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Abstract

  1. Top of page
  2. Abstract
  3. HOW DOES HABITAT STRUCTURE ALTER PREDATION RISK FOR FORAGING AND NESTING BIRDS?
  4. CAN PREDATION RISK ALTER HABITAT CHOICE AND IF SO BY HOW MUCH?
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

It has been suggested that increased predation rates may rival habitat alteration as a causal agent in farmland bird population declines. Such a view may be over-simplistic, however, as changes in habitat structure may influence habitat selection and foraging efficiency through their influence on perceived and actual predation risk. We review evidence from the literature on the effects of habitat structure on predation risk of foraging and nesting birds and apply these principles to investigate the likely effects on the 20 species that comprise the UK Government's ‘Farmland Bird Index’. Shorter vegetation is likely to enhance foraging efficiency and reduce predation risk (when ground foraging) for 15 of the 20 species. However, within grassland systems longer vegetation is known to enhance food supplies (e.g. Tipulid larvae and voles) of several farmland bird species and so mosaics of short and long vegetation may provide the optimum conditions for most species (e.g. Lapwing Vanellus vanellus, Starling Sturnus vulgaris, Barn Owl Tyto alba). Agricultural intensification has encouraged uniform dense swards, thus reducing habitat diversity, and agri-environment schemes that provide heterogeneous sward structure may thus facilitate farmland bird conservation. Intensification has also resulted in less dense hedgerows; although a reversal of this trend may improve foraging efficiency for many species, it may be detrimental to a smaller number of species that prefer shorter, less dense hedges for nesting. Before these tentative conclusions can be confirmed, more research is required that considers how the effects of habitat structure on individuals is likely to translate into population-level impacts.

Ecologists have traditionally considered the most obvious effect of predators, that of eating the prey, on predator–prey interactions and population dynamics. However, predators also have other indirect effects on their prey, including influencing where they choose to feed, breed and sleep. Although most of the studies in this area have focused on behavioural differences of animals under differing levels of predation risk, there are also a number of studies that have attempted to link these non-lethal effects of predation with demographic processes (see reviews in Lima & Dill 1990, Lima 1998a, 1998b, Evans 2004).

Increased predation rates are sometimes viewed as a cause of the widespread and severe declines in European farmland bird communities, others argue that habitat change is the causal factor and the debate has become somewhat polarized between these two views (e.g. Thompson et al. 1998). It is clear that controlling predators can result in population increases for some prey guilds, at least at a local scale (Tapper et al. 1996, Paradis et al. 2000, Stoate & Szczur 2001). On the other hand, loss of habitat or its degradation through changing management has been clearly linked to population decline for the Corncrake Crex crex (Green et al. 1997) and Cirl Bunting Emberiza cirlus (Evans & Smith 1994, Peach et al. 2001) and is strongly suspected to be causal for a number of other species (Potts 1986, Hudson et al. 1994, Donald & Forrest 1995, Wilson et al. 1997, Anderson et al. 2001). This polarized debate is naïve, however: the two processes are inextricably linked, as habitat degradation may make prey more vulnerable (Evans 2004; Table 1). For instance, invertebrate declines may have left chicks malnourished and hungrier chicks may beg for food more loudly in order to attract their parents’ attention and incur an associated cost of increased risk of detection by a predator (Evans et al. 1997). Equally, predation pressure may make existing habitat less suitable; for example, birds may reduce their use of food sources located far from cover when predation risk is high.

Table 1.  An outline of some of the mechanisms through which habitat change may interact with predation rates to generate farmland bird population declines. The examples given here are derived from Evans (2004) and from examples given in this paper.
Habitat changeEffect of habitat changeEffect on predationSource
Reduced abundance of high-quality nesting habitat, e.g. in Spain many farmland species prefer to nest in fallow fields, which are relatively rareHigh nest density in preferred habitat typesHigh nest densities attract predators and birds nesting in preferred habitat types thus experience increased nest predation ratesPescador and Peris (2001)
Reduction in volume of non-cropped vegetationNest concealment reducedPredator detection on nests increasedSee text
Habitat change reduces the length of the breeding season, e.g. drainage of wet meadows reduces the breeding season of Common Snipe Gallinago gallinagoFewer re-nesting opportunitiesThe predation risk per nesting attempt is unaffected, but predation is more likely to prevent each pair successfully rearing youngGreen and Robins (1993)
Habitat alteration reduces food abundanceHungrier nestlings beg moreIncreased detection of nests by predators leads to increased predation ratesEvans et al. (1994)
Habitat alteration reduces food abundanceAdults forage further from nestParents are less likely to detect nest predators, leading to reduced nest defence and higher nest predation ratesSchmidt (1999)
Habitat alteration reduces food abundanceForaging birds accept a higher risk of predation in order to maintain intake ratesPredation rates increaseVan der Veen (2000)
Habitat alteration alters probability of predator detection or avoidanceVigilance rates change, e.g. birds forage in areas of lower food availabilityPredator detection rates changeSee text

