The recent declines of farmland bird populations in ...

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© 2004 British Ornithologists’ Union

Blackwell Publishing, Ltd.

Review paper

The recent declines of farmland bird populations in Britain: an appraisal of causal factors and

conservation actions

IAN NEWTON*

Centre for Ecology and Hydrology, Monks Wood Research Station, Abbots Ripton, Huntingdon, Cambs. PE28 2LS, UK

In this paper, the main aspects of agricultural intensification that have led to population declinesin farmland birds over the past 50 years are reviewed, together with the current state of know-ledge, and the effects of recent conservation actions. For each of 30 declining species, attentionis focused on: (1) the external causes of population declines, (2) the demographic mechanismsand (3) experimental tests of proposed external causal factors, together with the outcome of(4) specific conservation measures and (5) agri-environment schemes. Although each specieshas responded individually to particular aspects of agricultural change, certain groups of speciesshare common causal factors. For example, declines in the population levels of seed-eating birdshave been driven primarily by herbicide use and the switch from spring-sown to autumn-sowncereals, both of which have massively reduced the food supplies of these birds. Their populationdeclines have been associated with reduced survival rates and, in some species, also withreduced reproductive rates. In waders of damp grassland, population declines have been drivenmainly by land drainage and the associated intensification of grassland management. This hasled to reduced reproductive success, as a result of lowered food availability, together withincreased disturbance and trampling by farm stock, and in some localities increased nest predation.The external causal factors of population decline are known (with varying degrees of certainty)for all 30 species considered, and the demographic causal factors are known (again with varyingdegrees of certainty) for 24 such species. In at least 19 species, proposed causal factors have beentested and confirmed by experiment or by local conservation action, and 12 species have beenshown to benefit (in terms of locally increased breeding density) from options available in oneor more agri-environment schemes. Four aspects of agricultural change have been the main driversof bird population declines, each affecting a wide range of species, namely: (1) weed-control,mainly through herbicide use; (2) the change from spring-sown to autumn-sown cereal varieties,and the associated earlier ploughing of stubbles and earlier crop growth; (3) land drainage andassociated intensification of grassland management; and (4) increased stocking densities, mainlyof cattle in the lowlands and sheep in the uplands. These changes have reduced the amounts ofhabitat and/or food available to many species. Other changes, such as the removal of hedgerowsand ‘rough patches’, have affected smaller numbers of species, as have changes in the timingsof cultivations and harvests. Although at least eight species have shown recent increases in theirnational population levels, many others seem set to continue declining, or to remain at a muchreduced level, unless some relevant aspect of agricultural practice is changed.

Widespread population declines of farmland birdsare of major conservation concern in Britain andother parts of Europe. Such declines have been rapid,

massive and widespread, with some species experi-encing more than 80% reductions in former numbersand range in less than 20 years (Tucker & Heath 1994,Fuller

et al

. 1995). Parallel declines have also occurredin other components of farmland biodiversity,

*Email: [email protected].

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including insects and wild plants (Wilson 1992,Donald 1998, Sotherton & Self 2000, Preston

et al

.2002). In this review, I shall assess the causal factorsunderlying the population declines of particular birdspecies in Britain, together with the effectiveness ofvarious conservation measures that have beenapplied in recent years.

In most species affected, the main period ofpopulation decline seems to have occurred between1970 and 1990 (especially 1975–85), although insome species (such as those affected by organochlo-rine pesticides) declines began earlier, and in othersthey continued into or beyond the 1990s (Table 1;see Fuller

et al

. 1995, Fuller 2000, Siriwardena

et al

.1998, Chamberlain

et al

. 2001, Gregory

et al

. 2004for Britain; Tucker & Heath 1994, Donald

et al

.2001b for Europe as a whole). Substantial changes inthe farmland bird populations of Britain are byno means new, however, as they have followedevery major change in agricultural practice that hasaffected bird habitats and food supplies, at least overthe 250 years for which documentation is available(Shrubb 2003). In the 19th century, the most markedchanges occurred as semi-natural habitats, such aswetland, heath and moorland, were brought intomore intensive use, but in the 20th century, the mostmarked changes occurred through the more intens-ive use of existing farmland. Compared with earlierchanges, recent population changes have affectedsimultaneously a wider range of species, almost all ofwhich have declined, only small numbers havingapparently benefited, and increased in numbers orrange (see later).

Farmland birds have been defined either narrowly,as those found primarily in farmed habitats, such asGrey Partridge and Corn Bunting (see Appendix forscientific names; only those of species not mentionedin Table 1 are given in the text), or broadly to includealmost all landbirds that breed or winter in openhabitats. The latter reflects the fact that almost allopen land in Britain (70% of total land area) is usedfor agricultural production. In this paper, I shalladopt a broad view of farmland birds, as any speciesthat have been conspicuously affected by recentchanges in the methods of crop, meat or milk pro-duction. To exclude such species as Sparrowhawkand Black Grouse, merely because they do not breedin cultivated habitats, would be to ignore some ofthe most dramatic examples of bird species beingaffected by recent farming practice. Discussion isconfined to 30 breeding species whose populationshave declined markedly since 1950, and which have

been studied in detail. Other declining breedingspecies are excluded, as are winter visitors.

SOURCES OF INFORMATION ON BIRD POPULATIONS

Evidence for the role of agricultural change in thedeclines of many bird populations has come partlyfrom temporal and spatial correlations between par-ticular types of agricultural change and particularbird declines (e.g. Chamberlain

et al

. 2000, Gates &Donald 2000, Shrubb 2003), but also from detailedfield studies designed to find the crucial causalfactors (see species notes and references to Table 1given in the Appendix). The main sources of infor-mation on the changes that have occurred in Britishbird populations are the various wide-scale, long-term data-sets held by the British Trust for Ornitho-logy (BTO). These include: (1) the Common BirdsCensus, recently (and after a period of overlap)replaced by the Breeding Bird Survey (BBS), both ofwhich were designed to measure year-to-year changesin breeding bird abundance in various habitats, andhave therefore revealed the extent and precise tim-ing of decline in most affected species; (2) two Atlasprojects, designed to map the breeding distributionof all British bird species in 10-km squares through-out the country, at about 20-year intervals (1968–72 and 1989–92) (Sharrock 1976, Gibbons

et al

.1993); (3) the Ringing Scheme, providing recoverydata from which the annual mortality rates of vari-ous species have been calculated, and thus enablingcomparisons between periods of numerical stability,increase and decrease; (4) the Nest Record Scheme,from which the performance of individual breedingattempts of various species has been calculated,again enabling comparisons between periods ofstability, increase or decrease; (5) periodic surveys ofspecies of interest, not well covered by the otherschemes.

All these schemes have strengths and weaknesses(Grice

et al

. 2004), but in our present context themain problem lies with nest record data (Crick

et al

.2003). In multi-brooded species, nest records giveno measure of the total seasonal production of youngper pair, which is the measure needed to understandpopulation changes. In some species, such as CornBunting and Turtle Dove, nest record data indicatedan increase in individual nest success during a periodof population decline, while more detailed dataresulting from field studies indicated a shortening ofthe breeding season, and hence in the total seasonal


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Recent declines of farmland bird populations in Britain

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Table 1. State of knowledge and outcome of conservation action in 30 bird species that have declined in Britain in association with recentagricultural change. See Appendix for species notes and references.

Species

External causal factor1

Demographic causal factor2

Experimentaltest3

Local conservation

action4

Agri-environment

scheme5

National policy

change6

Recent national population

trend7Surv. Rep.

