|Appendix 3. Factors influencing the use of fields and foraging patches by birds: a review of the literature
The available demographic evidence suggests that annual survival has been the key demographic rate underlying population change of birds on farmland over the last few decades for most species with nidicolous young (review in Siriwardena et al. 2000). If a bottleneck for survival exists, it is likely to fall, for many species, in mid-winter (when temperatures are lowest, days shortest and food availability perhaps restricted by frost or snow cover) or early spring (when over-winter food supplies have been depleted to a minimum). One conservation measure to address such a limiting factor for seed-eating farmland birds and thus to increase population growth rates would be to provide supplementary food in winter. The most efficient way to do this within the existing agricultural landscape may be to provide seed in the form of sacrificial crops or unsprayed stubbles grown or left under a government-sponsored agri-environment scheme. The cost-effectiveness of such a scheme would be maximized by ensuring the measures benefited the maximum number both of species and of individuals. We have addressed the issue of the influence of the spatial distribution of food sources elsewhere (Objective 2 and Appendix 2). The important factors determining the use of seed-rich habitats on a local scale are likely to include field size, field boundary characteristics, the distance from the crop to cover or to woodland and the amount of disturbance to which the crop or field is subjected. Identifying the optimal balance between these potential influences would allow agri-environment scheme payments to be targeted at fields or parts of fields most likely to benefit more birds. Here we review the existing published evidence on the influence of the factors listed above on the use of agricultural habitats by predominantly granivorous farmland birds in order to identify the habitat contexts in which providing supplementary food resources is most likely to succeed.
Studies of habitat selection by birds fall into two broad types: selection for breeding (including nesting per se) and selection for feeding. This review focuses on the latter: a range of factors unrelated to foraging will influence the selection of nest sites (nesting and foraging locations can be effectively independent for non-territorial species) and studies at the territory level will identify the combination of factors that are important, making it difficult to separate influences on foraging from others.
Even within studies of foraging behaviour, much of the work has been done on birds in the breeding season. The relevance of patterns in the breeding season to those in winter is likely to vary between species. Species that are territorial in the breeding season are usually less so at other times of year (or, at least, home ranges increase or territories are held by individuals instead of pairs) and flocks are often formed outside the breeding season, so foraging habitat selection will often be subject to different constraints. Diets also often change with season, with most finches, for example, taking only seeds in winter but predominantly invertebrates in the breeding season (Wilson et al. 1999). This will also influence habitat selection. However, species may well have certain adaptations for foraging and vigilance behaviour, for example, that pre-dispose them to use only open habitats or to select foraging sites that are close to cover, so studies of habitat choice in the breeding season may still be relevant to the situation in winter. This review focuses on studies conducted in the non-breeding season because these will be the most relevant; work from the breeding season is included here only where it informs and adds to the winter results.
3.2 Factors Influencing Field Selection
Probably the best studied aspect of habitat selection by farmland birds concerns crop type, perhaps simply because it is the most obvious way in which agricultural habitats differ. The key variations between fields include arable versus grass, winter- versus spring-sowing and cereal versus brassica versus root crops, with variation in management such as the use of over-wintered stubbles and crops planted to provide cover for game birds providing further complexity in the system.