In this review we discuss how variation in within-field and hedgerow habitat structure may impinge on foraging and nesting success through altering predation risk and discuss the potential for such scenarios to influence demography, an area that has received relatively little attention in farmland bird research. This paper is not an exhaustive review of all the studies that report effects of habitat structure on predation risk. Our aim is to provide examples from the literature that illustrate our argument that interaction between vegetation structure and predation risk may influence the demography of farmland bird populations.

HOW DOES HABITAT STRUCTURE ALTER PREDATION RISK FOR FORAGING AND NESTING BIRDS?

  1. Top of page
  2. Abstract
  3. HOW DOES HABITAT STRUCTURE ALTER PREDATION RISK FOR FORAGING AND NESTING BIRDS?
  4. CAN PREDATION RISK ALTER HABITAT CHOICE AND IF SO BY HOW MUCH?
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

Foraging

Taller or denser habitat structure generally increases visual obstruction for foraging birds (Butler & Gillings 2004), and so may increase predation risk (Metcalfe 1984, Cresswell 1994a); alternatively, it may reduce predation rates by improving crypsis (see below). Foraging animals can compensate for an increase in predation risk by being more vigilant (Elgar 1989, Lima & Dill 1990, Quenette 1990, Devereux et al. 2004). Vigilance is generally defined as time spent with the head raised during periods of foraging. Although ‘head-up’ periods may serve additional functions, such as scanning for conspecifics (Coolen et al. 2001) and handling food items (Illius & Fitzgibbon 1994), all models of anti-predator vigilance (e.g. Pulliam et al. 1982, Lima 1987, Packer & Abrams 1990, McNamara & Houston 1992) assume that during these head-up periods predators will be detected more quickly than when the head is lowered searching for food – which is supported by empirical evidence (Lima & Bednekoff 1999).

For species that rely on detecting predators by sight (Lima & Dill 1990), increased visual obstruction has been shown to increase vigilance in a variety of different species from many taxa (e.g. Metcalfe 1984, Goldsmith 1990, Cresswell 1994a, Arenz & Leger 1997a, 1997b). However, few studies have considered how effectively this increased vigilance compensates for reduced visibility and predator detection (Arenz & Leger 1997a). One such recent study was carried out on a farmland bird, the Chaffinch Fringilla coelebs, foraging on artificially created stubble substrates in aviary conditions (Whittingham et al. 2004). As predicted, Chaffinches increased their vigilance levels, by 13%, when foraging in obstructed patches (long stubble 13 cm tall), as compared with patches offering a clear field of view (short stubble 3 cm tall). Despite these differences in vigilance, detection of a moving stuffed Eurasian Sparrowhawk Accipiter nisus was approximately 24% slower in obstructed patches than in patches with a clear field of view. This effect of habitat structure remained even when analysis was confined to data from Chaffinches with their head raised when the model predator became visible in the obstructed patch. Additionally, birds that foraged on long stubble made on average 16.0 ± 0.80 (1 se) pecks/min compared with 18.3 ± 0.74 (1 se) pecks/min on short stubble, a 13% difference. These results suggest that foraging in patches with reduced visibility will impose the dual costs of reduced foraging efficiency and increased predation risk.