Seed eaters1 House Sparrow F+N S* (R) + + S2 Tree Sparrow F S* (R) + S3 Linnet F (R) + S4 Twite H+F S† D5 Bullfinch H+F D6 Yellowhammer F S* (R) + S7 Cirl Bunting F S R + + I8 Reed Bunting H+F S* (R) + + + S9 Corn Bunting F R + D

10 Turtle Dove H+F R* D

Other passerines11 Skylark H+F R + + D12 Meadow Pipit H(+F) D13 Yellow Wagtail H+F + D14 Starling F S R† + D15 Blackbird H+F S* – + I16 Song Thrush H+F S* (–) + I

Waders and others17 Lapwing H+F – R* + + + S18 Snipe H+F R + I19 Curlew H+F(+P) R* D20 Redshank H+F R + D21 Stone Curlew H+M R + + I22 Corncrake H+M R + + + I

Gamebirds23 Grey Partridge H+F(+P) S R + + + S24 Black Grouse H+F(+P) R + + + D25 Red Grouse H+F(+P+D) S

Birds of prey26 Sparrowhawk Pesticide S R (+) + + S27 Merlin Pesticide S R (+) + I28 Peregrine Pesticide S R (+) + + I29 Kestrel H+F+N + + D30 Barn Owl H+F+N + + D

1External causal factors: declines in habitat areas (H), food supply (F), nest-sites (N), or increases in predation or nest trampling (P),parasitic disease (D) and losses to machinery (M).2Demographic causal factors: survival (S) or reproduction (R). *In a mathematical model, a change in S or R could account on its ownfor the observed rate of decline. R values in parentheses indicate that change in nest success has been documented during the decline,but total seasonal productivity has not been studied. –indicates no change. †Continental study only.3Scientifically designed experiment to test the effect of a proposed limiting factor, which is changed in one or more treatment areas, butnot in control areas, and the response of the population is compared between areas.4Management action taken in one or more areas (often nature reserves) which was followed by an increase in breeding density.5A scheme in which farmers are compensated for loss of production in return for carrying out some conservation-friendly activity includingset-aside.6National policy changes, usually promoted by legislation or subsidy schemes. Bird-friendly changes include the growing of oilseed rape,which has probably benefited some seed-eating birds, notably Linnet and Reed Bunting (see text), and reduction in the use oforganochlorine pesticides, which benefited some predatory birds, and probably also some seed-eaters.7Based mainly on Breeding Bird Survey data, up until the year 2002: D, declining; I, increasing; S, stabilized at low level.

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production of young (compare nest record data forCorn Bunting and Turtle Dove from Siriwardena

et al

. (2000) with seasonal data from Brickle &Harper (2002) and Browne & Aebischer (2003)).

Because of this difficulty, other means have beensought to provide reliable estimates of seasonalproductivity. For example, Newton (1999) used end-of-breeding ratios of juveniles to adults in nettedsamples to assess the annual productivity of Bull-finches over a 5-year period (see also Proffitt

et al

.2004). Similar trapping data, as from the BTO Con-stant Effort or other netting schemes, might providereliable estimates of seasonal productivity in otherspecies (du Feu & McMeeking 1991; Peach

et al

. 1996),providing that such measures are restricted to a briefperiod at the end of the breeding season, and at datesappropriate to the species concerned. The advantageof such measures is that they incorporate the effects,not only of brood numbers, but also of immediatepost-fledging survival, which is another (usuallyunknown) component of reproductive success. Clearly,we need good measures of seasonal production, if weare fully to understand the demographic parametersthat have underlain population declines.

In addition to the BTO data, other wide-scale orlong-term information on some bird species is heldby other organizations, such as the Game Conserv-ancy Trust, the Centre for Ecology and Hydrologyand the Royal Society for the Protection of Birds.Additional (usually shorter-term) information isheld by the many individuals or university groupswho have conducted detailed studies of individualspecies in particular localities. In recent years, mostsuch field studies have been aimed mainly at address-ing conservation problems (see later examples).

COMPONENTS OF AGRICULTURAL INTENSIFICATION

The main problem in finding causes of bird popula-tion declines is that agricultural intensification is nota single process, but has several components, each ofwhich can affect different species. Moreover, thesevarious aspects of change have occurred more or lesssimultaneously and interdependently, which makesthe role of any one change hard to separate from theconfounding effects of others. Thirdly, the problemshave mostly been studied retrospectively, only afterthe agricultural changes and bird population declineshad been underway for several years.

The main components of recent agriculturalchange in Britain can be listed as follows, while data

on the extent or spatial scale of each type of changecan be found in Chamberlain

et al

. (2000), Fuller(2000) and Shrubb (2003):

1

Massive increases in the use of agro-chemicals,both pesticides and fertilizers. Rising pesticide use isreflected in increases in the range of chemicals used,the acreage treated and the numbers of applicationsper year. Some such chemicals, notably the organo-chlorine insecticides, have had direct effects on birds,causing reproductive failures or enhanced mortality(Newton 1979, 1986, 1998, Ratcliffe 1993), whileothers have affected birds indirectly through reduc-ing their food supplies (Potts 1986, Newton 1995,1998). Herbicide use reduces weed growth and seedproduction, which leads not only to loss of immedi-ate food supply, but to long-term depletion of theseed bank in the soil. Progressive increases in thenumbers of herbicides, modes of action and formu-lations available have gradually expanded the rangeof weed species that can be controlled effectively ineach crop type. Weeds also support insects, whichform important foods for some bird species (Potts1986). Direct pesticide effects were evident in somebirds of prey and (mainly in the years around 1960)some seed-eaters; indirect effects are evident in awide range of seed- and insect-eaters.

2

Removal of hedges and other uncultivated areas toproduce larger fields and more land for crop produc-tion. This activity has greatly reduced the amount ofsemi-natural habitat that existed within the agricul-tural matrix. Some bird species both nest and feedwithin such habitat, while others nest there but feedin neighbouring fields. In addition, mechanizationhas brought a change in hedgerow management, manyformer tall and thick hedgerows being converted toshort, narrow ones, with effects on bird populations(for relationships between hedgerow structure, vergefeatures, landscape context and bird populations, seeArnold 1983, Osborne 1984, Green

et al

. 1994, Parish

et al

. 1994, Macdonald & Johnson 1995, Gillings &Fuller 1998, Hinsley & Bellamy 2000). Such changesare likely to have affected most hedgerow-nestingspecies, and others using semi-natural habitats.

3

Change from spring ploughing to late summerploughing of cereal stubbles soon after harvest. Thischange has removed the supply of spilled grain andother seeds on the ground, on which many seed-eatersformerly fed in winter, as well as the spring-sowngrain itself, which was an important dietary compon-ent of several species at a time of year when otherfoods were scarce. It is likely to have affected all seed-eating species that fed in winter stubbles (for further


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discussion, see Potts 2003, Evans

et al

. 2004). Thisprocedural change also removed the invertebratefood supply and weed seeds provided by fresh tillin spring and the short-vegetation (including under-sown stubble) nesting and feeding habitat favouredby Lapwings, Skylarks and others (autumn-sowncereals having grown too tall and dense by spring).Spring-sown cereal crops were also managed withless chemical input.

4

Extensive land drainage, which, through loweringthe water-table, changed wet grassland to dry grass-land or enabled a change to cereal culture. This hasaffected many dampland species, notably certainwaders, but probably also other species, such as Euro-pean Starling, which feed on invertebrates from nearthe soil surface.

