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The potential value of managed cereal field margins as foraging habitats for farmland birds in the UK
Agriculture, Ecosystems and Environment 89 (2002) 41–52
The potential value of managed cereal field margins as foraging habitats for farmland birds in the UK Juliet Vickery∗ , Nick Carter, Robert J. Fuller British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK
Abstract Many farmland bird populations have exhibited marked declines in the last 20–30 years and there is growing evidence to link these declines to agricultural intensification. Field margin management is frequently proposed as a way of enhancing bird populations but there have been few attempts to assess the relative value of different management options. This paper aims to provide a preliminary assessment of the relative value of six different cereal field margin management practices in providing foraging habitats for farmland birds. It also briefly compares these with the benefits afforded by field and whole farm approaches such as set-aside and organic farming, to wildlife conservation on arable land. The field margins considered are: grass margins (separated into grass-only strips and grass/wildflower strips), naturally regenerated set-aside margins, uncropped wildlife strips, game cover crops and conservation headlands. Sympathetically managed field margins can provide a range of plant and invertebrate food resources for birds both in summer and winter. In general, the best winter food supplies (mainly seeds) will be provided by game cover crops and naturally regenerated rotational set-aside strips. The most abundant summer food supplies (invertebrates and seeds) will be provided by a diverse sward; grass/wildflower strips, uncropped wildlife strips and naturally regenerated rotational set-aside strips followed by conservation headlands. Field margin swards that are less diverse in terms of species composition and structural heterogeneity and that consequently support fewer invertebrates, can, nonetheless, provide higher quality foraging habitats for birds than an intensively managed crop up to the hedge base. A number of bird species such as yellowhammer (Emberiza citrinella) and tree sparrow (Passer montanus), prefer to forage in margins in winter and summer and many of the benefits of whole field approaches such as set-aside, overwinter stubbles and undersown cereals, could be gained from margins under the same management. Whole-field approaches are, however, required for boundary-avoiding species such as skylark (Alauda arvensis) and lapwing (Vanellus vanellus). Less is known about the use other birds make of margins, relative to their use of field centres in winter. Whole farm approaches such as organic farming, whilst being highly beneficial to birds, are likely to remain rather localised in the UK, whereas field margin management can be relatively easily incorporated into the farmed landscape on an extensive scale. © 2002 Published by Elsevier Science B.V. Keywords: Grass margins; Set-aside; Uncropped wildlife strips; Game cover; Conservation headland; Farming systems
1. Introduction Many farmland bird species have exhibited dramatic declines in numbers and marked range contractions ∗ Corresponding author. Tel.: +44-1842-7500-50; fax: +44-1842-7500-30. E-mail address: [email protected] (J. Vickery).
in the last 20–30 years (Fuller et al., 1995; Gibbons et al., 1993; Siriwardena et al., 1998). A number of them have been added to the red list of birds of conservation concern (Gibbons et al., 1996) or included in the UK’s biodiversity action plans (Anon., 1995a,b, 1998), identifying them as species requiring urgent action to halt, and ultimately reverse, their declines.
0167-8809/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 8 8 0 9 ( 0 1 ) 0 0 3 1 7 - 6
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An increasing body of evidence now exists to link the decline of these farmland birds to agricultural intensification, including changes in cropping practices (e.g. switches from spring to autumn sown cereals and loss of mixed farming), increased use of pesticides and inorganic fertilisers and loss of non-crop habitat (Fuller, 2000). There is also a growing acceptance that halting the declines of common and widespread birds can only be achieved through more sympathetic management of the wider countryside, particularly agricultural land. One of the consequences of arable agricultural intensification has been the removal and narrowing of field margins, defined as ‘strips of land lying between crops and the field boundary and extending for a limited distance into the crop’ (Anon., 1995a). In the remaining narrow margin, annual weeds such as barren brome (Bromus sterilis) and cleavers (Galium aparine) often replace the perennial flora (e.g. Dunkley and Boatman, 1994). The sympathetic management of field margins for wildlife is frequently proposed as a way of enhancing populations of farmland wildlife without significantly affecting the agricultural productivity (e.g. Smallshire and Cooke, 1999). As a result, some form of field margin management has been included as an option within most of the recent agri-environment schemes in the UK (e.g. countryside stewardship (MAFF, 1999); arable stewardship (MAFF, 1998); ESAs (MAFF, 1994; Dwyer, 1994)). The potential wildlife value of field margins is highlighted by the inclusion of cereal field margins as one of the first 14 key biodiversity habitats in the UK for which costed action plans were published (Anon., 1995a,b). The focus on margins of cereal fields, rather than other crops, is simply a reflection of the dominance of cereals in the British arable landscape. Assuming an average national field size of 12 ha, there would be approximately 400,000 km of cereal field edge in the UK. If all such boundaries included a 6 m managed margin, approximately 240,000 ha of land could be brought into sensitive management (Anon., 1995a,b). This equates to 5% of the UK’s arable area, a significant component of the farmed landscape, and one that could be incorporated extensively into the wider countryside. Appropriately managed, field margins provide nesting and feeding sites for gamebirds (Rands, 1985, 1986, 1988) and many ground feeding passerines (Parish et al., 1994;
Stoate, 1999; Bradbury and Stoate, in press), through supporting a diversity of arable plants (Wilson, 1994), and invertebrates (Moreby, 1994; Feber et al., 1995; Barker et al., 1997; Haysom, 1999). A number of different management options exist for field margins. However, despite their potential value for farmland birds, and the importance with which field margin options are considered in Britain and elsewhere in Europe (Kleijn et al., 1998; Marshall et al., 1994; Smallshire and Cooke, 1999), there have been few attempts to assess the relative value of the different option types. This paper provides a brief overview of the different options currently adopted in the UK and their potential value as foraging habitats for birds. In the following sections, six field margin management treatments are considered: grass margins, which include grass-only strips, grass/wildflower strips (elevated grass strips or beetle banks are not considered here since they are almost always established as mid-field strips rather than field margins); naturally regenerated set-aside margins; uncropped wildlife strips; game cover crops; and conservation headlands. Each one is defined and the resources they are likely to offer for farmland birds are outlined. These benefits are then compared with whole field and whole farm approaches to wildlife conservation such as set-aside, undersown cereal fields, retention of overwinter stubble and organic farming.
2. Types of cereal field margin and benefits to birds The suitability of different field margins as foraging habitat for farmland birds will be largely determined by the way in which their management influences the abundance of food and its accessibility (affected mainly by vegetation structure). A large number of bird species commonly inhabiting lowland farmland is likely to benefit from sympathetic management of field margins. These include seed and invertebrate feeders, passerines and gamebirds, residents and migrant species, several of which are red-listed as birds of conservation concern (Gibbons et al., 1996), e.g. grey partridge (Perdix perdix), turtle dove (Streptopelia turtor), song thrush (Turdus philomelos), tree sparrow, linnet (Carduelis cannabina), cirl bunting (Emberiza cirlus) and corn bunting (Miliaria calandra).