These findings are probably applicable to other granivorous species that locate food on the ground and raise their heads to consume it; examples of such farmland species that are of conservation concern include Yellowhammer Emberiza citrinella, Corn Bunting Miliaria calandra, Reed Bunting Emberiza schoeniclus and Common Linnet Carduelis cannabina. Stubbles are favoured field types of many farmland bird species (Wilson et al. 1996, Buckingham et al. 1999) and so simple manipulations of habitat structure could enhance habitats both in terms of increased foraging efficiency and lowered predation risk. The situation may be similar for insectivorous species that probe the ground for food, such as Northern Lapwing Vanellus vanellus and Eurasian Starling Sturnus vulgaris. Recent work on the latter has shown that foraging efficiency was 29% lower in long grass (2.5 ± 0.4 prey captured in 15 min) than in short grass (3.5 ± 0.5 prey captured in 15 min) (C.L. Devereux et al. unpubl. data).

Habitat structure may also influence aspects of foraging behaviour other than predation risk. For example, tall vegetation appears to limit movement of foraging Eurasian Starlings and may partially explain reduced foraging efficiency in such habitats (Devereux et al. 2004). Damp dense vegetation may impose additional energetic demands by wetting the plumage, and thus increasing heat loss, of birds moving through it (Dawson et al. 1992); such an effect seems particularly likely in grass fields during the winter. Dense grass swards may also result in food items becoming inaccessible or more difficult to detect (e.g. Nystrand & Granström 1997, Whittingham & Markland 2002, Butler & Gillings 2004). All of these factors suggest that short sparse vegetation not only lowers predation risk, by increasing visibility, but also benefits foraging birds in other ways. It should be considered, however, that sward structure can have profound effects on invertebrate communities and so mosaics of vegetation structure may hold the key to providing suitable landscapes with both high food abundance and good access for foraging birds (Benton et al. 2003, C.L. Devereux et al. unpubl. data). Red-backed Shrikes Lanius collurio provide a good illustration of this point. They forage at the boundary between long grass, which serves as a reservoir of large insects, and short grass, where the insects become accessible to their ‘perch and pounce’ foraging strategy. Vanhinsbergh (1999) showed experimentally that they select hunting perches with an interface between long and short vegetation over those with just long grass.

One important point about habitat structure and foraging is the way in which birds perceive cover. Proximity to cover can have two effects on a prey animal: it can provide a hiding place and protection from an attack, but it can also obstruct their view of approaching predators (Lazarus & Symonds 1992). All cover is probably both protective and obstructive to some degree (e.g. Lima 1990), although the examples we have used so far focus on species that mainly perceive within-field cover as obstructive (e.g. on attack by a predator Chaffinches typically fly away to cover such as a hedgerow, rather than trying to hide within field vegetation). Therefore, for species that retreat to protective cover such as a hedgerow, rapid detection of predators will be enhanced by short within-field vegetation that could be mediated through habitat manipulation. Other species may use within-field habitats as protective cover and can arguably be identified by their tendency to delay flight when approached by a predator and to adopt the alternative strategy of remaining still and relying on crypsis. Such species, which may include the Grey Partridge Perdix perdix and Corncrake, would probably benefit from longer within-field vegetation if not too dense.

Perhaps the best illustration of the differences in perception of cover between species comes from analysing variation in hedgerow use. Robinson and Sutherland (1999) found that wintering Skylarks Alauda arvensis depleted food resources in the middle of fields first and only moved closer to hedgerows once food became less abundant in field centres. Conversely, Yellowhammers were far more likely to forage close to hedgerows and seldom strayed into the central part of fields. The differences between the two species are most likely to be explained by their different predator avoidance strategies. Yellowhammers, like most other farmland granivores (e.g. Reed Bunting, Corn Bunting, Chaffinch – see Table 2), seek cover to protect them from attack and so select foraging areas close to hedges. Skylarks employ a very different strategy by taking to the air and attempting to out-fly their predators or by crouching (Cresswell 1994b) and so hedgerows are likely to increase predation risk as they may adversely affect the detection of predators (see also Whittingham et al. 2003). In addition, Canadian studies of granivores, all of which use cover to flee from predator attacks, suggest that subdominant species are forced to forage further from cover (Lima et al. 1987). Overall, we suspect that only a few species, such as Skylarks, would experience improved foraging conditions if hedgerows were kept short or removed entirely, and doing so would reduce nesting opportunities for numerous species.