5

The trend from mixed farms, producing a varietyof plant and animal products, to monoculture arable(mainly in the east) or monoculture grassland (mainlyin the west). This change has reduced the diversity ofhabitats available on individual farms, affecting spe-cies such as Lapwing, which depended on a localizedmixture of habitats, or Skylark, which benefited frombeing able to move from one crop-type to anotherduring the course of a single breeding season. It alsoled to a reduction in the area of root crops (importantbecause of weed growth, Hancock & Wilson 2003),oats and other minority crops (providing seed-richwinter stubble), and the almost total elimination ofbare fallow (an important habitat for several nestingspecies). Probably many species were affected by thischange, including sparrows, finches, buntings andSkylarks (Robinson 2001).

6

The trend to earlier harvesting dates, caused by acombination of earlier sowing dates, earlier ripeningcereal varieties and a change from hay to silage (grassharvested green at an earlier growth stage, allowingrepeated cuts through the season). This means thatmore cultivation procedures (including harvest)fall within bird breeding seasons, causing greaterdestruction of eggs and chicks of field-nesting species,such as Yellow Wagtail and Corn Bunting (e.g. Crick

et al

. 1994, Court

et al

. 2001, Henderson

et al

. 2004).In at least the Corncrake and Stone Curlew, destruc-tion of eggs and chicks can be sufficient to cause popu-lation declines (Norris 1947, Aebischer

et al

. 2000).

7

More intensive grassland management, involvinguse of inorganic fertilizer and more frequent re-seeding (Vickery

et al

. 2001). The fertilizer stimulatesgrass growth, making it no longer suitable for someground-nesting bird species, and eliminating throughcompetition many broad-leaved plant species. Loss

of plant diversity in turn reduces insect diversity andabundance. Re-seeding reduces invertebrate densitiesin the soil (which increase with age of grassland,Tucker 1992). The increase in grass growth can sup-port increased densities of cattle or sheep, or morefrequent cutting for silage, both of which result ingreater destruction of eggs and chicks. This change islikely to have affected all grassland species. More-over, the sward assumes a more uniform and denserstructure, and generally lacks the tussocks favouredby many species of ground-nesting birds, or the barepatches in which they can feed.

8

In upland areas, a massive increase in sheep-stockingdensities, which changes habitat structure, swardheight and composition, and reduces plant floweringand seeding, together with the densities of insects,many of which are eaten by birds. Probably almost allspecies of open upland habitats are affected.

Over the years, increasing mechanization and tech-nology have underpinned some of these changes, andat one time or another most have been encouragedby commodity price support and by payment ofgrants or subsidies, which made previously uneco-nomic practices profitable (for historical develop-ment of agricultural policy see Shrubb 2003). Whilesome changes occurred more or less simultaneouslyover the whole country, others occurred earlier inthe drier east than in the wetter west. The generaltrends were towards loss of grass to give more arablein the east, and loss of arable to give more ‘improved’grass in the west, producing generally less diverselandscapes in both regions.

PROCESSES LEADING TO POPULATION DECLINES

In studying population declines, it is important todistinguish between the external (environmental)factors that cause a decline and the intrinsic demo-graphic factors (such as reduced survival or repro-ductive rate) that underlie the decline (Newton 1991,1998, Green 1995). Thus, within suitable habitat, apopulation might be said to decline because offood shortage (the ultimate cause) or because of theresulting mortality (the proximate mechanism). Inattempts to manage bird populations, it is the exter-nal limiting factors that must be changed before anylasting change in population level can be achieved;the necessary demographic changes will follow nat-urally. However, knowledge of the demographicchanges can often help to pinpoint the externalcausal factors, or at least to distinguish whether

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population decline is associated with reduced breed-ing production or reduced survival.

The series of events that lead to some populationdeclines are short and simple. For example, thekilling of chicks during grass-cutting can reduce thereproductive rate of Corncrakes to such an extentthat it causes rapid population decline (Norris 1947,Green

et al

. 1997). This is a single-step process, whichcan be easily identified. Likewise, the destruction ofclutches during the rolling and harrowing of arablefields in spring can lead to population decline inStone Curlews (Green & Griffiths 1994). Populationdeclines of other species are due to longer, multi-stepsequences of events, which are much harder to workout. Some well-researched examples are given inFigures 1–4, and the Appendix.

CURRENT KNOWLEDGE OF POPULATION DECLINES AND EFFECTIVENESS OF CONSERVATION MEASURES

In this section, I discuss the state of current knowledgeand conservation action for 30 species of birds that arethought to have declined in Britain because of someaspect of agricultural change. They do not include allsuch declining species, but those discussed have beenwell enough studied to examine in detail. Some of these30 species, such as Corn Bunting and Grey Partridge,are obvious farmland specialists, while others, such asMerlin and Black Grouse, are not normally regardedas farmland species, but have nevertheless been greatlyaffected by changes in agricultural practice. For eachof these species, six main questions are addressed:

1

What have been the agricultural drivers of popu-lation decline, e.g. drainage, herbicide use, increasedstocking densities?

2

Have the above agricultural changes broughtabout reductions in habitat area, food or nest-sites,or increases in predation, parasitism/disease, human-induced mortality or competition, all of which fallunder the heading of ‘external limiting factors’?

3

What are the demographic changes that have broughtabout population decline: reduced survival, reducedreproduction or both? As we are dealing with nationalpopulations, the effects of any change in movementpatterns (immigration/emigration) can be ignored.

4

Has the proposed limiting factor (habitat, food orwhatever) been tested by experiment or by local con-servation action (as in nature reserve management ornest-site provision), resulting in increased breedingdensity?

5

Has the species concerned been shown to benefit byincreased breeding density from any agri-environmentscheme, including set-aside (leaving arable land fallow),or any other national policy change (but note thatsome existing schemes may not have operated for longenough to have yet promoted measurable changes inbird breeding densities)?

6

Has a previous substantial decline in population beenreversed, giving a sustained and statistically significantincrease over five or more recent years (up to 2002)?

My attempts to answer these questions, mostly frompublished research findings, are summarized in Table 1,which also gives footnotes on each species with sup-porting references. Although species have been stud-ied individually, they fall readily into distinct groups,according to the dominant agricultural drivers.

Seed-eaters

In all ten species considered, the main causal factoridentified was decline in food supply (Table 1, Fig. 1),but some species have also suffered considerablereduction in habitat (Bullfinch and Turtle Dovethrough loss of tall hedgerows, Twite through lossof saltmarsh wintering habitat, and Reed Buntingthrough drainage of wet areas), and the HouseSparrow perhaps also from reduction in nest-sites.Declines in food supply were caused mainly by:(1) herbicide use, which reduces current-year weed-seed production, and leads to long-term depletionof the seed bank in the soil; and (2) loss of winterstubble fields, with their associated weed-seeds andspilled grain, resulting from the switch from spring-sown to autumn-sown cereals. The relative import-ance of these seed sources varies between species,depending on their dietary needs, and the HouseSparrow has probably also suffered from the increasedbird-proofing of stores for grain and other animalfeed. Herbicide use also reduced the abundance ofweed-dependent insects that some seed-eaters feedto their young.

At least six seed-eating species have experienced adecline in survival rate during the period of decline;and in at least four of these, the measured declinein survival was found in a modelling exercise to becapable of accounting for the observed rate ofpopulation decline, without any concurrent changein reproductive rate. At least three species haveexperienced decline in seasonal reproductive rate,and in one (Turtle Dove) this was deemed sufficienton its own to account for the observed rate of decline.Changes in the success of individual nesting attempts


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were recorded in seven species (in five increases, intwo decreases), but as explained above, in multi-brooded species these measures do not necessarilytranslate into changes in seasonal production. Theyare therefore of limited value in our present context.

