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Abstract

  1. Top of page
  2. Abstract
  3. ARE CHANGES IN CROP STRUCTURE A CHARACTERISTIC OF AGRICULTURAL INTENSIFICATION?
  4. HOW DOES VEGETATION STRUCTURE AFFECT BIRDS?
  5. VEGETATION STRUCTURE: A GENERAL APPROACH TO REVERSING THE EFFECTS OF AGRICULTURAL INTENSIFICATION ON BIRDS?
  6. Acknowledgments
  7. REFERENCES

Agricultural intensification is accepted widely as a cause of bird population declines on farmland in Europe and North America. Although intensification is multivariate, one common theme is the impact on variation in crop structure, both within and between fields. Intensification creates simpler, more homogeneous and denser swards in both tillage crops and grassland. This influences predation risk, exposure to weather extremes and the diversity, abundance and accessibility of food. Birds trade off these pressures in different ways, so that the more uniform and dense the vegetation, the fewer the number of birds and range of species that are able to nest and forage successfully. Reversing recent trends towards dense, simplified and homogeneous swards will improve nesting and foraging habitat conditions for a wide range of species across farming systems, and may represent a cost-effective mechanism for the further improvement of agri-environment scheme options designed to assist the recovery of farmland bird populations.

Population declines in many farmland bird species have been documented during the last quarter of the 20th century in parts of Europe and North America where agricultural intensification has been most marked (e.g. Donald et al. 2001, Freemark & Kirk 2001). Agricultural habitats now support more bird species of European conservation concern than any other broad habitat type (Tucker & Evans 1997). These declines have not only affected birds; declines in the abundance and species diversity of mammals (Flowerdew 1997), arthropods and flowering plants (Sotherton & Self 2000) on farmland have also been severe.

There is now widespread consensus that agricultural intensification is either directly or indirectly the main cause of this widespread loss of biodiversity (e.g. Pain & Pienkowski 1997, Krebs et al. 1999, Aebischer et al. 2000, Robinson & Sutherland 2002, Grice et al. 2004, Newton 2004, Vickery et al. 2004). ‘Intensification’ is multivariate and difficult to define precisely (Chamberlain et al. 2000), but here we take it to subsume all those advances in crop and animal breeding, husbandry, nutrition, pest and disease control and harvesting methods (examples in Table 1) that have allowed farmers to increase primary productivity, and the proportion of it that is converted into harvested agricultural product (Krebs et al. 1999).

Table 1.  Elements of agricultural intensification (from Fuller 2000).
Land drainage
Hedgerow removal and mechanization of management
Earlier and more efficient harvesting
Re-seeding and fertilization of grassland
Increased pesticide use
Change from spring to autumn sowing
Multiple silage harvesting of forage grasses
Simplification of crop rotations
Increased livestock densities

The effects of agricultural intensification on many bird species have been studied in detail (Fuller 2000, Newton 2004). For example, the decline of Corn Crakes Crex crex has been caused because grassland improvement has permitted earlier and more frequent cutting of the meadows in which the birds nest, and the impact of increased pesticide use on arable land has been a key cause of Grey Partridge Perdix perdix decline through its impact on the availability of insect foods to chicks, and hence on reproductive success (Aebischer et al. 2000). Although this research effort has yielded some notable conservation successes in the UK (Table 2) through the development of conservation management measures within agri-environment schemes (Swash et al. 2000, Evans et al. 2002, Smallshire et al. 2004), national bird population recovery has only yet been achieved by targeting of specific management recommendations for species with restricted geographical ranges (Aebischer et al. 2000, Peach et al. 2001). Widespread species such as Grey Partridge and Sky Lark Alauda arvensis have remained in long-term decline (Gregory et al. 2004).

Table 2.  Examples of declining farmland bird species in the UK where research has led to successful conservation action (from Aebischer et al. 2000).
 Key environmental change linked to distribution or population changeProbable demographic mechanismRemedial action
Grey PartridgePerdix perdixIndirect effect of increasing pesticide use on insect food supply for chicks; loss of nesting cover; increasing predation pressureReduced survival of eggs, chicks and incubating femalesReduction in pesticide use in cereal headlands; retention of uncut field margins; predator control
Corn CrakeCrex crexDestruction of nests and young by harvesting of forage grassesReduced survival of eggs and chicksRetention of late-cut hay meadows, harvested from the centre of the field to the outside to allow escape of young
Stone-curlewBurhinus oedicnemusLoss of sparsely vegetated areas needed for nesting habitat due to lack of grazing on heaths and downland, and loss of spring- cultivated arable landFewer nesting opportunitiesSpring-sowing of arable crops; unsown nesting ‘scrapes’ in early-sown fields; protection of nests from field operations in spring
Cirl Bunting Emberiza CirlusImpact of increased herbicide use and autumn sowing on availability and seed food content of winter stubbles; effect of grassland improvement on availability of Orthoptera as key nesting food sourceReduced survival rates and nesting successRetain unsprayed winter stubbles; recreate rough grass field margins to provide invertebrate food source for nestlings