Seed-eating passerines can form large flocks in winter and are typically concentrated in the richest habitats available. In one study, for example, more than half of the birds found in a survey of 100 1km2 squares were in just 100 fields (1.4% of the area surveyed: Hancock & Wilson 2003). In addition, BTO (2003) found that the vast majority of stubble fields surveyed both in a national survey and in a more intensive, local study supported no birds at all, flocks being concentrated in a few high quality habitats at any one time. Among commercially grown crops, almost all seed-eating species select stubble fields over, for example, winter-sown cereals and grass (Wilson et al. 1996, Donald & Evans 1994, Gillings & Fuller 2001, Chaney et al. 1997, Watson & Rae 1997, Bradbury & Stoate 2000, Buckingham et al. 1999, Robinson & Sutherland 1999, Hancock & Wilson 2003). Woodpigeon is the exception: it often feeds on emergent crop vegetation and so selects fields with crops such as young winter cereals (Wilson et al. 1996). Among stubble fields, Corn Buntings reach densities in weedy stubbles, i.e. those with green, growing crop volunteers, that are double those found in clean (sprayed) stubbles (Donald & Evans 1994). They also tend to use stubbles with some tall vegetation (tall herbs, bushes or trees) or other elevated perches such as telegraph wires or fence posts either within or adjacent to them and can continue to feed on these fields after they are ploughed (Watson & Rae 1997). Oilseed rape stubbles provide the highest crop and weed seed availability for farmland birds but are usually ploughed in quickly in early winter (Robinson et al. 2002). Barley stubbles are preferred to wheat stubbles by many species, probably because of vegetation structure (see below) and the tendency for them to be subject to fewer herbicide treatments and to be harvested earlier, so that annual weeds therefore have longer to grow and set seed before they are killed by the onset of harsh winter weather (Moorcroft et al. 2002). Similarly, winter barley stubbles are richer in weed seed resources than spring barley or wheat stubbles, again probably reflecting a difference in harvesting time (Robinson et al. 2002). The most comprehensive study to date of the factors influencing the use of stubble fields has been conducted by BTO (2003). This study found, in both a national survey and a local, more intensive study, that stubbles varied in hugely in their value to, and thus use by, birds. Within an East Anglian study area incorporating 310 stubble fields, the vast majority of fields supported no birds at all, and very few supported more than two birds per hectare (BTO 2003). This was mirrored nationally where, for example, around 90% of cereal stubbles held no Skylarks and only 10-25% of fields helps more than two birds per hectare (ibid.). It was a general rule that bird densities varied hugely even within crop types, but the most consistently high densities were found in barley stubbles, while those in rape, linseed and beet stubbles could also be high, and wheat and maize stubbles tended to be low. BTO (op. cit.) concluded that factors such as the management practices applied to stubbles themselves as well as to the preceding crops are the most important factors determining field use. Examining field characteristics in detail in their local study, they found that nearly 80% of the variation in bird use could be explained by the density of two categories of weed seed (those from the Chenopodiaceae and the Polygonaceae) that are known to be important in farmland bird diets, together with the number of chemicals applied to the preceding crop (BTO 2003). Weed floras and the management that creates or impoverishes them therefore seem to be critical in determining stubble use. These results also indicate that, although it is true that stubbles can be a very valuable habitat for farmland birds in winter, this is only a part of the story and that promoting stubble retention is only likely to aid bird populations if appropriate management prescriptions are also stipulated.
Only two studies have recorded crops other than stubbles as most preferred by seed-eating passerines in winter. Hancock & Wilson (2003) found that densities in Scotland were highest (nearly four times higher than in stubbles, the next most preferred habitat) in fodder brassica crops, especially weedy ones, and Henderson et al. (2004) found that only seed-rich cover crops were preferred to stubbles (see below). In Hancock & Wilson’s (2003) study, farmyards were the next most important habitat component after stubbles and declining species avoided more wooded landscapes. The fodder brassica crops under strip grazing in their study allow birds gradual access to any weed seed within or beneath the crop as it is consumed through the winter and are usually not ploughed until spring (Hancock & Wilson 2003). Such crops have become rarer in Scotland in recent years, having been replaced by silage (ibid.), and are likely to be rarer still elsewhere in Britain, especially in predominantly arable areas and those in which mixed farming has declined. These crops are very like game cover crops in character, and indeed cover crops were included in the same category by Hancock & Wilson (2003), so this study is in general agreement with Henderson et al. (2004).
Henderson et al. (2004) found that crops sown specifically to provide cover and, often, food for game birds are also selected by granivorous passerines. These crops range from monocultures of maize to combinations of cover- and food-providing plants including sunflowers, quinoa, mustard and kale. In some cases, particular species are added to the cover mix specifically for passerines. The preferences of farmland birds for crops and crop mixes in these wild bird cover areas (usually sown in narrow strips along field boundaries or in patches within larger fields) depend on crop composition (Henderson et al. 2004) and tend to be species-specific (Boatman & Stoate 2002). Yellowhammers, for example, select sown cereal cover strips, reflecting the importance of cereal grain in their diet (Bradbury & Stoate 2000). Henderson et al. (2004) compared the bird use of wild bird cover crops with that of the surrounding fields. Differences in bird use were particularly marked late in the winter, but kale (especially in its second year) and quinoa supported high bird densities and sunflowers and buckwheat low densities (although greenfinches could be very abundant on sunflowers). The key crop plants in sown cover mixes were those that are seed-rich and that hold seed into late winter, namely kale, quinoa and seeding cereals.