At least five seed-eating species showed increasedbreeding densities after the local introduction ofagri-environment schemes. The spread of oilseedrape (from less than 1% of arable land in the 1950sto more than 8% in 2000) has almost certainlyprovided additional food for several species (notablyLinnet) and also additional nesting habitat for ReedBuntings. In some seed-eating species, local increasesresulting from management action have been insuf-ficient to reverse overall downward national trends,although numbers of five species seem to have stabi-lized in the last few years (Table 1). The only seed-eater that has shown substantial increase in responseto management is the Cirl Bunting, which is effectivelyconfined to parts of one county. Its numbers increasedabout four-fold over 10 years in response to increasedfood supplies provided by a targeted agri-environmentscheme.

Other passerines

The Skylark is one of the most studied species. Itsdecline is attributed to loss of habitat: the replace-ment of rough grassland by managed grassland, andof spring-sown by autumn-sown crops (which soongrow too tall for nesting Skylarks), and also to reduc-tion in food availability (mainly insects in summer,small weed-seeds in winter). There are insufficientring recoveries to check for possible changes inmortality rates, but the breeding season is curtailedin arable farmland because of rapid crop growth,through which the habitat becomes unsuitable.Although some Skylarks switch from one crop toanother during the course of the breeding season, notall pairs now have access to such alternative habitat,leading to a net reduction in offspring production.Skylarks have responded by increased breedingdensity and seasonal success to the provision ofunsown patches in cereal fields, to unsprayed set-aside fields and to local agri-environment schemes,but such local effects have not so far reversed thenational population decline.

Figure 1. Proposed sequence of events through which herbicide use causes population declines in seed-eating birds, some species ofwhich also require insects for their chicks. The relative contributions of seed reduction (route A) and insect reduction (route B) seem tohave varied between species, as shown. For references, see Appendix.

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The Meadow Pipit has suffered from loss ofhabitat (notably rough grassland), and probably alsofrom decline in insect food supplies consequentupon overgrazing (see studies of Black Grouse), butis the least known of all the species listed in Table 1,and is in obvious need of further study. The YellowWagtail nests both in wet grassland and in dry,sparsely vegetated arable land. It has suffered fromlarge reductions in nesting habitat, resulting fromthe drainage and more intensive management ofgrassland, and from the change from spring-sownto autumn-sown arable crops, which grow too talland thick by spring. It has responded by increaseddensity to raised water levels (and winter flooding)of some grassland nature reserves. The Starling hasexperienced a massive reduction in food supply, causedmainly by land drainage and more intensive grasslandmanagement, which has reduced the availability ofinvertebrates in the surface soil, and by reductions inthe number of accessible feed-sites for farm stock. Insome eastern areas, the conversion of former grasslandto arable has also reduced the acreage of potentialforaging habitat. Long-term population decline has beenassociated with reduced survival, and (in a Finnishstudy) with reduced breeding success. The Blackbirdand Song Thrush have suffered from loss of habitat(especially hedgerows and field margins), and mayalso have experienced reductions in food suppliesresulting partly from greater field drainage, whichmakes soil invertebrates less accessible. Both haveexperienced reduced survival, deemed sufficient in amodelling exercise to account for the observed rateof decline in the 1970s and 1980s, but both have shownsome population recovery since 1994. Various man-agement changes and predator control at the AllertonResearch and Education Trust farm in Leicestershirewere followed by substantial increases in the breed-ing density and seasonal productivity of both species,but as all changes were brought in simultaneously, itis as yet uncertain which were most important.

Waders, Corncrake and gamebirds

Four species have suffered mainly from the drainageof damp grassland. It is difficult to decide whether toput their declines down to loss of habitat or to lossof food, because as the ground dries, soil inverte-brates become less accessible (Fig. 2). Drainage isusually followed by ploughing, re-seeding andfertilizer application, which in turn leads to moreuniform sward structure, more rapid grass growth,decline in non-grass plant species (and associated

insect populations), earlier and more frequentmowing, or increased stocking densities with sheepor cattle. Increased livestock numbers lead to greaterdisturbance of nesting birds (which may in turn leadto increased egg and chick predation), and increasedtrampling of nest contents. Nest failure rates havebeen quantified in relation to stocking densities ofcattle and sheep from studies in The Netherlands(Beintema & Muskens 1987, Beintema

et al

. 1997).In addition to the effects of land drainage, Lapwingshave largely disappeared from former mixed farmingareas, which no longer offer the combination ofspring-sown arable and unimproved grass fields inclose proximity. In all four species, decline in popu-lation was associated with reduced reproductive suc-cess, and in two of them, reduced reproduction wasdeemed sufficient on its own to account for observedrates of decline.

At least three of the four dampland wader specieslisted in Table 1 have increased in numbers and nestsuccess following the raising of ground-water levelson grassland nature reserves, and the Lapwing hasresponded in a similar way to at least one agri-environment scheme. However, the overall nationalpopulations of at least three species are still indecline, the Snipe having increased slightly in BBS

Figure 2. Proposed sequence of events through which thedraining and re-seeding of wet grassland causes populationdeclines in waders. Re-seeding and fertilizing improves grassgrowth, which allows greater stocking densities or multiple grasscuts for silage. Both these effects can result in greater nestdestruction. For references, see Appendix.


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counts in the last 8 years (as opposed to declining insurveys of wet meadow birds).

In the Stone Curlew, population decline has beenattributed partly to loss from many areas of sparselyvegetated spring arable land, and short-sward semi-natural dry grassland, and partly to destruction ofeggs and chicks on remaining suitable arable areas byfarm machinery. Similarly, in the Corncrake, declineis attributed partly to loss of damp meadows, andpartly to the destruction in remaining meadows ofeggs and chicks during mechanized grass cutting. Inearlier centuries, the losses were smaller becausegrass was cut by hand and later in the year, as it wasused to make hay rather than silage. In both species,local conservation action, within the framework ofappropriate agri-environment schemes, has greatlyreduced nest losses, resulting in increased nationalpopulation levels.

The three gamebirds listed in Table 1 have all suf-fered from loss of habitat and food supplies. Whenpopulations decline, landowners tend no longer toemploy gamekeepers, and predation rates rise, furthercontributing to population declines. In the GreyPartridge, decline seems to result in the first instance

mainly from reduction in the abundance of insectseaten by chicks, but reduction in field margins resultsin a scarcity of nesting habitat (and the more weed-rich and insect-rich cropland at the edges of fields,Green 1984), while lack of predator control resultsin greater predation both on nest contents and onadults, the incubating females being especially vul-nerable. The series of events leading to populationdecline (Fig. 1) has been confirmed by experiment,and Grey Partridge populations have increased inresponse to local conservation measures (includingthe non-spraying of field edges) and agri-environmentschemes, and the overall national population seemsto have stabilized at a low level.

In Black Grouse, which are now mainly confinedto moorland and other rough grazing land in theuplands, numerical declines have been attributedprimarily to increased stocking densities, which reducethe sward height and the diversity of plant popula-tions (Fig. 3). Insects suitable for chicks decline inabundance, leading to greatly reduced chick survival,and hence to population decline. Populations in sev-eral experimental areas have responded to reducedlivestock densities (and greater insect abundance),

Figure 3. Proposed sequence of events through which the overgrazing of open upland vegetation causes population declines in grouseand other species. The relative contributions of food-plant reduction (route A) and insect reduction (route B) seem to have variedbetween species. For references, see Appendix.

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and in some (but not all) areas also to predator con-trol. The Red Grouse has declined mainly because ofreduction in the area of heather moorland. In someareas this is caused by afforestation, but in others byincreased sheep densities, through which heather isgradually converted to grassland. Other factors in thedecline of Red Grouse populations include reducedpredator control, and increased incidence of the viraldisease louping ill (see below). To my knowledge, norecent attempts to increase Red Grouse numbers(other than by predator control) have been pub-lished, and the national population shows no sign ofincreasing.