Cost-effective management to reverse losses of these more widespread species will be assisted if it is possible to identify measures that are beneficial across species and farming systems. Here, we review the relationship between agricultural practice and crop structure (height and density and their interactions with the specific plant architecture of the crop species) and the effects of differing crop structure on nesting and foraging birds. On the basis of this review, we argue that directional changes in crop structure are characteristic of agricultural intensification, that these changes have had pervasive and largely detrimental effects on bird populations and that management of the physical structure of crop swards offers cost-effective opportunities to help to reverse these effects for many bird species across a wide range of farming systems.

ARE CHANGES IN CROP STRUCTURE A CHARACTERISTIC OF AGRICULTURAL INTENSIFICATION?

  1. Top of page
  2. Abstract
  3. ARE CHANGES IN CROP STRUCTURE A CHARACTERISTIC OF AGRICULTURAL INTENSIFICATION?
  4. HOW DOES VEGETATION STRUCTURE AFFECT BIRDS?
  5. VEGETATION STRUCTURE: A GENERAL APPROACH TO REVERSING THE EFFECTS OF AGRICULTURAL INTENSIFICATION ON BIRDS?
  6. Acknowledgments
  7. REFERENCES

Agricultural crop swards consist typically of grasses (e.g. forage grasses and cereal crops), broad-leaved crops and, on unenclosed grazing land, mixes of grasses with woody shrubs. Intensification has had three general effects on these swards. It has increased their density (i.e. mass of vegetation per unit area prior to any grazing or harvesting impacts) and it has simplified and homogenized sward structure and architecture both directly and by reduction in the species diversity of swards. Mechanized, uniform sowing, agrochemical use, drainage, efficient harvesting and seed cleaning, re-seeding and increases in grazing and cutting intensity on grasslands, and rolling of tillage crops have all contributed, as illustrated in Table 3.

Table 3.  Aspects of agricultural intensification causing simplification and homogenization of sward structure in the UK(from Fuller 1987, Carter & Stansfield 1994, Fuller & Gough 1999, Hopkins 2000, Morris 2000, Vickery et al. 2001, Weiner et al. 2001, Robinson & Sutherland 2002, McCracken & Tallowin 2004).
Agricultural changeEffects
All fields
 Mechanization and increased  efficiency of field operationsMore uniform swards due to mechanized sowing by broadcast or in rows. More fields in the same state at any one time due to speed of field operations
Reduced incidence of crop failureAgrochemical nutrition and protection of crops and grassland increases uniformity of establishment and subsequent growth
Grasslands
 ‘Improvement’ by drainage, re-seeding with favoured forage grasses, fertilizer and herbicide useReduced species diversity by killing weeds, re-seeding with palatable, competitive species, and favouring these species through drainage and fertilizer use
 Increased duration and density of  grazing on improved fieldsReduced height and structural heterogeneity due to higher grazing intensity and lack of unpalatable species
 Increased frequency of harvesting  forage grasses on improved fieldsReduced species diversity due to impacts on slower-growing/seeding species
Tilled fields
 More efficient seed cleaning and  harvestingReduced species diversity as fewer weeds introduced at sowing, and left behind at harvest
 Increased herbicide useReduced species diversity by direct killing of weeds
 Increased fertilizer useReduced species diversity due to shading out of less-competitive weed species
 RollingGreater uniformity of crop growth
 Abandonment of undersowingLoss of structural diversity
 Crop breeding advancesIncreased competitive ability of crop relative to weed species
 Improved under-soil drainageGreater uniformity of crop growth
Unenclosed uplands
 Increased intensity and annual  duration of sheep grazing pressureGeneral trend towards replacement of shrub covers with grass covers as long-term grazing pressure increases. Reduced height and structural heterogeneity of grass covers at high grazing intensities