The discussion above focuses on arable fields. Unlike invertebrate-feeders, granivorous farmland birds tend to avoid grass fields in winter, especially when they are grazed (Tucker 1992, Wilson et al. 1996, Buckingham et al. 2001, Gillings & Fuller 2001). However, grass set-aside can be preferred by Skylarks in the absence of cereal stubbles (Chaney et al. 1997).
The selection of field-scale habitats in the breeding season is heavily influenced, for many species, by the availability of suitable nest sites nearby, so does not inform the situation in winter fields directly. Predominantly granivorous birds also tend to have an invertebrate-based diet during the breeding season (Wilson et al. 1999), so their foraging preferences might be expected to differ from that in winter. Nevertheless, two key factors for a range of species seem to be accessibility to bare ground, which leads to a selection of spring-sown cereal crops over denser autumn/winter sown crops (Morris et al. 2001, Wilson et al. 1997, Donald & Vickery 2000, Brickle & Harper 2000), and within-field habitat heterogeneity, which promotes seed and invertebrate richness in set-aside fields and leads to their frequent selection above fields under active cropping (Henderson & Evans 2000). The degree to which set-aside fields are used depends greatly on vegetation characteristics and composition and on the previous crop type (Henderson & Evans 2000); these issues are discussed further in the next section.
Vegetation structure, agricultural practices and food availability
Agricultural practices (sowing and cropping regimes, fertilizer and pesticide inputs) are a major influence on the vegetation structure and food availability for birds in arable crops. In grassland, the application of manure or inorganic fertilizer and the presence and type of grazing have significant effects on the foraging preferences of many invertebrate-feeding birds; gulls and plovers also prefer cultivated fields that have had applications of manure over other arable fields (Tucker 1992). Further, it seems that any management practices that increase the structural complexity of a sward will also increase invertebrate, and possibly also seed, food resources (Vickery et al. 2001). Similarly, vegetation characteristics, composition and previous crop types play a central role in determining the use of set-aside fields by birds (Henderson & Evans 2000). A more heterogeneous vegetation structure (providing bare ground and access to/shelter under larger plants) characterizes foraging patches selected by Skylarks in winter (Buckingham 2001), and indeed in summer (Odderskær et al. 1997, Henderson et al. 2001), and they reach higher densities in arable reversion grassland as the sward grows, until it closes, when they fall off again (Wakeham-Dawson & Aebischer 1998). In summer, Yellowhammers seem to select foraging patches on the basis of the availability of both cereal grain (for adults) and invertebrates (for nestlings) (Stoate et al. 1998). This means that they use areas in cereal crops with sparser, shorter vegetation and greater invertebrate densities when feeding nestlings (Morris et al. 2002). Corresponding with the findings described above, Whittingham & Markland (2002) showed experimentally that seed intake rates are higher when birds forage on bare ground than amongst vegetation, probably because there is a smaller surface area over which they need to scan with the former substrate.
All these preferences probably result from the combined influences of food abundance and availability. Fertilizer treatments can increase invertebrate biomass, providing more food for some species (Tucker 1992). Skylark densities are related to seed (especially weed seed) densities (Wakeham-Dawson & Aebischer 1998, Robinson & Sutherland 1999). Skylark intake rates are affected by both seed density and seed availability, the latter being lower when seeds are buried: surface seed availability controls bird density and these seeds are depleted first (Robinson 2001). For a range of seed-eaters, Robinson & Sutherland (1999) tested the relative influences of field characters such as position, aspect, crop structure and size on the selection of fields for foraging and found that the preferred stubbles had the highest seed densities and the largest reductions in seed density between November and March (presumably through depletion). The balance between seed depletion and replacement by seed set from growing weeds was also important. Conversely, Hart et al. (2002) found that seed density was unimportant in comparison to crop composition (till versus autumn-sown cereal versus stubble) and habitat features in determining the abundance of Skylark and Yellowhammer across arable fields. However, they sampled seed to a depth of 5mm (deeper than most birds could reach) and at only two points in most fields (although seed densities can be highly heterogeneous within fields: Robinson & Sutherland 1999). Hart et al.’s measure of seed density might not accurately reflect availability, therefore, and there an additional influence of perception may results from their approach of adding additional seed to fields: birds may not have discovered the augmented seed banks in certain winter cereal fields because they rarely sample habitats that appear unlikely to hold much seed.