Birds of prey

Three raptor species suffered massive populationdeclines in the late 1950s and 1960s through thedirect effects of organochlorines on breeding successand survival (Fig. 4). These chemicals are persistent,and highly fat-soluble, so they accumulated in thetissues of prey species, from which they concentratedto even higher levels in some raptor species. The

chemical DDE (the main metabolite of the insecti-cide DDT) caused eggshell thinning, which led toegg breakage and reduced breeding success, whilethe more toxic cyclodiene compounds (aldrin anddieldrin) killed birds outright. These various effectswere tested on captive birds of related species. As aresult of these and other findings, the use of organo-chlorines was progressively reduced over a periodof years, and almost ceased from 1986 onwards.Accordingly, the three species that had been mostaffected in Britain (Sparrowhawk, Peregrine andMerlin) steadily recovered through the 1970s and1980s, and in 20 years they had recolonized regionsfrom which they had been extirpated. This is a majorconservation success story, and the main threat tothese birds may now stem from declines in their var-ious prey species or from illegal persecution.

Two other birds of prey, the Kestrel and Barn Owl,also suffered from organochlorine use, but showedonly regional population declines, from which theyrapidly recovered in the 1970s (Newton

et al

. 1991,1999). Since then, they have declined more gener-ally, owing mainly to the loss and overgrazing of

Figure 4. Proposed sequence of events through which the use of organochlorine pesticides causes population declines in somepredatory birds. DDE is the main metabolite of the insecticide DDT, and acts primarily by causing eggshell thinning and reduced breedingsuccess. HEOD is the main metabolite of aldrin and dieldrin, is much more toxic than DDE and acts by increasing mortality rates. Therelative importance of DDE and HEOD in causing population declines seems to have varied between regions, depending on usagepatterns. For references, see Appendix, and Newton (1998).


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rough grassland that supports their principal preyspecies, the Field Vole

Microtus agrestris

. In addition,both have suffered, at least in some areas, fromreductions in the availability of nest-sites, mainlytree cavities, but (in the Barn Owl) also disusedcottages and farm buildings. Both species haveresponded by increased breeding density and successto improvements in food supply (brought about byfencing out sheep from rough grassland) and by pro-vision of artificial nest-sites in areas where naturalsites were scarce. Unless the trends in grassland man-agement and nest-sites can be reversed, these twospecies seem destined to decline further.

SUMMARY OF FINDINGS

Of the various changes on farmland in recent dec-ades, four emerge as the major drivers of declines inbird populations: (1) pesticide use (especially herbi-cides); (2) late summer ploughing of cereal stubbles,followed by rapid re-sowing and early crop growth;(3) drainage and intensification of lowland grasslandmanagement; and (4) increased stocking densities(mainly cattle in the lowlands and sheep in the uplands).Each of these changes has had serious effects on awide range of species. Other changes, such as theremoval of hedgerows and earlier harvesting of grassand cereals, have affected a smaller range of species.Most of these changes have resulted in reductionsin bird habitats and/or food supplies, which haveemerged as the main limiting factors underlying popu-lation declines in 27 of the 30 species considered(the only exceptions being the three raptor specieswhose populations collapsed following the introduc-tion of organochlorine insecticides) (Table 2). Otherevidence for the importance of food supplies hasemerged from other types of study, for examplewhere breeding or wintering densities, survival orbreeding success are greatest in localities with greaterfood supplies (for winter densities of seed-eatingbirds see Donald & Evans 1994, Wilson

et al

. 1996,Robinson & Sutherland 1999, Moorcroft

et al

. 2002,Hancock & Wilson 2003; for breeding success ofCorn Buntings see Brickle

et al

. 2000); on organiccompared with conventional farms (Christensen et al.1996, Wilson et al. 1997, Chamberlain et al. 1999b,Chamberlain & Wilson 2000); on first-year set-asidecompared with cropped fields (Watson & Rae 1997,Henderson et al. 2000); in localities with pesticideuse compared with localities with no pesticide use(Boatman et al. 2004); or in seed-bearing ‘game crops’compared with conventional crops (Stoate et al.

2003). Yet other studies have linked the temporalchange in regional or local agricultural practice withthe timing of declines in invertebrate and bird den-sities (Benton et al. 2002).

Predation has not emerged as the main causalfactor for any of the species considered, only as a sec-ondary or associated factor, chiefly in ground-nestingbirds. All the species listed, as with most other birdspecies in Britain, suffer from predation, and in somespecies predation rates have been recorded as higherthan in the past. Several species of predators, notablyRed Fox Vulpes vulpes, Carrion Crow Corvus coroneand Magpie Pica pica, have increased in numbersduring recent decades. Their increases have beenattributed to reductions in the numbers of game-keepers, and to increases in food supplies, contingentupon changes in land-use. In upland areas, increasedsheep densities have led to ecological overgrazing inmany areas, and to increased sheep mortality, whichhas provided more carrion. In lowland areas, thedecline in Grey Partridge numbers has encouragedmany landowners to rear and release large numbersof Pheasants Phasianus colchicus and Red-leggedPartridges Alectoris rufa, an activity which also pro-vides more food to sustain predator populations. Thegreater areas of short-sward grassland may also havefavoured corvid populations, as it provides addi-tional foraging habitat for these birds.

Table 2. Summary of existing knowledge and conservationaction for 29 species in Britain whose populations have declinedduring the past 50 years in association with agriculturalchanges. From data in Table 1.

Agricultural driver known 30External causal factor known1 30Demographic causal factor known2 24Experimental test positive3 15

10Local conservation action, positive response4

Agri-environment scheme positive response5 12Other policy change, positive response6 3 (+2)National population increase7 8 (27%)

1Habitat reduction 20 species, food reduction 25 species,nest-site reduction three species, pesticide poisoning threespecies, agricultural operations two species, predation four species,parasites/pathogens one species, competition no species (seetext).2Species: 1, 2, 4, 6–11, 14–28.3Species: 1, 2, 7, 11, 15–17, 22–24, 26–30.4Species: 13, 17, 18, 20–24, 29–30.5Species: 1, 6–9, 11, 14, 17, 21–24.6Species: 3, 8, 26–28.7Species: 7, 15, 16, 18 (see text), 21, 22, 27, 28.Species numbered as in Table 1.

}19

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Some land-use changes may predispose preyspecies to extra predation (Evans 2004): examplesinclude the draining and re-seeding of grassland,which supposedly makes ground nests more visibleor accessible to predators; hedge removal, which forsome species concentrates nests in smaller areas ofremaining habitat that are easier for predators tosearch; and reductions in food-supplies, which makehungry passerine broods call more, and attract theattention of predators (e.g. Evans et al. 1997, Brickleet al. 2000). Nest success of some species in differentareas has been correlated with densities of predators(notably corvids, Stoate & Szczur 2001), and predatorremoval studies have led to increased nest successand breeding density in a number of species, includ-ing Red Grouse, Golden Plover Pluvialis apricariaand Lapwing on heather moors (Tharme et al. 2001),and Grey Partridge, Song Thrush, Blackbird andother passerines on farmland (Tapper et al. 1996,Stoate & Szczur 2001, Donald et al. 2002). On theother hand, no relationships emerged between localtrends in songbird numbers and local trends in Magpieor Sparrowhawk numbers, two predators sometimessuspected of causing declines in songbird popula-tions (Gooch et al. 1991, Dix et al. 1998, Thomsonet al. 1998).