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There are several caveats in comparing the value of different field margin management practices for birds. The relative value of different options will vary with geographic location (reflecting soil type and seed bank), time since establishment and time of year (e.g. Marshall et al., 1994). For the purposes of this paper ‘ideal conditions’ are assumed (e.g. rich seed bank in the case of naturally regenerated margins and diverse seed mix in sown grass/wildflower strips) and compare treatments in the first 1–2 years after establishment. In addition, field margins as defined in this overview specifically exclude boundary features such as hedgerows and fencelines. It is important to note that the value of such margins will be heavily influenced by adjacent field boundary characteristics, e.g. through shading and as sources of certain invertebrates. Hedgerows are an extremely valuable habitat for birds providing food, shelter and nest cover (O’Connor, 1987). In general, bird species richness increases with overall size and shrub/tree species diversity of a hedgerow (e.g. Parish et al., 1994, 1995; Barr et al., 1995). Field margins adjacent to such hedges are likely to be used by, and so provide benefits for, a larger number of farmland birds. 2.1. Grass margins These may take the form of grass-only strips or grass and wild flower strips (subsequently referred to as grass/wildflower strips). These strips may be 2–6 m wide (Countryside Stewardship Scheme; MAFF, 1999) or 4–12 m wide (arable stewardship pilot scheme; MAFF, 1998) and established either through natural regeneration or sowing (MAFF, 1999). They may contain flowers or have them added at sowing (typically ox-eye daisy (Chrysanthemum leucanthemum), ribwort plantain (Plantago lanceolata), knapweeds (Centaurea spp.) and yarrow (Achillea millefolium)). They vary in grass length as a result of different species composition (short grass such as red fescue (Festuca rubra) or tall tussocky grass such as cocksfoot (Dactylis glomerata)) and management. Grass strips require little active management except some cutting to aid establishment and subsequently to prevent scrub encroachment. Grass-only sown strips help to protect hedgerow fauna and flora from farming operations such as pesticide and fertiliser applications and close ploughing,
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which could benefit bird species that feed or nest in hedgerow bottoms. They also allow year-round access for farmers to hedges enabling them to be trimmed late in the winter without damaging the adjacent crop. This could benefit birds, especially winter thrushes feeding on berries (Sparks and Martin, 1999). Reduced pesticide inputs will allow annual weeds such as chickweed (Stellaria species) and knotgrass (Polygonum species), to germinate. These can provide seed resources for birds (Wilson et al., 1999) and support a range of phytophagous insects (Morris and Webb, 1987). Grass seeds provided by grass-only strips are fed on by a range of species (Wilson et al., 1999) including starling (Sturnus vulgaris), dunnock (Prunella modularis) house sparrow (Passer domesticus), tree sparrow and yellowhammer (Cramp, 1988; Cramp and Perrins, 1994). Grass-only margins also provide suitable habitat for invertebrates such as elaterid and carabid beetles and tipulid larvae which are important dietary items for birds such as cirl and corn bunting (Evans, 1997a), grey partridge (Potts, 1996), yellowhammer (Emberiza citrinella) (Bradbury and Stoate, in press) and skylark (Alauda arvensis) (Donald, 1999). Compared with modern cereal fields, they support higher numbers of graminivorous sawfly larvae and other gamebird chick food insects such as plant bugs and hoppers and lepidopteran larvae (Barker and Reynolds, 1999). Compared with many other margin types, however, grass-only strips tend to support a low variety of broad-leaved weeds which may lead to a relatively low abundance and diversity of invertebrates (e.g. Morris, 2000; Vickery et al., in press). Furthermore, grass-only strips may form a tall dense sward that hampers foraging by birds (Weibel, 1998; Barker and Reynolds, 1999). Grass/wildflower strips, whether sown or naturally regenerated, are more likely to provide food for birds over a longer period of the year than grass-only swards, as different plants will flower and seed at different times. Most grasses flower in mid-summer, while some dicotyledonous species will flower before or after this period (Clapham et al., 1968; Fitter et al., 1980). In addition, grass/wildflower strips are more likely to support a greater range of invertebrate species than grass-only strips, as they provide a greater variety of host plants for phytophagous species and thus their invertebrate predators (e.g. Kirkham
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et al., 1994; Thomas et al., 1994; Baines et al., 1998). The presence of perennial herbs will promote numbers of Hemiptera, Hymenoptera and Aranaea which may colonise margins very quickly, usually within 12 months (Thomas et al., 1994). On grass-only and grass/wildflower strips, seed availability will be heavily influenced by the cutting regime. Both are usually mown in August or September to prevent scrub encroachment. This will also help to maintain biodiversity (Smith et al., 1993) and leave a short sward in autumn/winter which will facilitate foraging by birds such as thrushes that feed on ground dwelling invertebrates. However, such frequent cutting may reduce the value of grass margins for ground-nesting birds such as partridges (e.g. Rands, 1987, 1988) and yellowhammers (Stoate et al., 1998) both of which may benefit from leaving part of the strip, nearest the hedge, uncut. Grass-only and grass/wildflower strips may also benefit small mammals and hence increase hunting opportunities for raptors such as kestrels (Falco tinnunculus) and barn owls (Tyto alba) (Harris and Woollard, 1990; Tew et al., 1992). 2.2. Naturally regenerated (rotational) set-aside margins Set-aside margins, of a minimum width of 20 m, may be either rotational, where land is taken out of production and left as fallow for 1 year, or non-rotational, where land is taken out of production for a number of years (MAFF, 1997). In the latter case, the vegetation undergoes a typical secondary succession of annual weeds, followed by perennials. The vegetation cover on set-aside may be established either through natural regeneration or sowing with grass such as perennial ryegrass Lolium perenne. In practice, the latter is the norm on non-rotational set-aside and is considered in this overview as a form of grass margin (Henderson et al., 2000a). In this overview, the term naturally regenerated set-aside margins refers to rotational set-aside only. Naturally regenerated set-aside favours annual weed species and results in areas of winter stubble each year which are important feeding areas for birds (e.g. Buckingham et al., 1999). Most of this type of set-aside is sprayed in early summer with a broad-spectrum herbicide to control weeds in the
following crop (Henderson et al., 2000a,b). The diversity of plants growing on naturally regenerated set-aside depends on the soil type and the seed bank (Gates et al., 1997) and floristic diversity will, in turn, influence the invertebrate community. A number of studies suggest set-aside supports important food resources for insectivorous birds in summer that cannot be obtained from cereal fields (e.g. Moreby and Aebischer, 1992; Poulsen et al., 1998; Henderson, et al., 2000a,b) and the patchy sward will facilitate foraging at the sward base or on the ground (Clarke et al., 1997; Millenbah et al., 1996; Watson and Rae, 1997). Grey partridge, red-legged partridge (Cramp and Simmons, 1987), tree sparrow, goldfinch, greenfinch (Carduelis chloris) and linnet (Cramp and Perrins, 1994) are likely to benefit from the increased seed availability of weed seeds such as Chenopodium spp. and knotgrass, on set-aside compared with fields sown with intensively managed weed-free crops. Naturally regenerated set-aside also provides stubbles in winter. These are known to be important, but scarce, feeding habitats for birds such as corn and cirl bunting (Evans and Smith, 1994), linnet, house sparrow and chaffinch (Fringilla coelebs) (Buckingham et al., 1999; Evans, 1997a; Robinson and Sutherland, 1999; Wilson et al., 1996). Rotational set-aside may not, however, provide as good foraging opportunities as non-rotational for predatory birds such as kestrel and barn owl, since the sparse cover does not seem to favour small mammals (Green, 1994; Plesner Jensen and Honess, 1995). 2.3. Uncropped wildlife strips Uncropped wildlife strips comprise a 6 m wide strip of land adjacent to the arable crop together with a 1 m wide sterile strip between the wildlife strip and the crop. The wildlife strip comprises naturally regenerated vegetation cultivated once in every year or 2 years to encourage annual and biennial arable flowers (Critchley, 1994). The sterile strip is maintained so as to prevent aggressive arable weeds spreading into the adjacent crop. This type of field margin has, to date, been restricted to light and/or shallow soils and targeted at the conservation of rare arable weeds such as pheasant’s eye (Adonis annua) and cornflower (C. cyanus). If developed in the wider countryside,
