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Appendix 3.
3.1
Factors influencing the use of fields and foraging patches by birds: a
review of the literature
Introduction
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 governmentsponsored 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
Crop type
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.
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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
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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
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(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 boundaries
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).
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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 setaside 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 withinfield 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.
Landscape
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.).
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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.
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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.
3.4
Discussion
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 overwinter 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
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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.
Field Variable
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.
Crop type
Boundary
features
Vegetation
structure
Landscape
context
Field size
Predation risk
Disturbance
Travel time
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