Disease is hard to detect in wild bird populations,but ‘louping ill’ has emerged as a factor leading todecline in Red Grouse densities in some areas.This viral disease often kills grouse, but persists inalternative hosts, which include sheep. Its incidencein grouse has increased greatly on some moors onwhich sheep densities are high, leading to markeddeclines in grouse densities. Because of its depend-ence on high sheep densities, frequent louping ill canbe regarded as another legacy of recent agriculturalpolicy.

Most species of birds overlap in their dietary andother needs with other bird species and with otheranimals. Resources used by one species are notthen available to another. It is reasonable to supposetherefore that the densities of many species arelower than they might otherwise be, because of com-petition for shared resources, but the effects of com-petition are not necessarily greater now than in thepast. So far as I am aware, increased competitionhas not been suggested as a factor in farmland birddeclines. However, it is perhaps one additional factorworth examining in the House Sparrow. In formertimes, when House Sparrows were at their peakabundance, they were the only birds that lived intowns in such close association with people. They

had access to everything edible that people pro-vided. Their recent decline in urban areas hasoccurred over a period when increasing numbers ofother seed-eaters (including Collared Doves Strep-topelia decaocto) have been attracted into gardens byspecial food provision. It would be surprising if theseother species did not also remove some food itemsthat would formerly have been available to HouseSparrows. In addition, the effect on other birds ofreleasing 20–30 million Pheasants into the country-side each year has never been assessed. Pheasants arenormally fed with grain for most of the year (latesummer to spring), which could benefit other birds,but they also find some of their own food, whichcould adversely affect other species.

Agricultural operations, leading to destruction ofeggs or chicks, have been identified as a major causalfactor in the population declines of two species(Stone Curlew and Corncrake), but also contributeto nest failures in other ground-nesting species,especially during the harvest of silage (for Skylarksee Poulsen et al. 1998) or early ripening cereals (forCorn Bunting see Crick et al. 1994, Brickle & Harper2002; for Montagu’s Harrier Circus pygargus inEurope see Arroyo et al. 2002).

Apart from the raptors affected by organochlorinepesticides, only three species that had declinedhave shown substantial increases in at least the last5 years, namely Cirl Bunting, Corncrake and StoneCurlew. These three all have restricted distributionswithin Britain, and relatively small populations con-fined to farmland, which can be managed intensivelywithin the framework of agri-environment schemes.Without continued year-by-year management, thesespecies would all decline, and probably becomeconfined to nature reserves or disappear from Britaincompletely within a few years. For three other spe-cies, BBS indicates smaller, but significant, increasesover the past five or more years, namely Blackbird,Song Thrush and Snipe, although the evidence forthe Snipe is equivocal (see Appendix).

Although it is too early to assess properly the effectsof recently introduced agri-environment schemes,earlier schemes were most successful when targetedto the needs of particular species, such as CirlBunting, Lapwing, Stone Curlew and Corncrake, andof some seed-eaters through winter seed provision(Aebischer et al. 2000, Bradbury & Allen 2003). Morerecent schemes have led to localized increases in thebreeding and wintering densities of several otherspecies (Bradbury et al. 2004), but without detailedstudy, it is as yet uncertain whether these local


Recent declines of farmland bird populations in Britain 591

increases were due to immigration, rather than tothe improved survival and breeding success of localbirds. Only a general improvement in survival or off-spring production could lead to recovery of nationalpopulations. While reversing the declines in farm-land birds requires substantial changes to existingfarming systems, such changes do not necessarilyrequire a reversion to previous systems, providingthat appropriate management practices can be incor-porated in other ways.

Considering the massive changes that have occurredon farmland in recent decades, during the drive forintensive crop production, it is perhaps surprisingthat not all species have declined. However, the fewspecies involved have benefited in other ways, anda plausible reason for their lack of decline is usuallyapparent. For example, Woodpigeons Columbapalumbus were once largely dependent in winter onthe clover sown into cereal stubbles. As this cropdisappeared in the 1960s, Woodpigeons began todecline, but soon increased again when a substitutewinter food-supply, namely the young leaves of oilseedrape, became widely available (Inglis et al. 1990).Stock Doves Columba oenas have also increasedgreatly since the 1960s. Their numbers had previ-ously been greatly reduced by organochlorinepesticides (O’Connor & Mead 1984), so at least partof their increase represents population recovery fol-lowing reductions in organochlorine use, but latterlythey may also have benefited from the spread ofoilseed rape, the leaves of which form an importantwinter food. It is perhaps more surprising thatthree species of finches have not declined, namelyChaffinch Fringilla coelebs, Greenfinch Carduelischloris and Goldfinch Carduelis carduelis. However,Chaffinches are major beneficiaries of Pheasant feedsites, which are maintained throughout the winter,and Chaffinches and Greenfinches have fed increas-ingly in gardens since the 1960s, following the pro-vision of peanuts and other suitable seeds. The sameis true for Goldfinches since the early 1990s, but inany case a large proportion of Goldfinches that breedin Britain winter further south, mainly in Spainwhere some favoured food-plants grow and seedthroughout the winter (Newton 1972). However,further work is needed to determine whether theseplausible guesses can be turned into well-founded fact.

In conclusion, gaps still exist in our understandingof the causal factors behind the declines in somespecies affected by agricultural change, as indicatedin Table 1. In lesser-known species not discussed indetail here, such as Ring Ouzel Turdus torquata and

Whinchat Saxicola rubetra, population declines maywell be due to one or more of the same causal fac-tors, in which case these species could also benefitfrom remedial measures designed for better-studiedspecies. Despite the gaps in our knowledge, the maindrivers of bird population declines are now clear, andit is these drivers that must be addressed if recoveriesare to occur. One obvious way to reverse populationdeclines would be to revert to traditional systems ofland use, but such changes seem unlikely to happen,except on the small scale possible on nature reservesand other special sites. Alternatives include encour-aging farmers to adopt different procedures, or find-ing different ways, within the context of modernagriculture, of providing the food or other resourcesthat birds and other wildlife require. Both of theseoptions require a shift in the subsidy system, so thatfarmers do not suffer financially from adopting morebenign practices. Recent reforms to the CommonAgricultural Policy, notably the de-coupling of sub-sidies from crop and meat production, may provideopportunities for more widespread conservationmanagement. The main immediate requirements,however, are for continued monitoring of birdpopulations, at local as well as national levels, inorder to assess the effectiveness of existing and newagri-environment schemes. Only by careful appraisalof the application and consequences of these schemescan their prescriptions be modified in future toachieve greater environmental benefits.

I am grateful to James Goodhart, Rhys Green and JeremyWilson for helpful comments on the manuscript, andalso to Phil Grice and Juliet Vickery in their capacity asreferees.

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Received 27 April 2004; revision accepted 4 August 2004.

APPENDIX

Species notes and references to Table 1

House Sparrow Passer domesticus – Declined in urbanas well as in rural areas; several causal factors mooted,including decline of food supply (insects and seeds),increased predation from an expanding cat popula-tion, increased pollution from traffic in cities, and insome areas reduced availability of nest-sites (stem-ming from building repairs and conversions), butfurther work needed. In one experiment, respondedto provision of extra food on some farms wherenumbers previously declined, but not on others wherenumbers were previously stable or increasing (Holeet al. 2002). Decreased survival during the years of



decline (Siriwardena et al. 1999); also increased nestsuccess (Siriwardena et al. 2000), but no informationon possible change in duration of breeding season ortotal seasonal productivity. Other reference: Freemanand Crick (2002).

Tree Sparrow Passer montanus – National popula-tion decline has exceeded 95%. In local conservationprojects has benefited from provision of winter food(small seeds) and nestboxes, especially when sitednear water, where insects are still plentiful (Field &Anderson 2004). Showed increased nest success duringthe years of decline, but no information on possiblechanges in duration of breeding season or total sea-sonal productivity (Siriwardena et al. 2000).