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these may benefit a range of rare or declining weed species which germinate from seed banks that persist at field edges, e.g. rough (Papaver hybridum) and prickly poppy (P. argemone) and narrow-fruited cornsalad (Valerianella dentata) (Wilson and Aebischer, 1995). In the UK, this type of margin exists in three areas: (a) the Breckland environmentally sensitive area in East Anglia; (b) the arable stewardship pilot area in East Anglia; (iii) the arable stewardship pilot area in West Midlands (Critchley et al., 1999). These strips may provide areas of high seed availability, particularly where annual weeds are allowed to flower. Many of the plants that utilise this disturbed ground support a wide range of phytophagous insects, especially Hemiptera, Coleoptera and Lepidoptera (Hawthorne and Hassall, 1994; Hawthorne et al., 1999). They are also valuable pollen and nectar sources (Morris and Webb, 1987). Unlike conservation headlands which are ploughed, these strips are not necessarily rotovated every year, which will minimise the impact on ground dwelling invertebrates compared with annual cultivations (e.g. Hassall et al., 1992; Barker et al., 1997). This abundance and diversity of seed and invertebrate food, combined with the open patchy nature of the sward (e.g. Henderson et al., 2000a,b) means that uncropped wildlife strips are likely to provide good feeding opportunities for a range of farmland birds (Critchley, pers. comm.). 2.4. Game cover crops Farmers keen on encouraging gamebird populations often plant blocks or strips of game cover crops. These usually comprise species like kale (Brassica oleracea), quinoa (C. quinoa) and maize (Zea mays) (Kings, 1998) and can be included under set-aside as the wild bird cover (WBC) option (MAFF, 1997) and as wildlife seed mixtures under schemes such as arable stewardship (MAFF, 1998). The minimum requirement of WBC under set-aside is a 20 m wide strip and 0.3 ha extent, but the species composition may vary within this, e.g. with a mixture of nesting and brood cover. Where land remains set-aside for more than 1 year, WBC must be replaced during the first or second calendar year after sowing. Where it remains in place for 2 years, it should be left to reseed itself in autumn and not cut. In all the cases, it must
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be a mixture of at least two crops other than legumes (MAFF, 1997). The seed-rich open swards of game cover crops provide valuable foraging habitat, especially in winter, for a range of passerines such as yellowhammers and linnets, as well as gamebirds (e.g. Brickle, 1997; Stoate and Szczur, 1997). Seeds are not only provided by the sown plants but also by weeds that establish in the margin. Although game cover crops may not support the variety of invertebrates present in more botanically diverse habitats, some crops used support large numbers of pest insects such as aphids, while kale supports a variety of lepidopteran larvae (Carter, 1984). 2.5. Conservation headlands Techniques of modifying the management of arable, particularly cereal, field edges were developed in Germany to conserve rare arable weed species (Schumacher, 1987). These were modified in the UK by the Game Conservancy Trust to enhance gamebird populations, particularly those of the grey partridge (Sotherton et al., 1985; Rands, 1985; Rands and Sotherton, 1987). Conservation headlands comprise either a 6 or 12 m wide strip forming the outer margin of the crop often separated from the adjacent field boundary or other vegetation by a 1 m sterile strip. The conservation headland is cropped with cereals but is managed with limited insecticides (autumn cereals only) or no insecticides (spring cereals) and with reduced inputs of herbicides in order to favour wild arable plants, particularly broad-leaved weeds, and the invertebrates which live on them (Sotherton, 1991). Conservation headlands have been shown to benefit grey and red-legged partridge and pheasant (Phasianus colchicus) by providing good chick-feeding habitats and promoting chick survival rates (e.g. Potts, 1996; Rands, 1986; Chiverton, 1999). Reduced insecticide and herbicide use encourages a range of invertebrates, including sawflies (Hymenoptera) and plant bugs (Hemiptera) (e.g. Dover, 1996; Hassall et al., 1992) which are important food items for gamebirds. Conservation headlands may not support as great an abundance or diversity of invertebrates as uncropped wildlife strips for two reasons. First, they are usually ploughed, a disruptive management practice for soil-inhabiting and epigeal invertebrates (e.g. Potts,
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1996; Barker et al., 1997). Secondly, the vegetation cover comprises mainly crop plants and is often less diverse in terms of species and structure (Cardwell et al., 1994; Hawthorne and Hassall, 1994; White and Hassall, 1994). Conservation headlands support a relatively low-diversity sward. However, they are more likely to benefit gamebird chicks than planted grass strips, since they benefit chick food insects, weeds used by birds, e.g. chickweed (S. media) and have an open structured sward that is easy for birds to move through (Barker and Reynolds, 1999). The foraging benefits of conservation headlands to non-gamebird species have been less well-studied. The diversity and abundance of invertebrates and seeds may be less than that of uncropped wildlife strips. However, increased seed availability, both from broad-leaved weeds and later, after harvest, from the crop itself, and increased invertebrate abundance would be expected to benefit a range of bird species, though evidence is lacking of any effects of bird density in adjacent hedgerows (Green et al., 1994). Conservation headlands have been shown to benefit small mammal populations (Tew et al., 1992) and may benefit avian predators that hunt along hedgerows.
3. Margins compared with other approaches for enhancing farmland bird populations In addition to field margin management techniques, there are a number of whole field or farm approaches that are likely to benefit birds. The three most widely practiced whole-field management practices likely to benefit birds in arable farmland, are two options within agri-environment schemes, namely overwinter stubbles and undersowing. The third practice, set-aside, was introduced to limit production, although it also has important environmental benefits (MAFF, 1998). On an area-for-area basis, there are a number of reasons why field margins may be a more cost-effective way of providing food resources for birds than whole field options. First, in general, the abundance of plant and invertebrate prey decreases with distance from the field boundary (Marshall, 1989; Wilson and Aebischer, 1995; Gates et al., 1997). Secondly, many birds nest in hedgerows and may favour foraging close to the nest site which will also result in them utilising field margins more than field centres. This has been shown
to be the case on set-aside fields, where the majority of species occur more frequently than expected in field margins rather than field centres (Henderson et al., 2000a). A number of species such as yellowhammer and tree sparrow (Robinson, 1997) also seem to prefer margins in winter. However, very little is known about the within-field distribution of foraging birds in winter and hence the relative value of field margins and centres at this time of year. A number of species avoid field margins, particularly skylark, lapwing (Vanellus vanellus) and stone curlew (Burhinus oedicnemus) and whole field options are required for these species. In addition, it has been suggested that higher numbers of small predatory mammals in field margins may result in increased predation rates of ground nesting species like skylark (Donald et al., 1998). The widespread integration of whole-field set-aside into the arable landscape is likely to have provided an important refuge for many farmland birds that are declining in numbers. Set-aside increases habitat diversity and quality by effectively re-introducing fallow into arable systems and, through rotational set-aside, areas of stubble in winter (Evans, 1997a). The potential conservation benefits of set-aside are large, partly because of the habitat it creates and partly as a result of the scale over which the scheme operates. Like set-aside, field margins are a means by which habitat of considerable conservation value can be created on a broad geographical scale. However, at its peak, set-aside covered almost 750,000 ha of arable land in England and Wales (Vickery and Buckingham, 2001), a much greater area than the estimated 240,000 ha that would exist if all cereal fields had 6 m margins managed. It is still unclear whether habitat creation even at the scale of set-aside is sufficient to stem the declines of species such as the skylark (e.g. Vickery and Buckingham, 2001; Donald and Vickery, 2000). Agri-environment schemes promoting options such as field margin management need to be implemented in parallel with initiatives designed to reduce the intensity with which large areas of arable land are managed (Donald and Vickery, 2000; Vickery and Buckingham, 2001). Whole farm approaches such as organic farming may provide significant benefits for birds. Two major comparisons of birds on conventional and organic farms, one in Denmark (Christensen et al., 1996) and the other in Britain (Chamberlain et al., 1999)
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suggest that organic farms support higher breeding and wintering densities of a wide range of species than conventionally farmed land nearby. This conclusion is supported by single species studies of yellowhammer (Petersen, 1994) and skylark (Wilson et al., 1997), both of which exhibited higher breeding success (brood size and nest survival, respectively) on organic compared with conventional farms. These benefits of organic farming to birds are likely to derive from a range of factors (Fuller, 1997). These include use of rotations, mixed arable and livestock regimes, very limited pesticide usage and more extensive non-cropped habitat, e.g. hedgerows and farm woodland, as well as increased food availability (weeds and some invertebrate groups) in organic compared with conventional cereals (Brookes et al., 1995; Reddersen, 1997; Hald, 1999; Chamberlain et al., 1999). However, although substantial benefits may be derived from organic farming, it forms a relatively small part of the total farmed area in Britain (240,000 ha were farmed organically or in conversion in April 1999 (Anon., 1999)). This area is expanding but it is unlikely to provide the widespread changes in the farmed landscape comparable with the set-aside scheme. The conservation benefits associated with organic farming are likely, therefore, to be localised. Sensitively managed field margins involve reduced pesticide and fertiliser inputs and the reintroduction of grass to tillage systems, similar to some of the key features of organic farms. Field margin management offers the potential to integrate such changes over a very large geographical scale. On a local scale, however, field margins are unlikely to introduce the mosaic of habitats, of arable and livestock, associated with organic farms.