Increase in vegetation density on agricultural land is most apparent on grasslands where the whole standing biomass is cropped. Grassland improvement has allowed meadows and pastures to produce progressively greater yields, thus allowing more frequent cutting of forage crops and higher grazing intensities (Hopkins 2000). In tillage crops, where the harvested food product is usually just a part of the plant (e.g. the seeds, root or flowering head), the changes in total biomass may be less straightforward, but there are clear trends in crop structure. For example, cereal yield has increased throughout the 20th century because of advances in crop breeding, independently of any change in above-ground dry-matter production per plant (e.g. Austin et al. 1980, 1993). This has occurred because these advances have combined increased grain yield for a given level of plant nutrition with shorter, stiffer stems which reduce the likelihood of lodging (flattening of the mature crop in rain and wind) and allow the farmer to sow at a higher seed rate. However, nitrogen fertilization has also had a substantial additional effect in increasing above-ground biomass at maturity (Austin et al. 1993). The end result is that an intensively managed, mature cereal crop today has a higher density of stiffer, thicker stems than a crop grown 50 years ago (see Stamp 1955, Plate 10).

HOW DOES VEGETATION STRUCTURE AFFECT BIRDS?

  1. Top of page
  2. Abstract
  3. ARE CHANGES IN CROP STRUCTURE A CHARACTERISTIC OF AGRICULTURAL INTENSIFICATION?
  4. HOW DOES VEGETATION STRUCTURE AFFECT BIRDS?
  5. VEGETATION STRUCTURE: A GENERAL APPROACH TO REVERSING THE EFFECTS OF AGRICULTURAL INTENSIFICATION ON BIRDS?
  6. Acknowledgments
  7. REFERENCES

Crop structure has three key, interacting influences on birds. It may conceal a bird from a predator or vice versa (Elgar 1989, Lima & Dill 1990), it may protect a bird from, or expose it to, extremes of weather (Walsberg 1985), and it influences the diversity, abundance and accessibility of food items available at a given location. Each of these three influences is considered below in relation to both nesting and foraging requirements of birds exploiting agricultural swards.

Nesting

Two often inversely correlated properties of a nest-site amongst ground vegetation determine the risk of nest failure: its protection from weather or visually hunting predators and the view of the surroundings afforded to parent birds at the nest (Gotmark et al. 1995). Different bird species trade off the needs for protection and detection in different ways. For example, many studies of ground-nesting species have shown strong positive correlations between nest success rates and a variety of measures of vegetation cover and heterogeneity (e.g. Hines & Mitchell 1983, Rands 1986, Riley et al. 1992, Green & Stowe 1993, Norment 1993, DeLong et al. 1995). Even so, when preference for dense cover conflicts with a sward's value as a commercial crop, nest destruction by agricultural operations may cause severe population decline as has occurred, for example, in populations of Corn Crakes of Yellow Wagtails Motacilla flava and Whinchats Saxicola rubetra nesting in hay and silage meadows (Stowe et al. 1993, Court et al. 2001, Muller et al. 2005). Although concealing vegetation may generally be beneficial for prey of visual predators such as corvids, it may also increase predation risk for prey of olfactory predators such as mammals. For example, Wiebe and Martin (1998) showed that although concealed nests of White-tailed Ptarmigan Lagopus leucurus were less likely to be detected by predators, when a concealed nest was detected the incubating female was more likely to be killed. Similarly, species able to lure away or deter predators from visiting nests by the use of distraction displays or communal nest defence may benefit from nesting in less concealing vegetation. Thus, on agricultural land in the UK, Stone-curlews Burhinus oedicnemus, Northern Lapwings Vanellus vanellus, Golden Plovers Pluvialis apricaria and Sky Larks − all with well-developed antipredator behaviours (Delius 1963, Cramp & Simmons 1983) − select flat, open, relatively sparsely vegetated ground on which to nest (e.g. Galbraith 1988, Wilson et al. 1997, Wakeham-Dawson et al. 1998, Green et al. 2000, Whittingham et al. 2002). Stone-curlews in particular have excellent all-round vision in the horizontal plane and below the bill, but almost no vision above the head (Martin & Katzir 1994). This species is therefore particularly sensitive to vertical growth of vegetation around the nest, preferring very sparsely vegetated ground when selecting a nest site (Green et al. 2000).