Moorcroft et al. (2002) investigated the factors underlying foraging site selection among stubble fields for Grey Partridge, Woodpigeon, Skylark, Linnet, Chaffinch, Reed Bunting, Yellowhammer and Corn Bunting. The most important predictors of abundance were the preceding crop, percentage bare ground and percentage dietary weed cover (weeds were recorded as having seeds important in bird diets or “other”). Although these variables were highly inter-correlated, making interpretation complicated, one or more of them was significant for each of the species considered, and other potential influences such as field boundary characteristics were never significant. Moorcroft et al. also studied Linnets’ foraging site selection within fields and found that the birds chose areas with higher densities of dietary weed seeds (>300m-2) and greater percentages of bare earth. In general, it appears again that both weed seed abundance (as shown by the coverage of dietary weeds) and availability (as shown by the area of bare earth) determine foraging activity. Moorcroft et al. suggest that the lack of a positive response to bare ground in Skylark and Woodpigeon reflects their consumption of vegetation as well as seed; the other species need bare ground to gain access to their seed food and also potentially pay a physiological cost from moving through such dense, wet vegetation as characterizes undersown grass in a stubble. There may also be a cost in terms of a greater requirement for vigilance in denser vegetation (Devereux et al. 2004).
Field boundary structures provide cover and corridors for foraging birds in winter, and also determine how enclosed a field appears from a bird’s perspective. However, the lack of any need for nesting habitat in winter may mean that many species are less tied to well-developed hedgerows than they are in spring and summer. Nevertheless, Perkins et al. (2000) found that more enclosed grass fields (those with more, higher hedgerows) were selected by Blackbird Turdus merula and Magpie Pica pica, and that the preferences of six other species, including Skylark and Yellowhammer, were also affected by enclosure, although in less easily interpretable ways (enclosure was tested rather coarsely, as a fourlevel categorical variable). The same species, as well as other thrush species, were found to respond positively to the presence of more hedgerow around permanent grass and ley fields by Tucker (1992).
In arable areas, species richness in winter is generally higher in larger hedges with larger ditches and more trees, a pattern which also occurs in summer (Parish et al. 1994, 1995), and seven species, including Greenfinch, Yellowhammer, Tree Sparrow and Linnet occur at greater densities in wild bird cover crops with a higher percentage of hedgerow in their boundaries and/or higher boundaries (Henderson et al., 2004). Conversely, Skylarks prefer to forage further from field boundaries (Robinson & Sutherland 1999, Buckingham 2001, Thomsen et al. 2001) and Corn Buntings prefer wild bird cover crops with open boundaries and low hedgerows (Henderson et al., 2004). However, for Skylarks at least, this aspect of foraging behaviour is plastic, and they forage closer to hedgerows as the winter progresses, perhaps because of depletion in winter cereal and sprayed stubble field centres (Robinson & Sutherland 1999, Buckingham 2001). In these fields, seed densities are highest near to field margins throughout most of the winter and these areas form the primary feeding habitat for Yellowhammers (and perhaps also Linnets and Tree Sparrows: Robinson & Sutherland op. cit.). In set-aside stubbles, where seed distributions are more even, this foraging pattern leads to greater depletion from near to field edges by the end of winter (Robinson & Sutherland op. cit.) and the relatively abundant resource towards the centre of such unsprayed stubbles may explain the lack of any significant influence of boundary structure on their use (Moorcroft et al. 2002).
A combination of the proximity to nest sites and a similar pattern of greater weed seed and invertebrate abundance nearer to field boundaries leads to a frequent selection of field margin habitats by birds in summer (Holland et al. 2002, Marshall 1989, Wilson & Aebischer 1995, Vickery et al. 2002). The concentration of seed nearer to field edges may sometimes be the result of seed dispersal from parent plants among field boundary vegetation. In addition, a long-term lower efficiency of cultivation and herbicide application near to field boundaries allowing a build-up of the weed seed bank in these less intensively farmed peripheral areas is also probably important (Wilson & Aebischer 1995). The similar concentration of invertebrate species richness and density nearer to field edges (Holland et al. 2002) is probably driven by the seed and plant distribution. Although little is known currently about the within-field distribution of foraging farmland birds in winter (Vickery et al. 2002), food resources not derived directly or indirectly from the crop must presumably still be concentrated nearer field edges. Where birds actually forage will be determined by this distribution, but in an interaction with other factors such as the perceived risk of predation and food accessibility.
