Declining farmland birds:

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1 : evidence from large-scale monitoring studies in the UK Dan Chamberlain and Juliet Vickery Mike Langman ABSTRACT Several farmland bird species have undergone serious population declines during the past 30 years. Evidence from national monitoring schemes co-ordinated by the British Trust for Ornithology indicates that agricultural intensification has been responsible for these losses. A range of factors, including a decrease in the acreage of spring cereals, the loss of rough grazing, an increase in intensively managed silage and in livestock density, and the overall lack of crop diversity, have coincided with population declines of several species. Owing to the wide range of species affected and the multiple factors underlying the declines, it is difficult to identify those individual aspects of farm management which are of most importance to the recovery of farmland bird populations.we suggest that a general extensification of agriculture, affecting a range of management practices, rather than implementation of management prescriptions for individual species, will be most likely to benefit farmland bird communities.this could be achieved through agri-environment schemes in conjunction with widespread initiatives to reduce the intensity of farm management over large areas. 300 British Birds 95 June

2 During the past three decades, the British farmland bird community has undergone great changes. From the early 1970s onwards, there were large declines in already scarce species such as Corn Crake Crex crex, Stone-curlew Burhinus oedicnemus and Cirl Bunting Emberiza cirlus (although all of these have benefited from recent intensive conservation efforts; see Aebischer et al. 2000). At the same time, there was a marked contraction in the range of several formerly common species, such as Turtle Dove Streptopelia turtur and Corn Bunting Miliaria calandra, which are now mostly restricted to southeast Britain. Perhaps most worrying of all, however, was the decline of formerly abundant farmland species. Although the Sky Lark Alauda arvensis is still one of Britain s most common farmland birds, the population has halved in numbers over the past 30 years and similar trends have been observed in other common species such as Song Thrush Turdus philomelos, Linnet Carduelis cannabina, Common Bullfinch Pyrrhula pyrrhula and Yellowhammer E. citrinella (Siriwardena et al. 1998). Even two former pest species, Common Starling Sturnus vulgaris and House Sparrow Passer domesticus, are nowhere near the levels of abundance of 30 years ago. Monitoring the changes Detection of these population declines, particularly for the commoner species, was made possible largely by the efforts of volunteer ornithologists who carry out regular monitoring surveys co-ordinated by the British Trust for Ornithology (BTO). These include annual surveys of breeding birds (e.g. Noble et al. 2000), studies of species range across the UK (Gibbons et al. 1993), and one-off or irregular surveys designed to consider particular habitats (e.g. the Winter Farmland Bird Survey) or individual species (e.g. Lapwing Survey, Wilson et al. 2001). The scheme that has been largely responsible for detecting farmland bird declines, however, is the Common Birds Census or CBC (Marchant et al. 1990). To date, 599 farmland CBC sites have been surveyed since the survey s inception in 1961, involving over 40,000 hours of fieldwork. The CBC allowed us to measure the relative change in populations by gathering population data from a range of sites around the country, each surveyed annually, thus allowing the percentage change in breeding populations to be estimated. The CBC British Birds 95 June has now been replaced by the Breeding Bird Survey (BBS), which continues to monitor a range of bird species across Britain, but has the advantages of requiring less time and effort from surveyors, giving more complete geographical coverage, and covering a wider range of species (Noble et al. 2000). CBC data have shown that farmland bird communities have undergone far greater declines than those in other habitats. In addition, farmland bird specialists are more likely to have declined than those species which breed on farmland but which also use a range of habitats (table 1). Clearly, some factor(s) unique to farmland habitat are implicated, but can we gain any idea of what they might be from national monitoring data gathered by volunteers? In this paper, we assess the evidence for the effect of farming practices on bird populations using data from broad-scale BTO datasets. We demonstrate the value of these data for identifying likely impacts on bird populations, and how they may be used to make conservation recommendations or direct more intensive research. Why have farmland birds declined? The factors responsible must either be specific to farmland, or for some reason have a