Effects of weather may also mean that nest-sites are selected for their vegetation characteristics (Walsberg 1985, With & Webb 1993, Gloutney & Clark 1997, Hoekman et al. 2002). Typically, birds place nests with respect to nearby vegetation so as to ameliorate the impacts of extremes of insolation, wind and precipitation, and thereby reduce physiological stresses on adults, eggs and young. For example, With and Webb (1993) found that three breeding bird species of prairie grasslands in North America displayed different nest placement strategies as a result of their differing responses to insolation. McCown's Longspurs Calcarius mccownii and Horned Larks Eremophila alpestris nested early and were associated with sparse vegetation cover, while later-nesting Lark Buntings Calamospiza melanocorys were associated with overhanging shrubs and tussocky grasses and were less exposed. The authors suggest that these differences reflect the benefits of warmth for early nesting species, whereas for the later-nesting species there is a significant risk of overheating. In some cases, weather and predation constraints interact. For example, Marzluff (1988) demonstrated opposing selection pressures on nest placement in Pinyon Jays Gymnorhinus cyanocephalus with temperature conditions favouring exposed nests, but predation risk favouring concealed sites.

Overall, it is clear that preferred height, density and architectural diversity of vegetation differs between ground-nesting species. For example, Stone-curlews, Northern Lapwings and Sky Larks nesting on arable land all select vegetation cover for nesting that is ‘short’ and ‘sparse’ relative to that which is now typical for crops in the spring in the UK. However, Stone-curlews select nest-sites with vegetation preferably less than 5 cm and not more than 10 cm tall, and with sparse (< 10%) ground cover (Green et al. 2000), while Sky Larks nest typically in vegetation between 20 and 60 cm tall, with ground cover anywhere between 20 and 100% (Wilson et al. 1997). None the less, increases in the height, density and homogeneity of crop vegetation during the breeding season have played a significant part in the national population declines of all three of the above species in the UK. They have done this by limiting the availability of nesting opportunities spatially, with fewer spring fallows and late-sown crops, and temporally, with opportunities for later nesting attempts being especially limited. There are similar examples from other habitats. Norris et al. (1997) found the highest densities of saltmarsh-nesting Common Redshank Tringa totanus where moderate densities of grazing cattle created swards of great height diversity, with tussocks of Sea Couch Grass Elymus pycnanthus as suitable nest-sites. Conversely, salt-marshes dominated by intensive sheep grazing tend to have uniform grass swards unattractive to Common Redshank, and a long-term trend in this direction has probably contributed to the national decline of this species in the UK (Norris et al. 1998). Similarly, Delany and Linda (1994) suggest that the mechanism for the decline of the Florida Grasshopper Sparrow Ammodramus savannarum floridanus following conversion of dry prairie grasslands to improved pasture is that pastures do not offer a heterogeneous sward structure combining areas of bare earth for foraging with adequate vegetative cover for nesting. In upland habitats, Baines (1996) found that the breeding densities and nesting success of Black Grouse Tetrao tetrix, another species in long-term decline, were higher on lightly, rather than heavily, sheep-grazed moors in England. He attributed this to the protection from predation afforded by vegetative cover and high abundances of preferred insects on moors with low numbers of large grazing mammals. This case illustrates that management of swards to benefit one declining bird species may disadvantage others because, although Black Grouse densities responded positively to grazing reductions, numbers of breeding Northern Lapwings and Grey Partridges in the same areas fell. It seems that the use of grazing to generate vegetation mosaics is essential to achieving favourable vegetation management for all three species (Baines et al. 2002).