The features of the landscape surrounding a field are likely to influence the birds that are able to use it as a foraging site in winter. Corn Buntings in north-east Scotland illustrate the scale of “catchment” that small areas and a few fields can have: all the breeding birds from one 38km2 study area were found almost entirely within one 2km2 area in winter (Watson & Rae 1997).
Arnold (1983) found that bird numbers on central survey plots in winter were strongly influenced by habitat at the landscape scale. More recent studies have produced broadly consistent results, such as a positive effect of nearby permanent pasture on species richness in field margins (Parish et al. 1994). Henderson et al. (2004) found that the use of wild bird cover crops relative to that of the surrounding fields varied according to whether the latter were grassland or arable: the increase in density from the fields to the cover crops was greater in grassland for six species, including Tree Sparrow, Linnet, Chaffinch and Reed Bunting, and greater in arable farmland for four species, including Bullfinch and Yellowhammer. Where the difference is greater, it could mean either that the cover crops draw in more of the birds in the surrounding environment (implying that the latter is relatively poor in resources) or that there are more birds to be drawn in because the surrounding environment supports them.
Another important feature of the landscape is the presence of human habitation and farmyards near fields. In grassland in winter, five species including Chaffinch and Yellowhammer were found more often in fields adjacent to buildings than in those further away, probably in response to the presence of grain and cattle feed in farmyards (Perkins et al. 2000). Three species were more common in grass fields that were adjacent to stubble fields, but these were corvids rather than specialist granivores, so this may reflect these species’ use of both seed and invertebrate resources (ibid.).
The best studies of landscape-scale effects on arable field use in summer come from North America and give conflicting results. Best et al. (2000) found that the use of survey plots by species that are generally found in grassland or woodland was strongly related to the area of these habitats in a surrounding circle of radius 800m, while other species (aerial foragers and habitat generalists) responded only to the arable habitats themselves and were unaffected by the wider landscape. Conversely, Jobin et al. (2001) found that distances to the nearest building, woodlot, hedgerow and road had no significant influence on the bird use of three types of field boundary adjacent to arable fields. An important caveat to studies of landscape influences like this is that the results are likely to depend on the scale at which landscape is measured and the exact variables that are chosen; the scale and variables that are important are also very likely to vary between species.
Influence of field size
Effects of field size are likely to be confounded to large extent with effects of boundary structure and of field enclosure, but there may also be an effect of size per se. Skylarks avoid boundary vegetation in winter, as in spring and summer, and are more abundant on larger fields, those larger than 7.5ha being selected (Gillings & Fuller 2001, Thomsen et al. 2001, Moorcroft et al. 2002). Linnets may also prefer larger fields, but the only published evidence here might be explained by a random distribution across field sizes (Moorcroft et al. 2002). Among wild bird cover crops, larger plots supported lower bird densities throughout the winter, although this result could be an artefact of differences in detectability, which was lower in larger plots (Henderson et al., 2004). However, this suggests that there may be an optimum plot size to maximize both numbers and density of birds when a certain area is available for sacrificial crops within a given area of farmland. There was no indication of a threshold plot size below which densities declined (ibid.).
3.3 Supplementary Feeding
The review above focuses on the use of growing crops as foraging sites, but research is also increasingly being conducted into the provision of seed directly to birds. Gamebirds, especially Pheasants Phasianus colchicus, are routinely fed with grain throughout the winter and beyond wherever shooting is practised and the approaches used and their benefits in terms of weight gain and productivity are well-established (e.g. Draycott 2002). The seed is usually provided in hoppers designed to release seed on demand but to keep it dry and out of reach of non-target animals, but can nevertheless form a well-used food resource for other farmland birds such as Yellowhammers (Bradbury & Stoate 2000).
Where seed is supplied solely to feed birds of conservation concern, the target species can show clear preferences for particular seed types. Tree Sparrows select seed for their energy content, size and ease of husk removal (husks dramatically increase handling time), leading them to prefer sunflower hearts over all other seed types tested and grain and weed seeds over oilseed rape and grass seeds (Perkins & Anderson 2002). The results of Whittingham & Markland (2002) suggest that seed should be provided on a bare earth substrate if intake rates are to be maximized. This would benefit individuals in that they would need to spend less time foraging on an artificial food patch and therefore less time at risk from predators, particularly Sparrowhawks Accipiter nisus, but is perhaps unlikely to have any (even local) population effect because seed intake rate at a patch with effectively ad libitum availability is unlikely to be limiting: food will still probably be plentiful even if intake rates are slow. A further consideration for the provision of artificial seed supplies is that more seed in dense patches will not necessarily lead to a proportionately greater value for the birds being targeted. Johnson et al. (2001) found that, while handling time (the reciprocal of intake rate) tends to fall as House Sparrow group size on a food patch increases to three, it then rises again with larger groups because of vigilance to avoid interference competition. The later effect also seems to be greater at higher seed densities (ibid.). These results suggest that artificial seed sources will have to be constructed with attention to details such as seed density and substrate type if the efficiency of provision to the target species is to be maximized.