Linnet Carduelis cannabina – Reduced survival andnest success during the years of decline (Siriwardenaet al. 1999, 2000), but no information available onpossible change in duration of breeding season or ontotal seasonal productivity. In a French study, annualvariations in production of fledglings influenced year-to-year population increase (Eybert et al. 1995). Forimportance of oilseed rape as a food source, seeMoorcroft et al. (1997) and Moorcroft and Wilson(2000).

Twite Carduelis flavirostris – Has suffered insummer from loss of weeds from meadows near themoorland edge (through improved grassland man-agement), and in winter from loss of weeds fromarable land, and especially from loss of winterfeeding areas of coastal saltmarsh, which have beenconverted to farmland (Atkinson 1998, Dierschke2002). Additional reference: Brown et al. (1995).

Bullfinch Pyrrhula pyrrhula – Adverse habitat changesinvolve loss of tall thick hedgerows from farmlandand loss of understorey shrubs from many deciduouswoods (often through increased deer browsing). Thesechanges also involve loss of food-plants, in additionto loss of farmland weeds. Furthermore, recovery ofSparrowhawk numbers may have confined Bullfinchesto feeding close to cover (through a behaviouralresponse), and thus reduced the total land area overwhich they can forage, compared with the 1960s(Newton 1967). The demographic cause of declineis uncertain (Siriwardena et al. 2001), but higher nestsuccess was recorded during the decline (Siriwardenaet al. 2000), with no information on possible changein duration of breeding season or total seasonalproductivity. The latter is greatly influenced by theamount of late summer nesting (Table 1; Newton1999, Proffitt et al. 2004).

Yellowhammer Emberiza citrinella – Breeding den-sities in different parts of Britain strongly correlated

with the proportion of land under arable crops, withcrop diversity and with hedgerow length. Declinebegan later than in some other seed-eaters, and mostmarked following loss of minority cereal crops fromwestern areas now dominated by grass and non-cerealcrops (Kyrkos 1997, Kyrkos et al. 1998). Individualnest success was higher during years of populationdecline (Siriwardena et al. 2000), but in one studyon nine scattered farms total seasonal productivitywas too low to maintain a stable population (Bradburyet al. 2000).

Cirl Bunting Emberiza cirlus – Decline attributedto lack of insect food for chicks in breeding season,and seeds in winter. Appropriate conservation meas-ures, within the framework of an agri-environmentscheme, promoted an increase in food-supplies, lead-ing to a four-fold increase in the remnant populationwithin 10 years (Evans & Smith 1994, Evans 1997,Wotton et al. 2000, Peach et al. 2001).

Reed Bunting Emberiza schoeniclus – Probablysuffered from both land drainage and destruction of‘rank patches’ (reducing nesting habitat) and declinein food supply (insects in summer, weed and grassseeds in winter). During the years of decline (1975–83) breeding numbers fell rapidly on arable andmixed farms, but remained relatively stable onpastoral farms (Peach et al. 1999). The decline wasgreater in northern Britain than in the southeast.Annual survival was lower during the years of decline(Peach et al. 1999), as was nest success (Siriwardenaet al. 2000), and recent study suggests a decline in totalseasonal productivity in some areas (Brickle & Peach2004). Oilseed rape crops and small wetland features(such as ditches) now provide most nesting places infarmland. Additional reference: Burton et al. (1999).

Corn Bunting Miliaria calandra – National popu-lation decline has exceeded 80%. Fewer birds nowraise a second brood than in the past, reducing theseasonal production of young (Brickle & Harper2002), but success per nesting attempt was recordedas slightly higher during the years of decline (Crick1997; Siriwardena et al. 1998). Nest survival waslower in localities with little invertebrate chick food(owing to pesticide use), which led to chick starva-tion and predation (Brickle et al. 2000). Populationdecline in Schleswig-Holstein, Germany, was attrib-uted to increased chick starvation (Busche 1989).Benefits from low-input spring-sown cereals (barley),associated weedy winter stubbles and wide fieldmargins. Other references: Donald and Evans (1994),Brickle and Harper (1999), Crick (1997), Donaldand Forrest (1995), Donald et al. (1994).

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Turtle Dove Streptopelia turtur – Decline attributedto reduction of tall hedgerows used for nesting andweed-seeds used as food (Browne & Aebischer 2001).Increased nest success recorded during the years ofdecline (Siriwardena et al. 2000), but reductions induration of breeding season and in total seasonalproductivity (Browne & Aebischer 2001, 2003).

Skylark Alauda arvensis – Adverse habitat changesinclude loss of rough grassland, switch from spring-sown to autumn-sown cereals (which grow too tallfor nesting in spring), and conversion of mixed farmsto cereal or intensive grass monocultures (Chamberlain& Gregory 1999); food shortages involve summerinsects and winter seeds (Jenny 1990, Odderskaeret al. 1997). Breeding densities tend to be higher inpermanent grass and set-aside than in autumn-sownor spring-sown cereals (Poulsen et al. 1998; Wakeham-Dawson et al. 1998, Chamberlain et al. 1999a, Eraud& Boutin 2002); crops that do not favour breedingSkylarks, such as autumn-sown cereals and rape, havebecome increasingly prevalent in recent decades.Nest success was higher during the years of decline(Siriwardena et al. 2000), but several studies suggestreduction in the duration of the breeding season,because of earlier crop growth and lack of alternativehabitat locally (a consequence of greater arablemonoculture) (O’Connor & Shrubb 1986, Wilsonet al. 1997, Chamberlain & Crick 1999). No infor-mation on possible change in mortality. Breedingdensities and seasonal productivity increased follow-ing the provision of unsown patches (4-m squaresat two per hectare) in autumn-sown cereal fields(Morris et al. 2004). Other references: Green (1978),Shläpfer (1988), Donald et al. (2001a, 2002),Robinson (2001), Pierce-Higgins and Grant (2002).

Meadow Pipit Anthus pratensis – In lowland, declineassociated with loss of patches of rough grass. Inupland, reduced breeding densities evident in heavilygrazed, shorter swards (Vanhinsberg & Chamberlain2001, Pierce-Higgins & Grant 2002). Intensive graz-ing of heather moorland encourages the replacementof heather by grass, which favours Meadow Pipits,whose densities decline under further grazing, whichreduces sward height and diversity, and associatedinsect densities (Smith et al. 2001). No relevant dataon demographic changes during the years of decline.

Yellow Wagtail Motacilla flava – Breeds in dampgrassland and in dry sparsely vegetated arable land(Mason & Lyczynski 1980, Nelson 2001). Declineattributed to drainage and reductions in the area ofdamp grassland, and generally more intensive grasslandmanagement, and to change from spring-sown to

autumn-sown crops. Decline most marked in pastoralregions (Chamberlain & Fuller 2001). No publishedinformation on possible change in reproductive ormortality rates, or on possible effects on populationlevels of events in African wintering areas. YellowWagtails have responded with increased breedingdensity to raised water levels in some grasslandnature reserves. Additional references: Bradbury andBradter (2004), Henderson et al. (2004).

Starling Sturnus vulgaris – Reduction of food sup-ply caused by conversion of former grass to arable ineastern districts, and by more intensive managementof grassland generally, leading to reduction in soilinvertebrates, and also by reduction in the numbersof accessible feed-sites for farm stock. Reduced sur-vival during the years of decline recorded for Britain,and reduced reproductive rate recorded in Finland(Tiainen et al. 1989). Other references: Feare (1994),Whitehead et al. (1995), Freeman et al. (2002).