greater impact on farmland than in other habitats. Two possibilities are disease (including parasites) and climate change, both of which may affect the population ecology of birds (e.g. Crick et al. 1997; Tompkins et al. 2000). It is, however, not clear why either should be specific to farmland species or habitats. A third possibility is that increasing predation may have driven farmland bird declines. Although a number of predators have increased on farmland in recent years, for example corvids (table 1), a close examination of population data reveals that for most declining farmland birds the main period of decrease was between the mid 1970s and early 1980s. The population size of most avian predators increased sometime after that. At the population level, there is no evidence that avian predators have caused population declines (Thomson et al. 1998). We know little about population trends of mammalian predators, but few are really farmland specialists, so again their effect is unlikely to be specific to farmland. Although predation alone is perhaps unlikely to have had a significant impact on farmland bird populations, habitat changes may have made 301

3 Table 1. The percentage change in the breeding populations of 32 species on farmland between 1973 and , derived from the Common Birds Census, except where indicated (*). Farmland specialists are those which preferentially nest on farmland and find most of their food there (defined according to Fuller et al. 1995). When considering the numbers of increasing or decreasing species, it is apparent that 15 farmland specialists have declined (12 by more than 30%), while only six have increased. For the other species, which occur across a range of habitats, the number of gains and losses are evenly split, with six in each category. If this type of analysis is extended to other habitats, a far greater proportion of birds are found to be declining on farmland compared with any other habitat (Fuller et al. 1995). Farmland specialists Generalists Species % change Species % change Common Kestrel Falco tinnunculus -26 Wren Troglodytes troglodytes -13 Grey Partridge Perdix perdix -57 Hedge Accentor Prunella modularis -45 Northern Lapwing Vanellus vanellus -44 Robin Erithacus rubecula +3 Stock Dove Columba oenas +96 Blackbird Turdus merula -38 Wood Pigeon Columba palumbus +101 Song Thrush Turdus philomelos -69 Turtle Dove Streptopelia turtur -79 Blue Tit Parus caeruleus +18 Barn Owl Tyto alba -43* Great Tit Parus major +27 Sky Lark Alauda arvensis -53 Magpie Pica pica +57 Yellow Wagtail Motacilla flava -25 Carrion Crow Corvus corone +45 Common Whitethroat Sylvia communis +100 Common Chaffinch Fringilla coelebs +25 Eurasian Jackdaw Corvus monedula +55 Common Bullfinch Pyrrhula pyrrhula -70 Rook Corvus frugilegus +40* Common Starling Sturnus vulgaris -51 House Sparrow Passer domesticus -51 Tree Sparrow Passer montanus -93 Greenfinch Carduelis chloris +7 Goldfinch Carduelis carduelis -5 Linnet Carduelis cannabina -47 Yellowhammer Emberiza citrinella -45 Reed Bunting Emberiza schoeniclus -57 Corn Bunting Miliaria calandra Up-to-date population changes for a large number of species are available on the BTO website at: prey species more vulnerable or predators more efficient. Teasing apart the effects of predation and changes in farm management is difficult, and is likely to require an experimental approach. Research is currently being undertaken at the Game Conservancy Trust s experimental farm at Loddington, in Leicestershire, to address this issue (Stoate 2001). Agricultural intensification There is, in fact, overwhelming evidence that changes in farm management have been the major factor underlying farmland bird declines. Change in agriculture ( intensification ) during the past 40 years, brought on by technological and economic developments, has been substantial and has affected every aspect of farming, including the crops grown, the amount and type of pesticide and fertiliser applied, the management of grassland, the density of livestock and the number of mixed farms (farms tend nowadays to be specialised into either arable or grass). A number of these changes are illustrated in fig. 1. A common feature of these trends is that the periods of most rapid change often occurred at about the same time, in the 1970s. Birds and agricultural management The effects of agricultural management on bird abundance have been addressed by examining bird and habitat data collected in a range of BTO surveys for a whole suite of farmland species. The importance of such large-scale data in providing vital clues about the causes of the decline of individual species is illustrated here by three specific examples: two passerines Sky Lark (a widespread species which nests and feeds within the cropped habitat) and Reed Bunting E. schoeniclus (a less widespread species which may feed within the crop but nests in semi-natural vegetation at field boundaries); and a wader Northern Lapwing Vanellus vanellus. 302 British Birds 95 June