Foraging

Vegetation may affect the efficiency of foraging both directly, through its effects on the detectability and accessibility of food items, and indirectly through its impact on the trade-off between time allocated to feeding and time allocated to vigilance for predators. Direct effects include physical obstruction to the movement of foraging birds (e.g. Hilden 1965, Brodmann et al. 1997), additional energetic demands via chilling effects when vegetation is cold or wet (Dawson et al. 1992) or reduced detectability and accessibility of food items in dense swards (e.g. Nystrand & Granström 1997, Whittingham & Markland 2002, Butler & Gillings 2004). In an agricultural context, Odderskaer et al. (1997) found that Sky Larks foraging in barley fields favoured tractor wheelings (‘tramlines’) and experimental unsown plots within the field rather than the more densely vegetated main crop area, despite the fact that the absolute density of arthropod food items was higher in the crop than in these more sparsely vegetated areas. Perkins et al. (2002) obtained similar results for Yellowhammers Emberiza citrinella feeding in grass margins of arable fields, and Moorcroft et al. (2002) showed that both seed abundance and availability of bare ground on which to forage determined occupancy of winter stubble fields by Linnets Carduelis cannabina and other granivorous passerines. In grassland, Perkins et al. (2000), Barnett et al. (2004) and Atkinson et al. (2004) showed that grass fields are more likely to be used by foraging birds where they have sparser swards and more bare ground. Moreover, improved accessibility of food in sparse or heterogeneous swards may be complemented by the fact that structurally heterogeneous swards, especially where these are also species diverse, are associated typically with greater abundance and diversity of invertebrate and seed sources (e.g. Lawton 1983, Haysom & Coulson 1998, Morris 2000, Vickery et al. 2001, Kruess & Tscharntke 2002, McCracken & Tallowin 2004).

As with nesting, the trade-off between the benefit of vegetation as cover from predators and its cost as visual obstruction to the detection of predators will differ for species with different perceptual abilities, foraging techniques and escape responses. In some cases (e.g. many Galliformes), vegetation cover and foraging habitat are one and the same and the birds rely on the combination of vegetation cover and their own crypsis to avoid detection. In others, the birds may forage on the ground in herbaceous swards, but use nearby shrubs or trees for cover rather than relying on the sward itself (e.g. Vickery et al. 2002). Other species rely on escape behaviours (e.g. Cresswell 1994) and select foraging sites with unobstructed fields of view, often flocking to increase the probability of predator detection, and to reduce the probability of predation via dilution and confusion effects (Elgar 1989). For example, a recent study of Chaffinches Fringilla coelebs feeding on seed in experimental stubbles of differing length (13 cm vs. 3 cm) found that in the longer stubble, vigilance was increased by 13% and peck rate reduced by 13%, and yet there was a 24% slower escape response to a predator − a stuffed Eurasian Sparrowhawk Accipiter nisus‘flown’ nearby (Whittingham et al. 2004). In a further experiment, seed density in the long stubble had to be raised to approximately 2.5 times that in the short stubble before parity of use of the two stubble types by foraging Chaffinches was achieved (Butler et al. 2005). Similar studies in grassland have also shown that foraging Common Starlings Sturnus vulgaris and Northern Lapwings tend to select shorter grassland swards when foraging (Devereux et al. 2004).

These experimental studies help to explain why a wide taxonomic range of ground-feeding birds on agricultural land have been shown to forage preferentially in sites that are sparsely or heterogeneously vegetated, across a wide range of agricultural swards. These include studies of Sky Larks and other passerines on arable land (e.g. Diaz & Telleria 1994, Odderskaer et al. 1997, Schön 1999, Buckingham 2001, Anthes et al. 2002, Moorcroft et al. 2002, Morris et al. 2002, Perkins et al. 2002), geese, waders and passerines on grassland (Eiserer 1980, Whitehead et al. 1995, Colwell & Dodd 1997, Milsom et al. 1998, Vickery & Gill 1999, Perkins et al. 2000, Johnstone et al. 2002, Barnett et al. 2004, Bradbury & Bradter 2004, Devereux et al. 2004), and waders and passerines on heathland (Bowden 1990, Whittingham et al. 2000). Studies of aerial hunters such as kestrels Falco tinnunculus and F. sparverius (Shrubb 1980, Toland 1987) have also shown that foraging activity and hunting success are higher over less densely vegetated habitats, although other species adapt behaviourally to hunting in vegetation of different structure. For example, although Loggerhead Shrikes Lanius ludovicianus prefer to hunt using perch-and-wait tactics over short vegetation, Yosef and Grubb (1993) found that the birds’ hunting frequency and success over high vegetation could be maintained (perhaps at greater energetic cost) by switching to hunting by hovering and aerial chases.

VEGETATION STRUCTURE: A GENERAL APPROACH TO REVERSING THE EFFECTS OF AGRICULTURAL INTENSIFICATION ON BIRDS?