Blackbird Turdus merula – Decline associatedwith loss of hedgerows and field boundaries, andland drainage which reduces food availability. InThe Netherlands, the main demographic change wasreduced annual survival (Dix et al. 1998). Increasedbreeding density and success followed various con-servation measures (including predator control) atthe Game Conservancy Trust’s farm in Leicestershire(Stoate & Szczur 2001).

Song Thrush Turdus philomelos – Decline associatedwith loss of hedgerows and field boundaries, andland drainage, which reduces food availability.Main demographic cause assessed as reduced survival,especially in years of prolonged summer droughtor winter frost (Baillie 1990, Thomson et al. 1997,Robinson et al. 2004), judged as sufficient in a popu-lation model to account for decline at the observedrate, without any concurrent change in reproduction(Robinson et al. 2004). However, in one study, thebreeding season was shortened in a dry arable area,compared to a more mixed farming area, leading toreduction in seasonal production of young, also judgedas sufficient to cause population decline (Peach et al.2004). Increased breeding density and success followedvarious conservation measures (including predatorcontrol) at the Game Conservancy Trust’s farm inLeicestershire (Stoate & Szczur 2001).

Lapwing Vanellus vanellus – Main adverse habitatchange associated with the drainage of wet grassland(Fig. 2); also conversion of mixed farms to arableor ‘improved’ grass monocultures; change fromspring-sown to autumn-sown cereals (which reducesthe area of spring tillage, favoured for nesting); and



increased stocking densities on grassland (which leadsto more disturbance, nest predation and trampling)(Beintema & Muskens 1987, Baines 1988, 1989,Beintema 1988, Galbraith 1988, Shrubb 1990, Shrubb& Lack 1991, Hudson et al. 1994, Wilson et al. 2001,Hart et al. 2002). No change in annual survival dur-ing the years of decline (Peach et al. 1994), soreduced reproduction proposed as the demographiccause; and in some local populations annual produc-tion measured as insufficient to offset expected annualmortality (e.g. Galbraith 1988). Seasonal productivitylower on improved than on unimproved grassland,owing to fewer replacement clutches and poorerchick survival (Baines 1989). In some localities, sin-gle pairs and small groups are more vulnerable to eggpredation than larger groups, owing to the reducedeffectiveness of communal nest defence (Seymouret al. 2003). Lapwings have responded with increasedbreeding density to local reserve management andagri-environment schemes (Ausden & Hirons 2002,Bradbury & Allen 2003). Additional references: Baines(1988, 1989), Crick et al. (1998), Shrubb (1990),Henderson et al. (2001, 2003), Sheldon et al. (2004).

Snipe Gallinago gallinago – Main adverse habitatchange is the drainage of wet tussocky grassland (e.g.Baines 1988). Even in some areas where birds stillbreed, the nesting season is shortened because theground dries out earlier in the breeding season, lead-ing to reduction in seasonal productivity (Green1988a). Snipe have responded with increased breed-ing density to raised water levels in some grasslandnature reserves but not in others (Ausden & Hirons2002). Two count schemes gave conflicting results,BBS indicating an increase in recent years, andbreeding wader surveys in meadows a steep decline.Additional reference: Crick et al. (1998).

Curlew Numenius arquata – Main adverse habitatchange is the drainage of rough wet grassland (e.g.Baines 1988). High predation rates on eggs in someareas, such as Northern Ireland, where measuredreproductive rates were too low to sustain popula-tions and could account for recent rates of popula-tion decline (Grant et al. 1999). Fragmentation ofhabitats may also allow Foxes to search nestingplaces more efficiently. Other references: Berg (1992,1994).

Redshank Tringa totanus – Main adverse habitatchange is drainage of wet grassland (e.g. Baines 1988).On coastal saltmarsh, breeding densities declinedmore markedly during 1985–96 on sites that hadexperienced an increase in grazing pressure, fromungrazed/lightly grazed to moderately/heavily grazed,

although some grazing by cattle was deemed beneficialto sward structure (Norris et al. 1998). Redshankshave responded with increased breeding density toraised water levels in some grassland reserves (Ausden& Hirons 2002).

Stone Curlew Burhinus oedicnemus – On arableland, management has entailed finding nests, andprotecting them from farm machinery, and theprovision (through an agri-environment scheme) ofspring tillage for nesting (Aebischer et al. 2000). Otherreferences: Green (1988b), Green and Griffiths (1994).

Corncrake Crex crex – Main decline attributed toearlier and mechanized grass cutting (Norris 1947),but drainage and intensive grass management hasalso removed much previously suitable nesting hab-itat (Stowe et al. 1993). One of the last remainingpopulations of Corncrake in Ireland is in theShannon ‘callows’, where mowing is forced lateby unregulated winter and spring flooding (Shepherd& Green 1994). Breeding success and breedingdensity in various areas increased in response to laterand more careful grass cutting (Green & Stowe 1993,Green et al. 1997, Green 1999, Green & Gibbons2000).

Grey Partridge Perdix perdix – Population declinehas exceeded 85%. One of the most thoroughlystudied species, with proposed mechanisms tested byexperiment (Fig. 1, Potts 1986, Rands 1985, Potts &Aebischer 1995, Tapper et al. 1996). Other references:Green (1984), Bro et al. (2000).

Black Grouse Lyrurus tetrix – Heavy grazing ofmoorland habitat causes decline in food-plants (forfull grown birds) and insects (for chicks) (Fig. 3;Baines & Hudson 1995, Baines 1996, Jenkins &Watson 2001). Local populations responded, withimproved breeding success and breeding densities, toreduced grazing within the framework of an agri-environment scheme, and some also responded inthe same way to predator control (Calladine et al.2002, Warren & Baines 2004).

Red Grouse Lagopus l. scoticus – Predation byraptors (especially Hen Harriers Circus cyaneus) cancause substantial population decline (Redpath &Thirgood 1997), but because some raptors and otherpredators are controlled on most grouse moors, long-term declines in Red Grouse densities are more oftenassociated with loss of heather through overgrazing(Redpath & Thirgood 1997, Jenkins & Watson 2001).High sheep numbers in some areas also raise the incid-ence of louping ill, which is often lethal to grouse,accounting for some local declines (Duncan et al.1979, Hudson & Dobson 1991).

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Barn Owl Tyto alba – Main adverse habitat changeis the loss or overgrazing of rough grassland, whichsupports the main prey species, the Field Vole Microtusagrestis; also loss of nest-sites through hedgerow treeremoval and ‘barn conversion’, and collapse of old,disused cottages, previously suitable (Shawyer 1987,Ramsden 1998, Newton 2002). Population assessmentsin 1982–85 and 1995–97 put national numbers at4500 pairs and 4000 pairs, respectively, but numbersfluctuate greatly from year to year, and the two figuresresult from different methodology, so may not becomparable (Shawyer 1987, Toms et al. 2001). Hasresponded by increased density and breeding successto the fencing out of sheep from areas of grassland(as in afforestation projects), and by increased breed-

ing density to the provision of nestboxes in areaswith insufficient nest-sites (Newton 2002).

Sparrowhawk Accipiter nisus, Merlin Falco colum-barius, Peregrine F. peregrinus – Main declines fromlate 1950s to early 1960s, associated with the heavyuse of organochlorine pesticides (Fig. 4). Populationsrecovered in subsequent decades following progres-sive reductions in organochlorine use, and theirreplacement by less persistent insecticides (Newton1979, 1986, Ratcliffe 1993, Newton et al. 1997,1999). The Sparrowhawk has stabilized or may havebeen in regional decline since about 1994.

Kestrel Falco tinnunculus – As for Barn Owl, exceptthat old buildings are much less important as nest-sites(Village 1990).

The recent declines of farmland bird populations in ...

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