The aim of supplementary feeding in terms of conservation will probably always be to bring about increases in abundance, at least locally. Browne & Aebischer (2002) provided seed, ad libitum, to Turtle Doves on their breeding grounds, but found no effect on density or breeding success when experimental areas were compared with controls, despite the heavy use the birds made of the food provided. They concluded that they had not provided food at a large enough scale, given the areas over which Turtle Doves range, but it is perhaps more likely that food availability simply does not currently limit breeding success or density. The birds probably use the supplementary seed because it represents a more convenient food supply, but this just replaces their usual foraging habitat or resources. Other evidence suggests that the British Turtle Dove population is not limited by breeding success and that it is the situation in winter that may be critical (Siriwardena et al. 2000). This implies that the factors causing decline and preventing recovery in this species might best be sought in their migrations or on the wintering grounds. Another study has actually found that supplementary feeding (again during the breeding season) can actually cause local Western Bluebird Sialia mexicana populations to decline, probably because the mean fitness of individuals surviving after feeding is lower than it was before (Keyser & Promislow, submitted). However, modelling showed that this result of feeding was likely to occur only under rare conditions, namely when only a small proportion of the population was fed in poor years (Keyser & Promislow, op. cit.). A more positive result of supplementary feeding (in winter) was found by Hole et al. (2002), who demonstrated an increase in over-winter survival for House Sparrow at a farm site where survival was otherwise unsustainably low; three other sites experienced no such increase, but their local populations had higher initial survival rates and were therefore less likely to be limited by survival.
The success of a future agri-environment scheme based around providing supplementary food resources in the form of sacrificial crops grown specifically for birds or the retention of seed-rich over-winter stubbles is likely to depend, at least in part, on the context and characteristics of the fields or parts of fields concerned. To target such a scheme most effectively, the key factors influencing field use should first be identified, so that they can be incorporated in the planning of the scheme. We have reviewed above the evidence for those factors that we believe are likely to be the most important, where it exists, and our conclusions are summarized in Table 3.1.
There is currently no, or very little, published research on the influence of field size per se (i.e. independent of boundary structure), predation risk, travel time, disturbance or the landscape context on the selection of fields for foraging by farmland birds in winter (Table 3.1). However, reliable predictions can probably be made with respect to the roles of crop substrate, boundary features and vegetation structure. To a certain extent, these factors will have species-specific effects, so it will be important either to use a range of field types and contexts or to target only a few species (being aware that some species of conservation concern might then not benefit at all). Nevertheless, we can make general recommendations such as that both food abundance and availability should be considered and that heterogeneous vegetation is beneficial. Thus, crops planted specifically for birds should not be too dense and should have a variation in height and structure that might best be achieved by sowing two or more seed-bearing crops together, particularly if one or more of these includes a crop that retains seed late into winter e.g. quinoa. In addition, the management of fields to be retained as over-wintered stubbles should be carefully controlled so that they provide valuable seed-rich habitat. This is best done by promoting a “weediness” in stubbles, especially where some of those weeds germinate and set seed in the stubble, replenishing seed resources in late winter The selection of fields to be managed in this way should be contingent on characteristics such as field size and boundary features in order to maximize their value to whatever species are to be targeted. In general, fields with good quality hedgerows are likely to attract most birds of the widest range of species (with notable exceptions such as “open-field” species like Skylark) and relatively undisturbed sites are likely to be favoured over those that are close to farm buildings, for example.
