differences between One potential method to reduce predation is to provide individual predators with a bait that resembles their prey, but which is dosed with noxious chemicals to make them sick. Over time, the predator will learn to stop taking their too, will make them sick. real prey believing that they, (CTA), Aversion This method, known as Conditioned Taste eggs by has been shown to reduce predation on bird’s and Decker 1994; Bogliani and Bellinato corvids (Avery 1998; Dimmock and Nicolaus 1990; Nicolaus 1987; Nicolaus 4.e. Conditioned Taste Aversion 4.e. Conditioned Taste Foxes have been successfully conditioned using bitter chemicals in small-scale laboratory trials (Macdonald and some mammals are able to Baker 2004). Unfortunately, detect the chemicals and so avoid the baits (Massei wild in the US. For example, up to 10 ravens were killed annually over a three-year period to protect a colony of eggs injected Californian least terns. The following year, with methiocarb were set out for ravens and, no tern eggs were predated and no ravens consequently, had to be killed (Avery 44 MITIGATING THE IMPACTS OF PREDATORS not bepickedupbythistypeofstudy. reducing predationmayhavealreadyoccurred,andwould to fittheiranimalwithabell,soanyeffectofbells bell. Ownersofcatspronetokillbirdsmaybemorelikely by theircatsandnotewhetherthecatwasfittedwitha volunteers wereaskedtorecordthepreybroughtback on birdpredationrates.However, inthesestudies studies (e.g.Woods electronic bleeper)tothosecatsknownhunt.Several to attachacollar-mounted warningdevice(abellor by catswouldbewelcomeandprudent.Oneoptionis management techniquestoreducethemortalitycaused constitutes aconservationproblem.Nevertheless, in theUKeveryyear, althoughitisunclearwhetherthis Domestic catsundoubtedlykillalargenumberofbirds be practicalfordomesticpredators. how todothisinmostcircumstancesthewild,itmay their chancesofbeingkilled.Whileitishardtoimagine predators, theycouldtakeavoidingactionandreduce If preycouldbewarnedoftheapproachhunting predators 4.f. Warning preyofapproaching Figure 20 a bellornothing(plain).(DatafromNelson month bycatsfittedwithcollarsthathadasonicdevice, Number of birds per month 0.5 1 1.5 2 0 Average (+/-1SE)numberofpreyreturnedina li elSonic Bell Plain et al. et 2003) foundnoeffectofbells et al. et 2005). reduced birdkillsby41%and51%,respectively(Nelson ‘bleep’ everysevenseconds.Bellsandsonicdevices bells andelectronicsonicdevicesthatemitanaudible second experimentalstudybytheRSPBtestedboth be thecasewithwildpredators. have animpactontheirwelfareorsurvival.Thismightnot owners, makingthemlesseffectivekillersisunlikelyto domestic catsobtainmostoftheirfoodfrom usefully bedonetoimprovetheirefficacy. Because solution tocatpredation,andmoreresearchcould Clearly, collar-mounted warningdevicesofferapartial Sonic device(CatAlert et al. et quick-release catcollars. by preyreturns)abouthalf(Ruxton showed thatbellsreducedbirdpredation(measured when theywere,orwerenot,wearingabell,and the preybroughtbackbyindividualcatsduringperiods Experimental studiesaremorerevealing.Onerecorded Nigel Butcher 2005; seeFigure20). TM ) andbellon et al. et 2002). A

FUTURE RESEARCH NEEDS 45 Sue Tranter (rspb-images.com) Tranter Sue and has this led to increased predation pressure on and has this led to increased predation pressure ground-nesting birds? Several other non-lethal methods need further in quite development, despite only being applicable research is needed specific circumstances. In particular, predators, to improve the success of methods to exclude to provide them with diversionary food and to condition them not to kill eggs and chicks. more research to determine the identity of key Finally, predators and their foraging behaviour is needed, because both lethal and non-lethal solutions to reduce levels of predation can be markedly improved upon when these are known. Wren et al. 5. Future research needs research 5. Future The interactions between changes in habitat management and land-use practices and levels of predation need to be better understood, so that additional non-lethal solutions to reduce predation can be designed, tested and implemented. For example, can vegetation be managed to make predators less numerous and nests or chicks of prey less easily located? can the right resources, such as food and nest Similarly, sites, be provided to ensure breeding seasons of prey are not curtailed, allowing them to re-nest if predated to help compensate for losses to predation? Furthermore, to what extent have artificial food supplies and fragmented habitats boosted populations of generalist predators? For example, have fox numbers increased because of the growth in rear and release of gamebirds, An updated review of the effectiveness of removing An updated review of the effectiveness be valuable, predators to protect bird populations would as the existing reviews (Côté and Sutherland 1997; Newton 1998) are now also a decade old, and additional removal studies have been undertaken in the interim. Such a review should seek to determine the circumstances in which predator removal is most often effective, thus allowing better targeting of this method. 1998) is now a decade old, and trends of these and other 1998) is now a decade old, and trends of predators and their songbird prey have changed. Further studies that provide clear evidence of whether Further studies that provide clear evidence are needed. predators limit populations of their bird prey predator While such evidence is best obtained from removal experiments, these are not always is to straightforward, so an alternative approach or at times, compare populations of bird prey in areas, most studies with differing predator densities. So far, similar studies have been of ground-nesting birds, and urban and of songbirds are needed, for example in songbird suburban areas. In addition, some of the updating. In studies that have been undertaken need of the impacts of magpies and a classic study particular, (Thomson sparrowhawks on songbird populations Various suggestions for research have been highlighted suggestions for research have Various here throughout this review and these are summarised to help focus any future work. Most of the scientific evidence presented here has been Most of the scientific 50 or so years. Undoubtedly, collected over the last become available, and reviews such further evidence will periodically to ensure that as this should be undertaken practice is underpinned by the conservation policy and science. most contemporary 46 IMPLICATIONS FOR CONSERVATION supports thisapproach. their habitats;theevidencepresentedinthisreview spent onrecoveringsongbirdpopulationsbymanaging A greatdealofconservationresourceiscurrentlybeing sparrowhawks ormagpies,islikelytoproveineffective. managing predatorpopulations,suchasthoseof any attempttorecovertheUK’s songbirdnumbersby predation bysparrowhawksormagpies.Becauseofthis, over thelastfewdecadesshouldnotbeblamedon In particular, itshowsthatdeclinesinsongbirdnumbers that songbirdnumbersintheUKarelimitedbypredation. This extensivescientificreviewprovideslittleevidence 6. Implicationsforconservation

reduce thechancesofbirdpreybeingkilled. colonies, diversionaryfeedingandmanaginghabitatsto options includeplacingelectricfencesaroundnesting though theirefficacyislesswelldocumented.Such be successful,other, non-lethalsolutionsareavailable, predator numbersbykillingthemisoneoptionthatcan that haveapoorconservationstatus.Whilereducing reducing theimpactsofpredationonground-nestingbirds sites, ConservationManagersshouldconsiderwaysof as wellmanaginghabitatstoprovidefoodandnest or youngaremorevulnerabletopredation.Consequently, predation thanothergroups,perhapsbecausetheirnests as wadersandgamebirds,aremorelikelytobelimitedby breeding populationsofsomeground-nestingbirds,such By contrast,thereisgrowingscientificevidencethat

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Cover image: red fox by David Tipling (Alamy) Back cover image: domestic cat on top of nestbox by Susanne Danegger (nhpa.co.uk) Regd charity England & Wales no 207076, Scotland no SCO37654 230-0134-07-08