Table 2.  Possible effects of within-field and hedgerow habitat structure on foraging and nesting of 20 species of farmland bird designated as the ‘Farmland Bird Index’ by the UK Government (including Barn Owl see Gregory et al. 2004). This table is subjective and uses the principles outlined in this review coupled with knowledge of the behaviour of the 20 species in agricultural environments. Some of the species in the table often feed off the ground (e.g. Goldfinches often feed by perching on plants); here we are referring to the effects of habitat structure when they are on the ground. (1) Relationship based on experimental evidence. (2) Relationship based on correlational evidence. (3) Best guess based on other studies. ‘Yes’ means an observed (or presumed) positive effect and ‘No’ means an observed (or presumed) negative effect. *Symbols refer to the likely effects of short vegetation on foraging efficiency and predation risk: + (a probable increase in foraging efficiency), +? (a possible increase in foraging efficiency), = (no likely effect), –? (a possible negative effect), – (a probable negative effect). Note that these symbols are simplistic; the precise details of the relationships for each individual species are likely to differ and are highly unlikely to be linear, e.g. very short grass (c. 3 cm) enables Starlings to have higher foraging efficiency than taller grass (c. 13 cm) (Devereux et al. 2004), but once grass reaches a height that Starlings are no longer able to see over even when they stand in an alert posture (e.g. c. 20 cm) any subsequent increases in height are likely to have little effect on foraging efficiency. These species-specific foraging responses to vegetation height are likely to be mainly affected by body size.
SpeciesForagingNesting
Effect of shorter within-field vegetation on foraging efficiency and predation risk*Hedgerow generally used as protective cover when foraging (i.e. retreat to hedgerow when alarmed)Are taller, thicker hedgerows preferred for nesting?Source (references refer to one or more columns)
  • A

    Woodpigeons prefer stubble fields with more grass (Moorcroft et al. 2002).

  • B

    Longer grass (> 20 cm) is suitable for voles, the main prey of Barn Owls and Kestrels, but voles may be easier to catch in shorter vegetation (Askew pers. comm.).

  • C

    Although Yellow Wagtails will benefit from taller vegetation within fields for nesting (Bradbury & Bradter 2004).

  • D

    Associated with taller, thicker hedges, therefore more likely to perceive hedges as protective (Parish et al. 1995).

  • E

    Tipulids (whose larvae form the main prey of Starlings) are more likely to deposit eggs in or on the soil when the vegetation is taller (McCracken et al. 1995), therefore a mosaic of habitats may be preferable for Starlings (Devereux unpublished data).

  • F

    Associated with shorter, narrower hedges in winter, therefore more likely to perceive hedges as obstructive (Parish et al. 1995), although will sometimes retreat to hedges when alarmed (pers. obs).

  • G

    Assumed to behave in a similar manner to the granivorous Chaffinch (Whittingham et al. 2004).

  • H

    Fields surrounded by short hedges are used preferentially (Whittingham et al. 2003).

Kestrel Falco tinnunculusUncertainBNot generally applicable in lowland farmland in the UKSpecies does not generally nest in hedges.Village (1990)
Grey Partridge Perdix perdix–? (2)Sometimes (2)Species does not generally nest in hedges.M. Watson (pers. comm.). Personal observation
Lapwing Vanellus vanellus+ (1)No (2)Species does not generally nest in hedges.Devereux et al. (2004)
Woodpigeon Columba palumbus+? (3)AYes (3)Yes (2)Moorcroft et al. (2002), Hinsley and Bellamy (2000)
Stock Dove Columba oenas+? (3)Yes (3)Species does not generally nest in hedges.Cramp (1985)
Turtle Dove Streptopelia turtur+? (2)Yes (3)Yes (3)Browne and Aebischer (2001)
Barn Owl Tyto albaDependsBNot generally applicable in lowland farmland in the UKSpecies does not generally nest in hedges.Askew (pers. comm.)
Skylark Alauda arvensis+ (2)No (2)Species does not generally nest in hedges.HRobinson and Sutherland (1999), Morris et al. (2004)
Yellow Wagtail Motacilla flava+ (2)C?Species does not generally nest in hedges.Bradbury and Bradter (2004)
Whitethroat Sylvia communis= (does not generally feed on ground within fields) Yes (3)DPrefers intermediate hedges.Hinsley and Bellamy (2000)
Starling Sturnus vulgaris+ (1)ESometimes retreats to hedges but sometimes flocks will attempt to fly from predatorsSpecies does not generally nest in hedges.Devereux unpublished data
Jackdaw Corvus monedula+? (3)Not generally applicable in lowland farmland in the UKSpecies does not generally nest in hedges.Cramp and Perrins (1994a)
Rook Corvus frugilegus+? (2)Not generally applicable in lowland farmland in the UKSpecies does not generally nest in hedges.Perkins et al. (2000)
Tree Sparrow Passer montanus+? (3)GYes (2)Species does not generally nest in hedges.Robinson (1997)
Greenfinch Carduelis chloris+? (3)GYes (3)Yes (3)Cramp and Perrins (1994b)
Goldfinch Carduelis carduelis+ (1)Yes (3)Yes (3)Glück (1986), Cramp and Perrins (1994b)
Common Linnet Carduelis cannabina+? (3)GDepends (3)FNoRobinson (1997), Parish et al. (1995), Hinsley and Bellamy (2000)
Corn Bunting Miliaria calandra+? (3)GYes (2)Species does not generally nest in hedges.Robinson (1997)
Yellowhammer Emberiza citrinella+? (3)GYes (2)No (2)Robinson and Sutherland (1999), Hinsley and Bellamy (2000), Bradbury et al. (2000)
Reed Bunting Emberiza schoeniclus+? (3)GYes (3)Species does not generally nest in hedges.Cramp & Perrins (1994b)