4 Sky Lark Although Sky Larks are still widespread, their numbers have more than halved during the past 30 years with populations decreasing not only on farmland, but also in upland and coastal areas. In common with many declining species, the population trend of the Sky Lark closely matches changes in several key agricultural variables, with a particularly steep decline in the early 1980s. The 1997 Breeding Skylark Survey (Browne et al. 2000) provided detailed information on the habitat preferences of Sky Larks on farmland. The highest densities of Sky Larks occurred on spring-sown cereals and they usually avoided oilseed rape, silage fields and grazed pasture. The preference for spring cereals may be important since they have largely been replaced by winter-sown cereals (fig. 1). The latter are occupied by Sky Larks at a lower density than are spring cereals, especially later in the breeding season (from June onwards), when densities of territory-holding birds in winter cereals decline markedly, while those in spring-sown crops remain high. The cause of Fig. 1. Trends in selected agricultural management variables between 1962 and 1995, in England and Wales.The figures are adapted from Chamberlain et al. (2000), and include some interpolated data. Area (million ha) (a) Spring barley Dry matter production (thousand tonnes) (d) Silage production (b) Fallow 12 (e) Herbicide applications Area (million ha) Area sprayed (million ha) Number of animals (millions) (c) Sheep 0.00 Application rate (kg per ha) (f) Nitrogen applied to tilled land British Birds 95 June

5 189. Sky Lark Alauda arvensis, Norfolk, February this difference appears to be the growth stage of the crop vegetation. Sky Larks nest in crops which are less than about 30 cm tall, but rarely attempt to nest in taller crops. Crop density increases with crop height and this is also important because Sky Larks prefer to nest in more open crops. Winter cereals reach a height of 30 cm about a month earlier than spring cereals and Sky Larks are forced to abandon their territory before a second breeding attempt. Again, this may be associated with intensification. Modern farms have much larger fields and fewer crop types than they did three decades ago, so the opportunities for re-nesting are fewer. The evidence, therefore, points to a reduction in the length of the breeding season (as a result of the switch from spring to autumn sowing of cereals), as one of the main factors to have caused the Sky Lark decline on farmland. Robin Chittenden Robin Chittenden 190. Northern Lapwing Vanellus vanellus, Norfolk, October British Birds 95 June

6 Northern Lapwing The decline of the Northern Lapwing has perhaps been more noticeable to people who have been birding since the 1970s because it is now absent from many areas in southwest England and Wales. A national breeding survey in 1998 revealed a decline of almost 50% in just 11 years. On farmland, the highest densities of Northern Lapwings were on spring cereals that were adjacent to grass fields (Wilson et al. 2001). Grass fields seem to be a favourite feeding habitat for young Northern Lapwing chicks, whereas the species prefers to nest in cereals with relatively low vegetation (Galbraith 1988). Changes in the sowing times of cereals may have affected the population (nests in spring cereals, now a scarce habitat, tend to be more productive), but what may be more important is that the proportion of mixed farmland has declined and arable farms are now less likely to have grass fields than in the 1960s. In regions dominated by pasture, Northern Lapwings show a different habitat preference, preferring rough unimproved grassland and avoiding silage and heavily grazed grassland (Wilson et al. 2001). The intensification of grassland management in these areas, with the consequent reduction in rough grazing, is likely to have had a detrimental effect on the Northern Lapwing population. western Britain may be associated with the loss of mixed farming. A number of other species have also shown an association with arable pockets in pastoral landscapes (Robinson et al. 2001). There is no other evidence to suggest that the population size of Reed Buntings is being affected by factors operating during the breeding season. Rather, changes in both adult and juvenile survival between years, calculated from ring-recovery data, can explain the population decline (Peach et al. 1999). Outside the breeding season, Reed Buntings, in common with most granivorous passerines, prefer to feed on weed-rich stubbles. Stubble fields are largely associated with spring-sown crops, whereas winter cereals are preceded by only a short period of stubble, or perhaps none at all. The decline of spring-sown cereals in many areas has, therefore, also meant a decline in a key foraging habitat. Furthermore, herbicides are used with much greater frequency and efficiency