4. Future research The value of field margins for birds could be further increased with additional knowledge relating to a range of issues. A rigorous comparison of their relative value in relation to whole-field or farm approaches designed to encourage wildlife would be valuable. Research on farmland birds has focussed largely on their requirements in the breeding season and more work is needed on the species using these field margins in winter. In addition, the extent to which
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increased floristic diversity genuinely increases the value of margins as invertebrate food resources is very poorly known. Value may depend on biomass rather than taxonomic diversity and this issue merits further research. Very little is known about the extent of any accrued benefits to birds derived from leaving margin types in place for several years and there have been no quantitative studies of the optimal width of field margins with respect to the costs and benefits in agronomic and environmental terms. Finally, the benefit of tailoring margin treatments to meet the specific needs of individual species of birds has been highlighted by research on the cirl bunting (Evans, 1997b) and grey partridge (e.g. Potts, 1996). Current and future research on the key dietary and habitat needs of many of the farmland birds, especially those of high conservation concern, should provide the basis for similar approaches to be adopted for other bird species.
5. Conclusions In summary, the potential value of wide-scale uptake of field margins managed sympathetically for wildlife in general and birds, in particular, is extremely high. This is particularly true of management that creates diverse food-rich swards in summer and stubbles in winter. In general, it is expected that the best winter food supplies will be provided by options such as naturally regenerated rotational set-aside, that creates stubble in winter, or game cover, designed specifically to provide winter food for farmland birds (Table 1). In summer, plant (mainly seeds and green material) and invertebrate food resources are likely to be highest on those options that are botanically most diverse, namely grass/wildflower strips, uncropped wildlife strips and naturally regenerated rotational set-aside, followed by conservation headlands. Overall, systems that promote botanical diversity will stimulate arthropod diversity. However, availability of these prey to birds may be reduced in dense-sown swards compared with open, naturally regenerated vegetation. The overall benefits of a given margin treatment will vary considerably depending on soil type, existing seed bank, adjacent land use, time after establishment, etc. (see Vickery and Fuller (1998) for full discussion of these factors). However, it seems likely that the highest availability of plant food will be derived from
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Table 1 The likely relative importance of different types of field margins as food sources to birds in summer and wintera Type of field margin
Food resources by season and typeb Summer
Grass Grass-only strips Grass/wildflower strips Naturally regenerated set-aside Uncropped wildlife strips Game cover crops Conservation headlands a b
Winter (seeds/green material)
Invertebrates
Seeds/green material
L/M H H H L/M M
L H H H M M
L L/M H M H L
Note all field margins are likely to provide greater food resources than intensively managed crop margins. L: relatively low importance; M: medium importance; H: relatively high importance.
naturally regenerated set-aside margins (assuming ideal conditions and good seed banks). These provide food in summer (a diverse sward of cereals, grasses and dicotyledenous weeds) and winter (stubble). Grass-only strips, with lower plant species and sward structural diversity, will be less valuable in terms of increasing the foraging opportunities for birds, certainly with respect to plant food resources. Assessing the value in terms of invertebrate food supply is more difficult due to the lack of quantitative data (R. Feber, pers. comm.; W. Powell, pers. comm.). However, grass-only strips are often the most likely to be taken up by farmers (e.g. Van der Meulen et al., 1996; Smallshire and Cooke, 1999). Even if the invertebrate/seed availability is limited, compared to other margin options, they are likely to offer a substantially better foraging resource for birds than the alternative of growing intensively managed cereal crop right up to the hedge base. On an area-for-area basis, field margins are likely to be more effective at providing food resources for birds than either whole field or whole farm options. Field margin practices can be relatively easily introduced over a wide geographical scale and can be integrated with whole field management practices. Further reading Feber, R.E., Smith, H., 1995. Butterfly conservation on arable farmland. In: Pullin, A.S. (Ed.), Ecology and Conservation of Butterflies. Chapman & Hall, London, pp. 84–97.
Acknowledgements This work is based on a review funded by the Ministry of Agriculture Fisheries and Food. We are grateful to Jon Marshall (IACR—Long Ashton) and Wilf Powell (IACR—Rothamsted) for major contributions, Ruth Feber for extensive comments on the original review and to two referees for their comments on this paper. References Anon., 1995a. Actions Plans, Vol. 2. Biodiversity. The UK Steering Group Report. HMSO, London. Anon., 1995b. Meeting the Rio Challenge, Vol. 1. Biodiversity. The UK Steering Group Report. HMSO, London. Anon., 1998. UK Biodiversity Group Tranche 2 Action Plans. Vertebrates and Vascular Plants, Vol. 1. English Nature, Peterborough. Anon., 1999. The Organic Food and Farming Report. Soil Association, Bristol. Baines, M., Hambler, C., Johnson, P.J., MacDonald, D.W., Smith, H., 1998. The effects of arable field margin management on the abundance and species richness of Araneae (spiders). Ecography 21, 74–86. Barker, A.M., Reynolds, C.J.M., 1999. The value of planted grass field margins as a habitat for sawflies and other chick food insects. In: Boatman, N.D., Davies, D.H.K., Chaney, K., Feber, R., de Snoo, G.R., Sparks, T.H. (Eds.), Field Margins and Buffer Zones: Ecology, Management and Policy. Aspects Appl. Biol. 54, 109–116. Barker, A.M., Vinson, S.C., Boatman, N.D., 1997. Timing and cultivation or rotational set-aside for grass and weed control to benefit chick-food insects. In: Weeds. The 1997 Brighton Crop Protection Conference. British Crop Protection Council, Farnham, Surrey, pp. 1191–1196.