Most of the research to-date that is summarized here relates to whole fields, but sacrificial crops to feed birds are likely often to be sown as patches in fields, as is currently the practice with cover crops for game birds. There has been some useful research into the use of plots like this by birds (Henderson et al., 2004), but key issues such as the effects of the surrounding cropping regimes and the distance from a sacrificial crop area within a field to a source of cover for small birds remain unaddressed. This could have important implications for the practicalities of establishing sacrificial crops within fields under agri-environment schemes, where a limited area is likely to be supportable financially for planting the crops and where feeding the maximum number of birds will be a key aim of the organizing bodies. This is one area where the results of our winter feeding experiment 9Objective 2 and appendix 2) should help to inform field selection. At a larger scale, our analyses of breeding bird abundance data and information on breeding and wintering habitat availability (Appendices 1a-1d) will aid the selection of regions in which to prioritize the provision of supplementary food in winter.
Table 3.1 Summary of the factors likely to influence field use by granivorous farmland birds in winter.
Effects on field selection by birds
Crops and management regimes providing high seed abundance and availability are preferred by granivorous birds. Game cover (and similar) crops are selected where available, but stubbles are preferred elsewhere. However, stubbles vary hugely in value for birds with management. Some species will use small plots of cover crops but are less common in stubbles, whilst others will use only open stubble fields (c.f. Greenfinch and Skylark). Crops in which seed remains available into late winter are particularly valuable.
Many species use the cover afforded by well-developed hedgerows and also benefit from foraging near field boundaries because food availability is higher there. However, other species (notably Skylark) prefer to avoid boundary features and are thus disinclined to use more enclosed fields.
A more heterogeneous vegetation structure increases both food abundance and accessibility for many species, especially if there are areas of bare ground between clumps of vegetation.
Likely to be important, but mechanisms currently unclear, partly because of variations in the scale at which landscape has been studied. Factors such as the effects of adjacent cropping and nearby hedgerow structure and connectivity need to be studied in more detail.
Confounded with boundary presence/field enclosure, but some species prefer large fields (e.g. Skylark) and others are likely to prefer access to cover near food.
Yet to be studied, but likely to be affected by the availability of cover (for feeding birds and predators) and flock size that food supply supports.
Yet to be studied, but the balance between the severity of disturbance and need for food is probably critical.
Yet to be studied – its influence will depend on the roosting and flocking behaviour of birds in different landscapes.
Arnold, G.W. (1983) The influence of ditch and hedgerow structure, length of hedgerows, and area of woodland and garden on bird numbers on farmland. J. Appl. Ecol. 20: 731-750.
Best, L.B., Bergin, T.M. & Freemark, K.E. (2000) Influence of landscape composition on bird use of rowcrop fields. J. Wildl. Manage. 65: 442-449.
Boatman, N.D. & Stoate, C. (2002) Growing crops to provide food for seed-eating farmland birds in winter. pp 229-238 in Boatman, N.D., Carter, N., Evans, A.D., Grice, P.V., Stoate, C. & Wilson, J.D. (eds) Aspects of Applied Biology 67: Birds and Agriculture. Association of Applied Biologists, Warwick.
Bradbury & Stoate 2000 pp in Aebischer, N.J., Evans, A.D., Grice, P.V., Vickery, J.A. (eds) Ecology and Conservation of Lowland Farmland Birds. British Ornithologists’ Union, Tring.
Brickle, N.W. & Harper, D.G.C. (2000) Habitat use by Corn Buntings Miliaria calandra in winter and summer. pp 156-164 in Aebischer, N.J., Evans, A.D., Grice, P.V. & Vickery, J.A. (eds) Ecology and Conservation of Farmland Birds. Proceedings of the 1999 British Ornithologists’ Union Spring Conference. British Ornithologists’ Union, Tring.
Browne, S.J. & Aebischer, N.J. (2002) The effect of supplementary feeding on territory size, territory density, and breeding success of the Turtle Dove Streptopelia turtur: a field experiment. pp 21-26 in Boatman, N.D., Carter, N., Evans, A.D., Grice, P.V., Stoate, C. & Wilson, J.D. (eds) Aspects of Applied Biology 67: Birds and Agriculture. Association of Applied Biologists, Warwick.
BTO (2003) The effects of different crop stubbles and straw disposal methods on wintering birds and arable plants. Final Project Report to DEFRA, project BD1610. British Trust for Ornithology, Thetford, Norfolk.
Buckingham, D.L. (2001) Within-field habitat selection by wintering skylarks Alauda arvensis in southwest England. pp 149-158 in Donald, P.F. & Vickery, J.A. (eds) The Ecology and Conservation of Skylarks Alauda arvensis. RSPB, Sandy.
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GS/Winter Feeding II/Appendices CSG15/Appendix 3