in cereals, so the abundance of weeds in crops has declined, leading to fewer seeds available in Reed Bunting In the two previous cases, species-specific survey data has been used in conjunction with more general knowledge about these species ecology to make inferences about the likely mechanisms underpinning population declines. For most species, such detailed survey data are not available, but we can still gain a valuable insight into the possible cause of population changes by considering trends and habitat associations from regular long-term monitoring schemes. The Reed Bunting has declined by more than 50% on farmland since the early 1970s. Its range has also contracted, with local extinctions reported in a number of northern and western regions of Britain. In the breeding season, Reed Buntings prefer farmland with an arable component so that, like the Northern Lapwing, declines in 191. Reed Bunting Emberiza schoeniclus, location unknown, Eddie Dunne British Birds 95 June

7 Table 2. Components of agricultural change which potentially cause population changes in selected farmland bird species. Note that this table only considers evidence from monitoring surveys, and is not intended to be a definitive statement on potential key factors affecting population change.there is a great deal more evidence for the effects of agricultural change on bird populations from more intensive studies (e.g. Aebischer 2000). KEY: F Evidence or hypothesised link to population change in the literature. M Significant association in the expected direction from analysis of monitoring data. Autumn sowing = switch from spring- to autumn-sown cereals with an associated reduction in overwinter stubbles. Ley grass = reduction in rotations, involving a ley grass phase. Improvement of rough grazing = drainage, reseeding and increased nitrogen input. Hay to silage = switch from hay to silage, with associated earlier cutting of grass and reduction in seed availability. Livestock = increased stocking rates, especially of sheep. Manure = reduction in use of farmyard manure, with consequences for soil invertebrates. Artificial fertiliser = increase in use of synthetic fertiliser, with associated increase in density of vegetation. Pesticides = increase in applications and diversity of pesticides. Hedgerow = reduction in non-crop habitat, especially hedgerows. Crop diversity = reduction in crop diversity as a consequence of the increasing specialisation of farming enterprises, the loss of mixed farming and the simplification of rotational systems. Species Autumn Ley Improvement of Hay to silage Livestock Manure Artificial Pesticides Hedgerow Crop sowing grass rough grazing fertiliser diversity 1 Potential effect 1 Fw,N * Fa Fa Fa,N * N Fa N Fa N Fa,N Northern Lapwing Vanellus vanellus M M F M F F F M Turtle Dove Streptopelia turtur F Sky Lark Alauda arvensis M F F M F M Yellow Wagtail Motacilla flava F M Common Starling Sturnus vulgaris F F F Common Whitethroat Sylvia communis F Blackbird Turdus merula F F F F Song Thrush Turdus philomelos M F F F Tree Sparrow Passer montanus F F F M Greenfinch Carduelis chloris M F F F Goldfinch Carduelis carduelis M F F F Linnet Carduelis cannabina M F F F F Common Bullfinch Pyrrhula pyrrhula F F F Corn Bunting Miliaria calandra M F F M Reed Bunting Emberiza schoeniclus M F F F F M Yellowhammer Emberiza citrinella F F F F M Fa = food (all year), Fw = food (outside breeding season), N = nesting habitat, N * = nesting habitat for Northern Lapwing and Skylark only 306 British Birds 95 June

8 Ian Henderson 192. Rape stubble, Colworth Park, Bedfordshire, April Stubbles such as this, which remain over winter, provide a good foraging habitat for a range of farmland species. those stubbles which do occur. A final factor may be that less grain is spilt due to increased harvesting efficiency. Loss of good feeding habitats may therefore have decreased over winter survival in this species. This loss of winter feeding habitat may be important for a number of other granivorous species, as shown by lower survival rates (Siriwardena et al. 1999) and a preference for cereal stubbles (e.g. Gillings & Fuller 2001). Discussion The three examples discussed here do not cover all the potential factors underlying population declines in these species, but they illustrate how we can use the BTO s long-term national datasets to home in on the problem. Such analyses have been undertaken for a wide range of species, and several of these have shown population declines which closely match certain changes in agricultural practices (table 2). There is strong evidence for the probable underlying causes of decline in several species. The decrease in spring-sown cereals and consequent replacement by winter cereals has caused the loss of high-quality nesting habitat (affecting Sky Lark and Northern Lapwing), and the loss of important feeding habitat outside the breeding season (granivores); the increasing use of