Nesting

Increased vegetation density and heterogeneity may reduce nest predation rates as both increase nest crypsis (reviewed by Newton 1998, Willson et al. 2001). Predation rates are not always related to the degree of nest concealment (Howlett & Stutchbury 1996, Burhans & Thompson 1998, Braden 1999), probably because some predators use non-visual stimuli for nest detection (e.g. Rangen et al. 1999) or because a trade-off exists between nest crypsis and the ability of parents, at the nest, to detect predators (Götmark et al. 1995, Cresswell 1997, King et al. 1999).

Stone Curlews Burhinus oedicnemus, Northern Lapwings, Golden Plovers Pluvialis apricaria and Skylarks all have well-developed anti-predator behaviours (Cramp & Perrins 1994a). They thus select flat relatively sparsely vegetated ground on which to nest (e.g. Galbraith 1988, Wilson et al. 1997, Wakeham-Dawson et al. 1998, Green et al. 2000, Whittingham et al. 2002). Other ground-nesting species such as Corncrakes and Grey Partridges nest in dense herbaceous cover and rely on concealment from predators (e.g. Rands 1986, Green & Stowe 1993). The tendency for summer swards to be denser and taller than in the past, for example owing to a reduction in spring-sown cereals, is likely to increase the predation risk experienced by the first set of species, but may reduce it for the latter suite of species (Wilson, unpublished data).

Many declining farmland birds prefer to nest in hedgerows, e.g. Common Linnet, Yellowhammer, Bullfinch Pyrrhula pyrrhula and Cirl Bunting (Newton 1993, Evans et al. 1997, Bradbury et al. 2000, Moorcroft 2000). Some of these species prefer tall dense hedges for nesting (e.g. Bullfinch) whereas others prefer shorter hedges (e.g. Linnet, which also shows no preference for hedge width) (Hinsley & Bellamy 2000). Hedgerow density may be linked to nest detectability and thus with predation rates for all of these species, with the possible exception of linnet. Although few studies have investigated this hypothesis it is supported by Hatchwell et al. (1996), who found that successful Blackbird Turdus merula nests were less conspicuous than nests that were predated. The reduced quality of hedgerows, in terms of an increased number of gaps and reduced thickness (Joyce et al. 1988), may have thus increased the rates of nest predation experienced by many declining farmland birds.

CAN PREDATION RISK ALTER HABITAT CHOICE AND IF SO BY HOW MUCH?