  1. Top of page
  2. Abstract
  3. ARE CHANGES IN CROP STRUCTURE A CHARACTERISTIC OF AGRICULTURAL INTENSIFICATION?
  4. HOW DOES VEGETATION STRUCTURE AFFECT BIRDS?
  5. VEGETATION STRUCTURE: A GENERAL APPROACH TO REVERSING THE EFFECTS OF AGRICULTURAL INTENSIFICATION ON BIRDS?
  6. Acknowledgments
  7. REFERENCES

The physical structure of crop swards is clearly an important determinant of the value of tillage crops and agricultural grassland as nesting and feeding habitat for terrestrial birds. Structural heterogeneity of herbaceous ground cover is more likely than uniform and dense cover to meet the needs of birds with differing food, antipredator behaviours, microclimate preferences and lengths of breeding season. For example, in the UK a recent review of the effects of sward height on foraging behaviour for the 20 species included in the ‘Farmland Bird Index’ (a UK government biodiversity indicator of quality of life) suggested that 15 are likely to benefit from shorter swards in terms of both foraging efficiency and improved detection of predators (Whittingham & Evans 2004).

Benton et al. (2003) argued that loss of ecological heterogeneity across multiple spatial and temporal scales was a universal consequence of multivariate agricultural intensification, and suggested that management solutions that re-create heterogeneity are key to restoring biodiversity in temperate agricultural systems. Management options within UK agri-environment schemes that are targeted at bird conservation tend to re-create heterogeneity at large scales (e.g. between fields and farms), and between cropped and uncropped habitats, rather than creating it within cropped fields. For example, of 17 options made available recently under the pilot Arable Stewardship Scheme in England (Bradbury & Allen 2003), 10 are concerned with the provision of over-winter stubbles, spring cereals, and ley grassland. The remainder (grass field margins, beetle banks, uncropped wildlife strips, wildlife seed mixtures, plus three options for limited inputs on field headlands) all aim to manage for richer biodiversity at the edges of fields (or as a raised grass strip through the field centre in the case of beetle banks), to improve protection against crop pests through encouraging populations of natural enemies, and to provide more abundant invertebrate and seed food resources for birds. On the basis of this review we suggest that more explicit attention to the potential benefits of managing to create structural heterogeneity within these management options, and within tillage and grass crops, may offer opportunities to improve further their cost-effectiveness for biodiversity. This will probably require greater agri-environment scheme financial support to account for the greater complexity of management required. In particular, we suggest that studies of the potential benefits of manipulating the physical structure of over-winter stubbles (e.g. through cutting, light grazing or light cultivation), grassland (through variation in fertilizer, flooding and grazing regimes), and cereal crops (see below) would be valuable. An excellent example of this approach is provided by the ‘Sustainable Arable Farming for an Improved Environment’ (SAFFIE) research project (http://www.saffie.info). This study has shown that providing undrilled patches in winter wheat crops or sowing the crops at wider row spacing can allow winter wheat fields to support higher densities of successfully nesting and foraging Sky Larks for a higher proportion of the breeding season, at very low cost to the farmer, mitigating the known impacts of tall, dense cereal growth on this species (Morris et al. 2004). Options such as these ‘Sky Lark patches’ are being included in the new Environmental Stewardship Scheme in England.

We anticipate that general beneficial effects across species and farming systems may be achieved by management of in-field vegetation structure, set alongside established measures designed primarily to restore bird food resources on farmland. This approach also has the potential to foster close co-operation between stakeholders simply because many husbandry practices (e.g. agrochemical usage, cultivation, grazing regimes, choice of crops, varieties and seed mixes, sowing practices) can be viewed as management tools to achieve desired structural effects. In the most general terms, management should strive to create structural heterogeneity in agricultural swards and to reverse the trend towards dense, simplified and homogeneous sward structures that have characterized recent agricultural intensification. This re-creation of structural heterogeneity will improve nesting and foraging habitat conditions for a wide range of ground-nesting and -feeding birds (Table 4).