fertiliser has increased crop British Birds 95 June growth rates and so made crop structure less suitable for Sky Lark; the decline in rough grazing has also had effects on nesting habitat (Northern Lapwing) and removed invertebraterich habitats (Song Thrush) when it has been replaced with intensively managed grassland; the increase in livestock density has affected ground-nesting species (Northern Lapwing and Yellow Wagtail Motacilla flava); and reductions in crop diversity and mixed farming have led to a decrease in suitable breeding habitat for Northern Lapwing, Sky Lark, Tree Sparrow Passer montanus, Corn Bunting, Reed Bunting and Yellowhammer (the last four species in pastoral landscapes). It is important to note that a close correlation between two variables does not necessarily mean that they are causally linked, but for some of these species (e.g. Sky Lark, Northern Lapwing, Corn Bunting) many of these associations have been supported by small-scale intensive studies, while research is ongoing for a number of others (e.g. Linnet, Tree Sparrow, Yellowhammer). An important message from table 2 is that a range of different factors may be affecting bird populations in different ways, so that, rather than targeting specific management practices, a more general extensification of agriculture, where a suite of appropriate practices are implemented, may be the best way to benefit a 307

9 range of farmland birds. There are a number of ways in which our farmed land may be made more bird friendly. These range from creating small open patches within arable fields to managing whole fields (e.g. stubble management) or even whole farms (e.g. rotation management) in environmentally sensitive ways, and may be achieved within agri-environment schemes or as part of novel farming techniques such as Integrated Crop Management (table 3). Apart from management practices tailored to the requirements of individual species, there is little evidence that any of these have influenced population trends in isolation, but a combination of measures that affect both cropped and noncropped areas may be more beneficial to the whole farmland bird community, at least on a local scale (Stoate 2001). Table 3. Agri-environment schemes and management techniques which have the potential to increase bird populations. Note that caveats involving economic constraints are not dealt with in this table. Organic Farming Scale: Whole farm. Characteristics: Specific features include rotations incorporating grass leys and legumes, reliance on animal and green manures rather than synthetic fertilisers, with no use of synthetic pesticides. Potential benefits: There is evidence that a range of species occur at higher densities on organic farmland, mostly in autumn and winter. Caveats: Despite recent government incentives and an increasing organic market, this is likely to remain a minority farming technique in the UK. Key reference: Chamberlain et al. (1999). Integrated Crop Management (ICM) Scale: Whole farm. Characteristics: A combination of farming practices which are designed to balance the economic production of crops, through the application of rotations, cultivations, choice of seed variety and judicious use of crop production inputs, with measures to preserve and protect the environment (i.e. minimising pesticide and energy inputs). Potential benefits: Lowered pesticide inputs and the use of traditional rotations have potential benefits for several species. Caveats: There are no clear-cut definitions of ICM and so the spectrum of farm types is broad. To date, there is no field research on the value of ICM to farmland birds. Key reference: LEAF (1995). Conservation headlands Scale: Field margin. Characteristics: A wide margin in arable crops which is left pesticide-free. Potential benefits: Increased reproductive success of Grey Partridge Perdix perdix. Benefits to non-game birds are also likely. Key reference: Potts (1986). Game cover strips Scale: Field margin. Characteristics: Strips planted with mixtures of seed-rich plants (e.g. sunflowers, quinoa, millet) to encourage gamebirds. Potential benefits: Provides important foraging habitat for a range of species in winter. Key reference: Henderson & Vickery (2001). Countryside/Arable Stewardship Scale: Field margin/field. Characteristics: Margin management includes: grass margins (grass-only strips, grass/wildflower strips); naturally regenerated set-aside margins; uncropped wildlife strips; game cover crops; and conservation headlands. Some options may be taken up at a whole- or part-field level. Potential benefits: Likely to have similar benefits to those of conservation headlands and game cover strips. Caveats: A new scheme, the benefits of which are unlikely to be detected for several years. Key reference: MAFF (1998; 1999). Set-aside Scale: Field. Characteristics: Arable land left fallow, either as a stubble (rotational set-aside) or grass (non-rotational