  1. Top of page
  2. Abstract
  3. HOW DOES HABITAT STRUCTURE ALTER PREDATION RISK FOR FORAGING AND NESTING BIRDS?
  4. CAN PREDATION RISK ALTER HABITAT CHOICE AND IF SO BY HOW MUCH?
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

It has been demonstrated, in a wide variety of natural systems, that predation risk can alter habitat choice (Lima 1998a). Most of this work has been conducted on aquatic animals and has demonstrated increases, sometimes striking, in the use of habitats that contain less food, but provide a reduced predation risk (e.g. Werner et al. 1983, Brabrand & Faafeng 1993, Persson & Eklöv 1995). These predator-induced shifts in habitat selection have been shown to influence growth (Werner et al. 1983, Leibold 1991), survival (Tonn et al. 1992, Brabrand & Faafeng 1993), community structure, ecosystem dynamics (Brabrand & Faafeng 1993) and population levels, both locally and at larger scales, i.e. entire lakes (Persson et al. 1996).

The difficulty of conducting such studies in more open terrestrial systems, in which the range of both predator and prey is less easy to define, has probably contributed to the relative paucity of similar studies in such habitats. However, some studies of mammals have shown habitat shifts away from areas of high predation risk, e.g. Hedgehogs Erinaceus europaeus (Doncaster 1994) and Mountain Sheep Ovis canadensis (Berger 1991) (see also Evans 2004). Few studies exist for birds. Hilton et al. (1999) demonstrated that Redshanks Tringa totanus visited saltmarsh, the habitat with the highest density of food, only when they were energetically stressed during periods of harsh weather. They avoided saltmarsh areas during periods of milder weather because they were closer to cover, which increased the likelihood of predation by Sparrowhawks. The difference in foraging efficiency of Redshanks between the saltmarsh and other intertidal areas was not presented and so it was not possible to determine the extent to which predation risk influenced foraging efficiency. Lima and Valone (1991) experimentally manipulated cover of shrubs in open grasslands in winter. They found that the addition of cover caused a marked increase in abundance of species with cover-dependent escape strategies, while the abundance of ‘cover-independent’ species reduced markedly. Neither changes in food abundance nor in microclimate, associated with shrub manipulation, appeared to explain these results. Butler et al. (in press) investigated the magnitude of the difference in food density necessary to equilibrate use of two patches (low risk – short 3 cm stubble; high risk – long 13 cm stubble) typical of both the food and the stubble densities found under natural conditions. Food density was approximately two and a half times greater in the longer stubble, with higher predation risk, before use of the two patches was similar. Predators can alter the habitats chosen to place nests. Bald Eagles Haliaeetus leucocephalus in the Aleutians use accessible nest-sites only when predators are absent (Sherrod et al. 1977). Similarly, Buzzards Buteo buteo and Kestrels Falco tinnunculus regularly nest on the ground on Scottish islands where there are no wild mammalian predators, but not when they are present (Newton 1979).

The difficulties involved with measuring how habitat structure may affect populations at the local scale, or beyond, have severely restricted avian studies in this area. One exception is Thirgood et al. (2002) who found that although the cover of heather significantly influenced Red Grouse Lagopus lagopus density, vegetation structure itself had no effect on survival. In addition, there was no difference in the success of attacks by Hen Harriers Circus cyaneus on Grouse broods in areas of differing vegetation height. We think two points are pertinent here. First, Hen Harriers hunt using mainly auditory cues (Rice 1982, Simmons 2000) and so vegetation height may be of far less importance in avoiding capture than in predator–prey systems where the predator uses visual cues (e.g. Sparrowhawks hunting on farmland). Secondly, Thirgood et al. (2002) compared natural variation in cover with survival. Habitats on grouse moors are far less variable both spatially and temporally than crops on lowland farmland, e.g. imagine the changes in density and height in a typical arable field over the course of a year. We believe that habitat-mediated predation risk may be more likely to have a significant impact on avian population dynamics in lowland farmland than in other habitats, such as moorland. More generally, however, it seems likely that by restricting the habitats available for nesting, predators can indirectly contribute to changing population density levels (Newton 1998).

DISCUSSION

  1. Top of page
  2. Abstract
  3. HOW DOES HABITAT STRUCTURE ALTER PREDATION RISK FOR FORAGING AND NESTING BIRDS?
  4. CAN PREDATION RISK ALTER HABITAT CHOICE AND IF SO BY HOW MUCH?
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

It seems plausible that habitat structure affects the choice of habitats used by foraging and nesting birds in agricultural environments. To illustrate this point we have attempted to summarize the effects of habitat structure on foraging (when on the ground) and nesting on those species included in the Farmland Bird Index, an index of 20 species compiled by the BTO and RSPB and used by the UK Government to indicate the health of our farmland birds (Table 2). Most of the relationships in Table 2 were drawn from correlative studies so they may not reflect a causal link with predation risk. However, the body of this review has suggested mechanisms by which predation risk is likely to be causal in these situations, although there is a general lack of relevant research to support the conclusion that predation risk and habitat quality have interacted to generate declines in farmland bird populations (Evans 2004).