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The potential value of managed cereal field margins as foraging habitats for farmland birds in the UK Juliet Vickery∗ , Nick Carter, Robert J. Fuller British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU, UK
Abstract Many farmland bird populations have exhibited marked declines in the last 20–30 years and there is growing evidence to link these declines to agricultural intensification. Field margin management is frequently proposed as a way of enhancing bird populations but there have been few attempts to assess the relative value of different management options. This paper aims to provide a preliminary assessment of the relative value of six different cereal field margin management practices in providing foraging habitats for farmland birds. It also briefly compares these with the benefits afforded by field and whole farm approaches such as set-aside and organic farming, to wildlife conservation on arable land. The field margins considered are: grass margins (separated into grass-only strips and grass/wildflower strips), naturally regenerated set-aside margins, uncropped wildlife strips, game cover crops and conservation headlands. Sympathetically managed field margins can provide a range of plant and invertebrate food resources for birds both in summer and winter. In general, the best winter food supplies (mainly seeds) will be provided by game cover crops and naturally regenerated rotational set-aside strips. The most abundant summer food supplies (invertebrates and seeds) will be provided by a diverse sward; grass/wildflower strips, uncropped wildlife strips and naturally regenerated rotational set-aside strips followed by conservation headlands. Field margin swards that are less diverse in terms of species composition and structural heterogeneity and that consequently support fewer invertebrates, can, nonetheless, provide higher quality foraging habitats for birds than an intensively managed crop up to the hedge base. A number of bird species such as yellowhammer (Emberiza citrinella) and tree sparrow (Passer montanus), prefer to forage in margins in winter and summer and many of the benefits of whole field approaches such as set-aside, overwinter stubbles and undersown cereals, could be gained from margins under the same management. Whole-field approaches are, however, required for boundary-avoiding species such as skylark (Alauda arvensis) and lapwing (Vanellus vanellus). Less is known about the use other birds make of margins, relative to their use of field centres in winter. Whole farm approaches such as organic farming, whilst being highly beneficial to birds, are likely to remain rather localised in the UK, whereas field margin management can be relatively easily incorporated into the farmed landscape on an extensive scale. © 2002 Published by Elsevier Science B.V. Keywords: Grass margins; Set-aside; Uncropped wildlife strips; Game cover; Conservation headland; Farming systems
1. Introduction Many farmland bird species have exhibited dramatic declines in numbers and marked range contractions ∗ Corresponding author. Tel.: +44-1842-7500-50; fax: +44-1842-7500-30. E-mail address: [email protected] (J. Vickery).
in the last 20–30 years (Fuller et al., 1995; Gibbons et al., 1993; Siriwardena et al., 1998). A number of them have been added to the red list of birds of conservation concern (Gibbons et al., 1996) or included in the UK’s biodiversity action plans (Anon., 1995a,b, 1998), identifying them as species requiring urgent action to halt, and ultimately reverse, their declines.
0167-8809/02/$ – see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 1 6 7 - 8 8 0 9 ( 0 1 ) 0 0 3 1 7 - 6
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An increasing body of evidence now exists to link the decline of these farmland birds to agricultural intensification, including changes in cropping practices (e.g. switches from spring to autumn sown cereals and loss of mixed farming), increased use of pesticides and inorganic fertilisers and loss of non-crop habitat (Fuller, 2000). There is also a growing acceptance that halting the declines of common and widespread birds can only be achieved through more sympathetic management of the wider countryside, particularly agricultural land. One of the consequences of arable agricultural intensification has been the removal and narrowing of field margins, defined as ‘strips of land lying between crops and the field boundary and extending for a limited distance into the crop’ (Anon., 1995a). In the remaining narrow margin, annual weeds such as barren brome (Bromus sterilis) and cleavers (Galium aparine) often replace the perennial flora (e.g. Dunkley and Boatman, 1994). The sympathetic management of field margins for wildlife is frequently proposed as a way of enhancing populations of farmland wildlife without significantly affecting the agricultural productivity (e.g. Smallshire and Cooke, 1999). As a result, some form of field margin management has been included as an option within most of the recent agri-environment schemes in the UK (e.g. countryside stewardship (MAFF, 1999); arable stewardship (MAFF, 1998); ESAs (MAFF, 1994; Dwyer, 1994)). The potential wildlife value of field margins is highlighted by the inclusion of cereal field margins as one of the first 14 key biodiversity habitats in the UK for which costed action plans were published (Anon., 1995a,b). The focus on margins of cereal fields, rather than other crops, is simply a reflection of the dominance of cereals in the British arable landscape. Assuming an average national field size of 12 ha, there would be approximately 400,000 km of cereal field edge in the UK. If all such boundaries included a 6 m managed margin, approximately 240,000 ha of land could be brought into sensitive management (Anon., 1995a,b). This equates to 5% of the UK’s arable area, a significant component of the farmed landscape, and one that could be incorporated extensively into the wider countryside. Appropriately managed, field margins provide nesting and feeding sites for gamebirds (Rands, 1985, 1986, 1988) and many ground feeding passerines (Parish et al., 1994;
Stoate, 1999; Bradbury and Stoate, in press), through supporting a diversity of arable plants (Wilson, 1994), and invertebrates (Moreby, 1994; Feber et al., 1995; Barker et al., 1997; Haysom, 1999). A number of different management options exist for field margins. However, despite their potential value for farmland birds, and the importance with which field margin options are considered in Britain and elsewhere in Europe (Kleijn et al., 1998; Marshall et al., 1994; Smallshire and Cooke, 1999), there have been few attempts to assess the relative value of the different option types. This paper provides a brief overview of the different options currently adopted in the UK and their potential value as foraging habitats for birds. In the following sections, six field margin management treatments are considered: grass margins, which include grass-only strips, grass/wildflower strips (elevated grass strips or beetle banks are not considered here since they are almost always established as mid-field strips rather than field margins); naturally regenerated set-aside margins; uncropped wildlife strips; game cover crops; and conservation headlands. Each one is defined and the resources they are likely to offer for farmland birds are outlined. These benefits are then compared with whole field and whole farm approaches to wildlife conservation such as set-aside, undersown cereal fields, retention of overwinter stubble and organic farming.
2. Types of cereal field margin and benefits to birds The suitability of different field margins as foraging habitat for farmland birds will be largely determined by the way in which their management influences the abundance of food and its accessibility (affected mainly by vegetation structure). A large number of bird species commonly inhabiting lowland farmland is likely to benefit from sympathetic management of field margins. These include seed and invertebrate feeders, passerines and gamebirds, residents and migrant species, several of which are red-listed as birds of conservation concern (Gibbons et al., 1996), e.g. grey partridge (Perdix perdix), turtle dove (Streptopelia turtor), song thrush (Turdus philomelos), tree sparrow, linnet (Carduelis cannabina), cirl bunting (Emberiza cirlus) and corn bunting (Miliaria calandra).