Table 4.  Examples of farmland bird species in the UK where there is evidence that manipulation of sward structure and heterogeneity will yield conservation benefit. All these species have suffered population and/or range declines in the UK and are Red or Amber listed (Gregory et al. 2002).
SpeciesResponse to variation/manipulation of sward structureReference
Kestrel Falco tinnunculusForaging activity and hunting success are higher over less densely vegetated habitatsShrubb (1980)
Lapwing Vanellus vanellusLapwings nest on a wide range of farmland types including arable land, lowland wet grasslands and upland pastures. In all cases, however, short, sparsely vegetated ground created either by grazing or late-sowing of crops is preferred for nesting and foragingGalbraith (1988), Milsom et al. (2000), O’Brien (2002), Sheldon et al. (2004)
Stone-curlew Burhinus oedicnemusStone-curlews require bare or very sparsely vegetated ground in spring on which to nest. The creation of tilled plots within arable fields is a key part of management to restore breeding Stone-curlew populations in the UKAebischer et al. (2000)
Redshank Tringa totanusModerate densities of grazing cattle on saltmarshes create swards of high structural height diversity, including grass tussocks that are ideal nest-sites for Redshank. Heavy sheep grazing, in contrast, creates uniform, short swards that are unsuitable for nestingNorris et al. (1997)
Black Grouse Tetrao tetrixBreeding densities and nesting success of Black Grouse are higher on lightly than heavily sheep-grazed moors. This probably reflects protection from predation by vegetative cover, and higher abundances of preferred insects in the more complex vegetation structure of moors with low numbers of large grazing mammalsBaines (1996), Baines et al. (2002)
Sky Lark Alauda arvensisOn arable farmland, Sky Larks nest in crops between 20 and 60 cm tall, and are thus excluded from many mature, autumn-sown crops by the height and density of vegetation. Sky Larks also preferentially select relatively sparsely vegetated ground when foraging in arable fields both during the breeding season and in winterOdderskaer et al. (1997), Wilson et al. (1997), Donald & Vickery (2001)
Yellow Wagtail Motacilla flavaYellow Wagtails nest in dense crops, but in arable crops nests in crops that grow > 1 m tall (e.g. autumn-sown oilseed rape) are abandoned. Sparsely vegetated ground is selected for foraging both in arable crops and grasslandStiebel (1997),Anthes et al. (2002), Bradbury & Bradter (2004)
Chough Pyrrhocorax pyrrhocoraxChoughs select short, grazed grassland swards when foraging within their territories during the breeding seasonJohnstone et al. (2002)
Starling Sturnus vulgarisHas greater feeding success in short swardsWhitehead et al. (1995), Devereux et al. (2004)
Linnet Carduelis cannabinaWhen foraging in stubble fields in winter, Linnets select areas that are both richer in preferred weed seeds, but also with greater areas of bare ground than randomly selected locations in the same fieldsMoorcroft et al. (2002)
Yellowhammer Emberiza citrinellaLocations within cereal fields and field margins selected by adult birds when collecting food for nestlings have shorter and sparser vegetation than randomly selected locations in the same habitat units. In winter, foraging Yellowhammers are associated with areas containing relatively high proportions of unvegetated ground within stubble fieldsMorris et al. (2002), Perkins et al. (2002), Moorcroft et al. (2002)
Reed Bunting E. schoeniclusIn winter, foraging Reed Buntings are associated with areas containing relatively high proportions of unvegetated ground within stubble fieldsMoorcroft et al. (2002)
Corn Bunting Emberiza calandraIn winter, foraging Corn Buntings are associated with areas containing relatively high proportions of unvegetated ground within stubble fieldsMoorcroft et al. (2002)

Acknowledgments

  1. Top of page
  2. Abstract
  3. ARE CHANGES IN CROP STRUCTURE A CHARACTERISTIC OF AGRICULTURAL INTENSIFICATION?
  4. HOW DOES VEGETATION STRUCTURE AFFECT BIRDS?
  5. VEGETATION STRUCTURE: A GENERAL APPROACH TO REVERSING THE EFFECTS OF AGRICULTURAL INTENSIFICATION ON BIRDS?
  6. Acknowledgments
  7. REFERENCES

We thank the Biotechnology and Biological Research Council for funding support, and Dr Simon Butler and an anonymous referee for very helpful comments on an earlier draft of the manuscript.

REFERENCES

  1. Top of page
  2. Abstract
  3. ARE CHANGES IN CROP STRUCTURE A CHARACTERISTIC OF AGRICULTURAL INTENSIFICATION?
  4. HOW DOES VEGETATION STRUCTURE AFFECT BIRDS?
  5. VEGETATION STRUCTURE: A GENERAL APPROACH TO REVERSING THE EFFECTS OF AGRICULTURAL INTENSIFICATION ON BIRDS?
  6. Acknowledgments
  7. REFERENCES
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  • Austin, R.B., Bingham, J., Blackwell, R.D., Evans, L.T., Ford, M.A., Morgan, C.L. & Taylor, M. 1980. Genetic improvements in winter wheat yields since 1900 and associated physiological changes. J. Agric. Sci. Cambridge 94: 675689.
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