Short within-field vegetation may enhance foraging efficiency when ground-feeding (via reducing the time spent looking for predators and improving access to food) and reduce actual predation risk (via more rapid detection of predators) for 15 of the 20 species included within the ‘Farmland Bird Index’, with likely negative effects on only the Grey Partridge (Table 2). Some species are likely to prefer short within-field vegetation only at particular times of year. For example, foraging Yellowhammers and Corn Buntings are likely to prefer shorter ground vegetation during the winter (where, in England, they favour stubble fields –Moorcroft et al. 2002) but during the breeding season they frequently forage in tall cereals (Cramp & Perrins 1994b, Morris et al. 2001). During the breeding season taller crops seem likely to be perceived as protective rather than obstructive cover.

Within grassland systems longer vegetation is known to enhance food supplies (e.g. Tipulid larvae, McCracken et al. 1995; and voles, Jacob & Brown 2000) of several farmland bird species and so mosaics of short and long vegetation may provide the optimum conditions for such species (e.g. Lapwing, Starling, Barn Owl). Therefore, at least within grassland systems, mosaics of short and longer vegetation are likely to hold the maximum benefit for many farmland birds (Benton et al. 2003, Atkinson et al. 2004, Buckingham et al. 2004). Agricultural intensification has encouraged homogenous, dense swards (Benton et al. 2003 Wilson, unpublished data) and the increasing prevalence of grass leys is likely to have increased sward height for much of the breeding season. Intensive grazing encourages short dense swards but these are likely to be detrimental to both above-ground and below-ground invertebrate populations (McCracken et al. 1995, Wakeham-Dawson et al. 1998, Atkinson et al. 2004) – it is no use providing accessibility if the food is not there. Agri-environment schemes that facilitate access to healthy invertebrate food populations are likely to aid farmland bird conservation.

It is less clear how to manage hedgerows to optimize the benefit to farmland birds. We found an equal number of nesting species are likely to benefit from large hedgerows as would benefit from small ones; yet, when foraging, most species will probably benefit from large hedgerows (Table 2). How best to integrate the various needs of species with differing ecological requirements is an area that has received only limited attention (Swetnam et al. in press).

Providing abundant food supplies for farmland birds is a key prerequisite to conservation. What we suggest here is that a change in habitat structure has the potential to provide significant additional benefits in terms of reducing perceived, and probably actual, predation risk and hence enhancing foraging rates. Careful targeted management of habitat structure thus has the potential to affect habitat choice, but the crucial question is ‘could it affect population size?’. We cannot answer this question with current data, but if the response is affirmative then targeted habitat management of micro- and meso-scale habitats may be a key part of the design of future agri-environment schemes.

Acknowledgments

  1. Top of page
  2. Abstract
  3. HOW DOES HABITAT STRUCTURE ALTER PREDATION RISK FOR FORAGING AND NESTING BIRDS?
  4. CAN PREDATION RISK ALTER HABITAT CHOICE AND IF SO BY HOW MUCH?
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES

Many thanks to Mark Watson, Chris Perrins and Andy Evans for helpful discussions and comments and to Nick Askew for providing information on Barn Owls. Thanks also to two anonymous referees and to Nicholas Aebischer for useful comments. M.J.W. was supported by a BBSRC grant (no. 43/D13408).

REFERENCES

  1. Top of page
  2. Abstract
  3. HOW DOES HABITAT STRUCTURE ALTER PREDATION RISK FOR FORAGING AND NESTING BIRDS?
  4. CAN PREDATION RISK ALTER HABITAT CHOICE AND IF SO BY HOW MUCH?
  5. DISCUSSION
  6. Acknowledgments
  7. REFERENCES
  • Anderson, G.Q.A., Bradbury, R.B. & Evans, A.D. 2001. Evidence for the Effects of Agricultural Intensification on Wild Bird Populations in the UK. RSPB Research Report no. 3. Sandy: RSPB.
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