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There are several caveats in comparing the value of different field margin management practices for birds. The relative value of different options will vary with geographic location (reflecting soil type and seed bank), time since establishment and time of year (e.g. Marshall et al., 1994). For the purposes of this paper ‘ideal conditions’ are assumed (e.g. rich seed bank in the case of naturally regenerated margins and diverse seed mix in sown grass/wildflower strips) and compare treatments in the first 1–2 years after establishment. In addition, field margins as defined in this overview specifically exclude boundary features such as hedgerows and fencelines. It is important to note that the value of such margins will be heavily influenced by adjacent field boundary characteristics, e.g. through shading and as sources of certain invertebrates. Hedgerows are an extremely valuable habitat for birds providing food, shelter and nest cover (O’Connor, 1987). In general, bird species richness increases with overall size and shrub/tree species diversity of a hedgerow (e.g. Parish et al., 1994, 1995; Barr et al., 1995). Field margins adjacent to such hedges are likely to be used by, and so provide benefits for, a larger number of farmland birds. 2.1. Grass margins These may take the form of grass-only strips or grass and wild flower strips (subsequently referred to as grass/wildflower strips). These strips may be 2–6 m wide (Countryside Stewardship Scheme; MAFF, 1999) or 4–12 m wide (arable stewardship pilot scheme; MAFF, 1998) and established either through natural regeneration or sowing (MAFF, 1999). They may contain flowers or have them added at sowing (typically ox-eye daisy (Chrysanthemum leucanthemum), ribwort plantain (Plantago lanceolata), knapweeds (Centaurea spp.) and yarrow (Achillea millefolium)). They vary in grass length as a result of different species composition (short grass such as red fescue (Festuca rubra) or tall tussocky grass such as cocksfoot (Dactylis glomerata)) and management. Grass strips require little active management except some cutting to aid establishment and subsequently to prevent scrub encroachment. Grass-only sown strips help to protect hedgerow fauna and flora from farming operations such as pesticide and fertiliser applications and close ploughing,
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which could benefit bird species that feed or nest in hedgerow bottoms. They also allow year-round access for farmers to hedges enabling them to be trimmed late in the winter without damaging the adjacent crop. This could benefit birds, especially winter thrushes feeding on berries (Sparks and Martin, 1999). Reduced pesticide inputs will allow annual weeds such as chickweed (Stellaria species) and knotgrass (Polygonum species), to germinate. These can provide seed resources for birds (Wilson et al., 1999) and support a range of phytophagous insects (Morris and Webb, 1987). Grass seeds provided by grass-only strips are fed on by a range of species (Wilson et al., 1999) including starling (Sturnus vulgaris), dunnock (Prunella modularis) house sparrow (Passer domesticus), tree sparrow and yellowhammer (Cramp, 1988; Cramp and Perrins, 1994). Grass-only margins also provide suitable habitat for invertebrates such as elaterid and carabid beetles and tipulid larvae which are important dietary items for birds such as cirl and corn bunting (Evans, 1997a), grey partridge (Potts, 1996), yellowhammer (Emberiza citrinella) (Bradbury and Stoate, in press) and skylark (Alauda arvensis) (Donald, 1999). Compared with modern cereal fields, they support higher numbers of graminivorous sawfly larvae and other gamebird chick food insects such as plant bugs and hoppers and lepidopteran larvae (Barker and Reynolds, 1999). Compared with many other margin types, however, grass-only strips tend to support a low variety of broad-leaved weeds which may lead to a relatively low abundance and diversity of invertebrates (e.g. Morris, 2000; Vickery et al., in press). Furthermore, grass-only strips may form a tall dense sward that hampers foraging by birds (Weibel, 1998; Barker and Reynolds, 1999). Grass/wildflower strips, whether sown or naturally regenerated, are more likely to provide food for birds over a longer period of the year than grass-only swards, as different plants will flower and seed at different times. Most grasses flower in mid-summer, while some dicotyledonous species will flower before or after this period (Clapham et al., 1968; Fitter et al., 1980). In addition, grass/wildflower strips are more likely to support a greater range of invertebrate species than grass-only strips, as they provide a greater variety of host plants for phytophagous species and thus their invertebrate predators (e.g. Kirkham
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et al., 1994; Thomas et al., 1994; Baines et al., 1998). The presence of perennial herbs will promote numbers of Hemiptera, Hymenoptera and Aranaea which may colonise margins very quickly, usually within 12 months (Thomas et al., 1994). On grass-only and grass/wildflower strips, seed availability will be heavily influenced by the cutting regime. Both are usually mown in August or September to prevent scrub encroachment. This will also help to maintain biodiversity (Smith et al., 1993) and leave a short sward in autumn/winter which will facilitate foraging by birds such as thrushes that feed on ground dwelling invertebrates. However, such frequent cutting may reduce the value of grass margins for ground-nesting birds such as partridges (e.g. Rands, 1987, 1988) and yellowhammers (Stoate et al., 1998) both of which may benefit from leaving part of the strip, nearest the hedge, uncut. Grass-only and grass/wildflower strips may also benefit small mammals and hence increase hunting opportunities for raptors such as kestrels (Falco tinnunculus) and barn owls (Tyto alba) (Harris and Woollard, 1990; Tew et al., 1992). 2.2. Naturally regenerated (rotational) set-aside margins Set-aside margins, of a minimum width of 20 m, may be either rotational, where land is taken out of production and left as fallow for 1 year, or non-rotational, where land is taken out of production for a number of years (MAFF, 1997). In the latter case, the vegetation undergoes a typical secondary succession of annual weeds, followed by perennials. The vegetation cover on set-aside may be established either through natural regeneration or sowing with grass such as perennial ryegrass Lolium perenne. In practice, the latter is the norm on non-rotational set-aside and is considered in this overview as a form of grass margin (Henderson et al., 2000a). In this overview, the term naturally regenerated set-aside margins refers to rotational set-aside only. Naturally regenerated set-aside favours annual weed species and results in areas of winter stubble each year which are important feeding areas for birds (e.g. Buckingham et al., 1999). Most of this type of set-aside is sprayed in early summer with a broad-spectrum herbicide to control weeds in the
following crop (Henderson et al., 2000a,b). The diversity of plants growing on naturally regenerated set-aside depends on the soil type and the seed bank (Gates et al., 1997) and floristic diversity will, in turn, influence the invertebrate community. A number of studies suggest set-aside supports important food resources for insectivorous birds in summer that cannot be obtained from cereal fields (e.g. Moreby and Aebischer, 1992; Poulsen et al., 1998; Henderson, et al., 2000a,b) and the patchy sward will facilitate foraging at the sward base or on the ground (Clarke et al., 1997; Millenbah et al., 1996; Watson and Rae, 1997). Grey partridge, red-legged partridge (Cramp and Simmons, 1987), tree sparrow, goldfinch, greenfinch (Carduelis chloris) and linnet (Cramp and Perrins, 1994) are likely to benefit from the increased seed availability of weed seeds such as Chenopodium spp. and knotgrass, on set-aside compared with fields sown with intensively managed weed-free crops. Naturally regenerated set-aside also provides stubbles in winter. These are known to be important, but scarce, feeding habitats for birds such as corn and cirl bunting (Evans and Smith, 1994), linnet, house sparrow and chaffinch (Fringilla coelebs) (Buckingham et al., 1999; Evans, 1997a; Robinson and Sutherland, 1999; Wilson et al., 1996). Rotational set-aside may not, however, provide as good foraging opportunities as non-rotational for predatory birds such as kestrel and barn owl, since the sparse cover does not seem to favour small mammals (Green, 1994; Plesner Jensen and Honess, 1995). 2.3. Uncropped wildlife strips Uncropped wildlife strips comprise a 6 m wide strip of land adjacent to the arable crop together with a 1 m wide sterile strip between the wildlife strip and the crop. The wildlife strip comprises naturally regenerated vegetation cultivated once in every year or 2 years to encourage annual and biennial arable flowers (Critchley, 1994). The sterile strip is maintained so as to prevent aggressive arable weeds spreading into the adjacent crop. This type of field margin has, to date, been restricted to light and/or shallow soils and targeted at the conservation of rare arable weeds such as pheasant’s eye (Adonis annua) and cornflower (C. cyanus). If developed in the wider countryside,