set-aside). Potential benefits: Preference for set-aside (especially rotational) shown by several species in both summer and winter. Caveats: Little evidence for an effect on population trends (may have slowed the population decline of some species) despite widespread and relatively long-term introduction. Key reference: Henderson et al. (2000). Species-specific management Scale: Variable, but usually field/margin. Characteristics: Management is tailored to the needs of individual species. Examples include the provision of overwinter stubble for Cirl Buntings Emberiza cirlus, adopting mowing regimes to minimise Corn Crake Crex crex mortality, and creating bare patches in winter cereals for nesting Sky Larks Alauda arvensis. Potential benefits: Uses detailed knowledge of species ecology to improve habitats and has shown impressive results for Cirl Bunting, Corn Crake and Stone-curlew Burhinus oedicnemus. Caveats: Species-specific, so may not benefit whole farmland bird community. Has mostly been used for rare species. Key reference: Aebischer et al. (2000). 308 British Birds 95 June

10 The Future While a consideration of historical changes in agricultural management has been useful in understanding more about the factors affecting bird populations, it is not necessarily the best premise on which to base current conservation practice. Times have changed, and a widespread reversion to farming practices of 40 years ago is not a realistic, nor necessarily desirable, option. Furthermore, there is no reason to suspect that those factors which originally caused populations to decline are those which are currently affecting population trends the most. What is needed now is the testing of solutions through novel management techniques. There is also a need to focus on those aspects of bird species life-cycles which are less well understood, for example post-fledging survival, length of breeding season, over winter survival. In this respect, targeted studies are needed in relation to experimental management techniques. We are currently at a crucial stage for farmland birds in Britain. It is quite clear that agricultural intensification has been a major factor in the decline of so many species, but it has only been in the last few years that any steps have been taken to remedy the situation. There is good news in the fact that the Government is taking these changes seriously and that funding for agri-environment schemes has been substantially increased. In 2000, the Government made an important commitment to the conservation of farmland birds by adopting a Public Service Agreement (PSA) target to reverse the long-term decline in the Farmland Bird Index (the population trends of 20 farmland specialists shown in table 1, with the exception of House Sparrow) by the year 2020 (statistical reseach is currently being undertaken to establish the precise magnitude of population increase which will constitute this target). This PSA should mark an important turning point in the fortunes of many declining farmland birds. It is an ambitious target given the scale of the declines and the widespread distribution of many of the species involved. But how realistic is this target? For some of our rarest farmland birds (Corn Crake, Stone-curlew, Cirl Bunting), great progress has undoubtedly been made (Aebischer et al. 2000). For tackling the widespread declines of Farmland Bird Index species, however, the situation is less hopeful. Many species continue to decline (e.g. Grey Partridge Perdix perdix, Turtle Dove, Sky Lark, Corn British Birds 95 June Bunting, Reed Bunting, Yellowhammer) despite the increase in environmental schemes and the introduction of set-aside. The latter is particularly interesting because set-aside was introduced on a large scale and is clearly a preferred habitat for several declining species, yet population declines do not appear to have been halted, merely slowed. If even set-aside could not reverse the declines, then we are likely to need major changes in the way farmland is managed on a large scale if we want to increase biodiversity. A range of conservation measures (table 3), incorporating not only agri-environment schemes but also widespread initiatives to reduce the intensity of farm management on a large scale, may achieve this. Acknowledgments We are most grateful to the many volunteer ornithologists who have contributed immeasurably to bird conservation by taking part in monitoring surveys. Many people have been involved in the work cited in this paper and we would like to thank all staff at the BTO who have in some way contributed. We would also like to thank Dr Jeremy Greenwood and Dr Ian Henderson for some valuable comments on an early draft of the manuscript. References Aebischer, N. J., Green, R. E., & Evans, A. D From science to recovery: four case studies of how research has been translated into conservation action in the UK. 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