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these may benefit a range of rare or declining weed species which germinate from seed banks that persist at field edges, e.g. rough (Papaver hybridum) and prickly poppy (P. argemone) and narrow-fruited cornsalad (Valerianella dentata) (Wilson and Aebischer, 1995). In the UK, this type of margin exists in three areas: (a) the Breckland environmentally sensitive area in East Anglia; (b) the arable stewardship pilot area in East Anglia; (iii) the arable stewardship pilot area in West Midlands (Critchley et al., 1999). These strips may provide areas of high seed availability, particularly where annual weeds are allowed to flower. Many of the plants that utilise this disturbed ground support a wide range of phytophagous insects, especially Hemiptera, Coleoptera and Lepidoptera (Hawthorne and Hassall, 1994; Hawthorne et al., 1999). They are also valuable pollen and nectar sources (Morris and Webb, 1987). Unlike conservation headlands which are ploughed, these strips are not necessarily rotovated every year, which will minimise the impact on ground dwelling invertebrates compared with annual cultivations (e.g. Hassall et al., 1992; Barker et al., 1997). This abundance and diversity of seed and invertebrate food, combined with the open patchy nature of the sward (e.g. Henderson et al., 2000a,b) means that uncropped wildlife strips are likely to provide good feeding opportunities for a range of farmland birds (Critchley, pers. comm.). 2.4. Game cover crops Farmers keen on encouraging gamebird populations often plant blocks or strips of game cover crops. These usually comprise species like kale (Brassica oleracea), quinoa (C. quinoa) and maize (Zea mays) (Kings, 1998) and can be included under set-aside as the wild bird cover (WBC) option (MAFF, 1997) and as wildlife seed mixtures under schemes such as arable stewardship (MAFF, 1998). The minimum requirement of WBC under set-aside is a 20 m wide strip and 0.3 ha extent, but the species composition may vary within this, e.g. with a mixture of nesting and brood cover. Where land remains set-aside for more than 1 year, WBC must be replaced during the first or second calendar year after sowing. Where it remains in place for 2 years, it should be left to reseed itself in autumn and not cut. In all the cases, it must
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be a mixture of at least two crops other than legumes (MAFF, 1997). The seed-rich open swards of game cover crops provide valuable foraging habitat, especially in winter, for a range of passerines such as yellowhammers and linnets, as well as gamebirds (e.g. Brickle, 1997; Stoate and Szczur, 1997). Seeds are not only provided by the sown plants but also by weeds that establish in the margin. Although game cover crops may not support the variety of invertebrates present in more botanically diverse habitats, some crops used support large numbers of pest insects such as aphids, while kale supports a variety of lepidopteran larvae (Carter, 1984). 2.5. Conservation headlands Techniques of modifying the management of arable, particularly cereal, field edges were developed in Germany to conserve rare arable weed species (Schumacher, 1987). These were modified in the UK by the Game Conservancy Trust to enhance gamebird populations, particularly those of the grey partridge (Sotherton et al., 1985; Rands, 1985; Rands and Sotherton, 1987). Conservation headlands comprise either a 6 or 12 m wide strip forming the outer margin of the crop often separated from the adjacent field boundary or other vegetation by a 1 m sterile strip. The conservation headland is cropped with cereals but is managed with limited insecticides (autumn cereals only) or no insecticides (spring cereals) and with reduced inputs of herbicides in order to favour wild arable plants, particularly broad-leaved weeds, and the invertebrates which live on them (Sotherton, 1991). Conservation headlands have been shown to benefit grey and red-legged partridge and pheasant (Phasianus colchicus) by providing good chick-feeding habitats and promoting chick survival rates (e.g. Potts, 1996; Rands, 1986; Chiverton, 1999). Reduced insecticide and herbicide use encourages a range of invertebrates, including sawflies (Hymenoptera) and plant bugs (Hemiptera) (e.g. Dover, 1996; Hassall et al., 1992) which are important food items for gamebirds. Conservation headlands may not support as great an abundance or diversity of invertebrates as uncropped wildlife strips for two reasons. First, they are usually ploughed, a disruptive management practice for soil-inhabiting and epigeal invertebrates (e.g. Potts,
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1996; Barker et al., 1997). Secondly, the vegetation cover comprises mainly crop plants and is often less diverse in terms of species and structure (Cardwell et al., 1994; Hawthorne and Hassall, 1994; White and Hassall, 1994). Conservation headlands support a relatively low-diversity sward. However, they are more likely to benefit gamebird chicks than planted grass strips, since they benefit chick food insects, weeds used by birds, e.g. chickweed (S. media) and have an open structured sward that is easy for birds to move through (Barker and Reynolds, 1999). The foraging benefits of conservation headlands to non-gamebird species have been less well-studied. The diversity and abundance of invertebrates and seeds may be less than that of uncropped wildlife strips. However, increased seed availability, both from broad-leaved weeds and later, after harvest, from the crop itself, and increased invertebrate abundance would be expected to benefit a range of bird species, though evidence is lacking of any effects of bird density in adjacent hedgerows (Green et al., 1994). Conservation headlands have been shown to benefit small mammal populations (Tew et al., 1992) and may benefit avian predators that hunt along hedgerows.
3. Margins compared with other approaches for enhancing farmland bird populations In addition to field margin management techniques, there are a number of whole field or farm approaches that are likely to benefit birds. The three most widely practiced whole-field management practices likely to benefit birds in arable farmland, are two options within agri-environment schemes, namely overwinter stubbles and undersowing. The third practice, set-aside, was introduced to limit production, although it also has important environmental benefits (MAFF, 1998). On an area-for-area basis, there are a number of reasons why field margins may be a more cost-effective way of providing food resources for birds than whole field options. First, in general, the abundance of plant and invertebrate prey decreases with distance from the field boundary (Marshall, 1989; Wilson and Aebischer, 1995; Gates et al., 1997). Secondly, many birds nest in hedgerows and may favour foraging close to the nest site which will also result in them utilising field margins more than field centres. This has been shown
to be the case on set-aside fields, where the majority of species occur more frequently than expected in field margins rather than field centres (Henderson et al., 2000a). A number of species such as yellowhammer and tree sparrow (Robinson, 1997) also seem to prefer margins in winter. However, very little is known about the within-field distribution of foraging birds in winter and hence the relative value of field margins and centres at this time of year. A number of species avoid field margins, particularly skylark, lapwing (Vanellus vanellus) and stone curlew (Burhinus oedicnemus) and whole field options are required for these species. In addition, it has been suggested that higher numbers of small predatory mammals in field margins may result in increased predation rates of ground nesting species like skylark (Donald et al., 1998). The widespread integration of whole-field set-aside into the arable landscape is likely to have provided an important refuge for many farmland birds that are declining in numbers. Set-aside increases habitat diversity and quality by effectively re-introducing fallow into arable systems and, through rotational set-aside, areas of stubble in winter (Evans, 1997a). The potential conservation benefits of set-aside are large, partly because of the habitat it creates and partly as a result of the scale over which the scheme operates. Like set-aside, field margins are a means by which habitat of considerable conservation value can be created on a broad geographical scale. However, at its peak, set-aside covered almost 750,000 ha of arable land in England and Wales (Vickery and Buckingham, 2001), a much greater area than the estimated 240,000 ha that would exist if all cereal fields had 6 m margins managed. It is still unclear whether habitat creation even at the scale of set-aside is sufficient to stem the declines of species such as the skylark (e.g. Vickery and Buckingham, 2001; Donald and Vickery, 2000). Agri-environment schemes promoting options such as field margin management need to be implemented in parallel with initiatives designed to reduce the intensity with which large areas of arable land are managed (Donald and Vickery, 2000; Vickery and Buckingham, 2001). Whole farm approaches such as organic farming may provide significant benefits for birds. Two major comparisons of birds on conventional and organic farms, one in Denmark (Christensen et al., 1996) and the other in Britain (Chamberlain et al., 1999)
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suggest that organic farms support higher breeding and wintering densities of a wide range of species than conventionally farmed land nearby. This conclusion is supported by single species studies of yellowhammer (Petersen, 1994) and skylark (Wilson et al., 1997), both of which exhibited higher breeding success (brood size and nest survival, respectively) on organic compared with conventional farms. These benefits of organic farming to birds are likely to derive from a range of factors (Fuller, 1997). These include use of rotations, mixed arable and livestock regimes, very limited pesticide usage and more extensive non-cropped habitat, e.g. hedgerows and farm woodland, as well as increased food availability (weeds and some invertebrate groups) in organic compared with conventional cereals (Brookes et al., 1995; Reddersen, 1997; Hald, 1999; Chamberlain et al., 1999). However, although substantial benefits may be derived from organic farming, it forms a relatively small part of the total farmed area in Britain (240,000 ha were farmed organically or in conversion in April 1999 (Anon., 1999)). This area is expanding but it is unlikely to provide the widespread changes in the farmed landscape comparable with the set-aside scheme. The conservation benefits associated with organic farming are likely, therefore, to be localised. Sensitively managed field margins involve reduced pesticide and fertiliser inputs and the reintroduction of grass to tillage systems, similar to some of the key features of organic farms. Field margin management offers the potential to integrate such changes over a very large geographical scale. On a local scale, however, field margins are unlikely to introduce the mosaic of habitats, of arable and livestock, associated with organic farms.
4. Future research The value of field margins for birds could be further increased with additional knowledge relating to a range of issues. A rigorous comparison of their relative value in relation to whole-field or farm approaches designed to encourage wildlife would be valuable. Research on farmland birds has focussed largely on their requirements in the breeding season and more work is needed on the species using these field margins in winter. In addition, the extent to which
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increased floristic diversity genuinely increases the value of margins as invertebrate food resources is very poorly known. Value may depend on biomass rather than taxonomic diversity and this issue merits further research. Very little is known about the extent of any accrued benefits to birds derived from leaving margin types in place for several years and there have been no quantitative studies of the optimal width of field margins with respect to the costs and benefits in agronomic and environmental terms. Finally, the benefit of tailoring margin treatments to meet the specific needs of individual species of birds has been highlighted by research on the cirl bunting (Evans, 1997b) and grey partridge (e.g. Potts, 1996). Current and future research on the key dietary and habitat needs of many of the farmland birds, especially those of high conservation concern, should provide the basis for similar approaches to be adopted for other bird species.
5. Conclusions In summary, the potential value of wide-scale uptake of field margins managed sympathetically for wildlife in general and birds, in particular, is extremely high. This is particularly true of management that creates diverse food-rich swards in summer and stubbles in winter. In general, it is expected that the best winter food supplies will be provided by options such as naturally regenerated rotational set-aside, that creates stubble in winter, or game cover, designed specifically to provide winter food for farmland birds (Table 1). In summer, plant (mainly seeds and green material) and invertebrate food resources are likely to be highest on those options that are botanically most diverse, namely grass/wildflower strips, uncropped wildlife strips and naturally regenerated rotational set-aside, followed by conservation headlands. Overall, systems that promote botanical diversity will stimulate arthropod diversity. However, availability of these prey to birds may be reduced in dense-sown swards compared with open, naturally regenerated vegetation. The overall benefits of a given margin treatment will vary considerably depending on soil type, existing seed bank, adjacent land use, time after establishment, etc. (see Vickery and Fuller (1998) for full discussion of these factors). However, it seems likely that the highest availability of plant food will be derived from
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Table 1 The likely relative importance of different types of field margins as food sources to birds in summer and wintera Type of field margin
Food resources by season and typeb Summer
Grass Grass-only strips Grass/wildflower strips Naturally regenerated set-aside Uncropped wildlife strips Game cover crops Conservation headlands a b
Winter (seeds/green material)
Invertebrates
Seeds/green material
L/M H H H L/M M
L H H H M M
L L/M H M H L
Note all field margins are likely to provide greater food resources than intensively managed crop margins. L: relatively low importance; M: medium importance; H: relatively high importance.
naturally regenerated set-aside margins (assuming ideal conditions and good seed banks). These provide food in summer (a diverse sward of cereals, grasses and dicotyledenous weeds) and winter (stubble). Grass-only strips, with lower plant species and sward structural diversity, will be less valuable in terms of increasing the foraging opportunities for birds, certainly with respect to plant food resources. Assessing the value in terms of invertebrate food supply is more difficult due to the lack of quantitative data (R. Feber, pers. comm.; W. Powell, pers. comm.). However, grass-only strips are often the most likely to be taken up by farmers (e.g. Van der Meulen et al., 1996; Smallshire and Cooke, 1999). Even if the invertebrate/seed availability is limited, compared to other margin options, they are likely to offer a substantially better foraging resource for birds than the alternative of growing intensively managed cereal crop right up to the hedge base. On an area-for-area basis, field margins are likely to be more effective at providing food resources for birds than either whole field or whole farm options. Field margin practices can be relatively easily introduced over a wide geographical scale and can be integrated with whole field management practices. Further reading Feber, R.E., Smith, H., 1995. Butterfly conservation on arable farmland. In: Pullin, A.S. (Ed.), Ecology and Conservation of Butterflies. Chapman & Hall, London, pp. 84–97.
Acknowledgements This work is based on a review funded by the Ministry of Agriculture Fisheries and Food. We are grateful to Jon Marshall (IACR—Long Ashton) and Wilf Powell (IACR—Rothamsted) for major contributions, Ruth Feber for extensive comments on the original review and to two referees for their comments on this paper. References Anon., 1995a. Actions Plans, Vol. 2. Biodiversity. The UK Steering Group Report. HMSO, London. Anon., 1995b. Meeting the Rio Challenge, Vol. 1. Biodiversity. The UK Steering Group Report. HMSO, London. Anon., 1998. UK Biodiversity Group Tranche 2 Action Plans. Vertebrates and Vascular Plants, Vol. 1. English Nature, Peterborough. Anon., 1999. The Organic Food and Farming Report. Soil Association, Bristol. Baines, M., Hambler, C., Johnson, P.J., MacDonald, D.W., Smith, H., 1998. The effects of arable field margin management on the abundance and species richness of Araneae (spiders). Ecography 21, 74–86. Barker, A.M., Reynolds, C.J.M., 1999. The value of planted grass field margins as a habitat for sawflies and other chick food insects. In: Boatman, N.D., Davies, D.H.K., Chaney, K., Feber, R., de Snoo, G.R., Sparks, T.H. (Eds.), Field Margins and Buffer Zones: Ecology, Management and Policy. Aspects Appl. Biol. 54, 109–116. Barker, A.M., Vinson, S.C., Boatman, N.D., 1997. Timing and cultivation or rotational set-aside for grass and weed control to benefit chick-food insects. In: Weeds. The 1997 Brighton Crop Protection Conference. British Crop Protection Council, Farnham, Surrey, pp. 1191–1196.
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