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The potential energetic gain and predation risk associated with a foraging patch is likely to be determined by an animal's foraging requirements and predator escape strategy (Watts 1990; Suhonen, Norrdahl & Korpimäki 1994). To maximize fitness, many animals must trade-off the conflicting demands of energetic gain and predator avoidance when choosing where to forage (Sih 1980). They are therefore likely to select patches with characteristics that allow them to optimize this trade-off.
For some species, vegetation is perceived as being largely protective, reducing the risk of predator detection, and patches with more complex vegetation structure are likely to have a lower associated predation risk (Ekman 1987; Högstad 1988). Conversely, some species perceive vegetation as largely obstructive, reducing the likelihood of early predator detection, and patches with increased vegetation structure are likely to have a higher associated predation risk (Metcalfe 1984; Sharpe & van Horne 1998). It follows that habitats with more heterogeneous vegetation structure will contain patches that suit the foraging requirements and predator escape strategies of a broader range of species and will therefore probably support a greater species richness.
However, aviary experiments have shown that granivorous passerines alter their foraging and vigilance behaviour in response to vegetation height manipulation (Whittingham et al. 2004). Chaffinches Fringilla coelebs L. foraging in short vegetation (3 cm) had shorter vigilance periods and faster peck rates than those foraging in long vegetation (13 cm). Individuals foraging in the short vegetation also responded significantly faster to simulated attack by a model sparrowhawk Accipiter nisus L. than those foraging in the long vegetation (Whittingham et al. 2004). These findings suggest that both the starvation risk and predation risk an individual of this species associates with a foraging patch could be reduced by lowering vegetation height in that patch. Butler et al. (2005) quantified this relationship between risk and vegetation structure in terms of food abundance. Using a similar experimental set-up, they showed that there needed to be approximately 2·5 times the density of food in a patch of long stubble before individual chaffinches showed parity of use between it and a patch of short stubble (Butler et al. 2005).
These aviary studies (Whittingham et al. 2004; Butler et al. 2005) suggest that stubble height manipulation could be used as a simple management option for increasing the value of stubbles as a foraging habitat for farmland birds. We tested this prediction under natural conditions on over-wintered wheat stubbles. Using a split-field design, we focused on the effects of stubble height reduction on bird abundance and distribution.
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Topping resulted in a mean stubble height reduction of 58·3 ± 1·9%, with a mean stubble height on the treatment plots of 5·9 ± 0·29 cm compared with 14·1 ± 0·33 cm on the control plots. The percentage of bare earth on the treatment plots (mean = 20·9 ± 4·4%) tended to be slightly lower than that on the control plots (25·7 ± 4·4%; t19 = −1·59, P= 0·13), possibly because of increased stubble residue resulting from the topping process.
A total of 28 bird species (4941 individuals) was recorded at the study sites during the survey period (see the Appendix). Of these, seven species were recorded only in the treatment plots and one only in control plots.
When all bird species were aggregated, there was no difference in their relative abundance on treatment and control plots (χ2 = 1·78, P > 0·1). However, when the relative abundances of each of the six functional groups were analysed separately, it was evident that the various groups responded differently to the stubble height manipulation. The abundance of granivorous passerines and invertebrate feeders was higher on treatment plots than on control plots, while the abundance of skylark Alauda arvensis L. and partridges was higher on the control plots (Table 1, Fig. 1). Neither corvid nor pigeon distribution was influenced by stubble height reduction. Field was a significant predictor of relative bird abundance on treatment and control plots for all functional groups, but this result was of little biological importance for this within-field study. No functional group showed a significant change in relative abundance on treatment and control plots between early and late-season surveys (Table 1).
Table 1. Results of logistic regression analyses for six functional groups surveyed on 20 wheat stubble fields at 12 lowland farms in central England. Probability values are presented for the effects of field identity, season and treatment (stubble manipulation) on within-field distribution. Significant effects of treatment are shown in bold. Directions of any association (C, more on control plots; T, more on treatment plots) between predictor variable and abundance are provided
| ||Model goodness-of-fit (res. dev./res. d.f)||Field||Season||Treatment|
|All birds||1·93||> 0·9||> 0·5||> 0·1|
|Granivorous passerines||0·79||< 0·001||> 0·25||< 0·05 (T)|
|Invertebrate feeders||0·74||< 0·025||> 0·5||< 0·01 (T)|
|Skylark||1·84||< 0·01||> 0·1||< 0·001 (C)|
|Corvids||1·57||< 0·05||> 0·1||> 0·1|
|Partridges||0·53||< 0·01||> 0·75||< 0·001 (C)|
|Pigeons||1·22||< 0·001||> 0·1||> 0·25|
Figure 1. The number of visits on which each of the six functional groups were recorded on treatment plots (open bars) and control plots (closed bars). A total of 120 visits was made, with each of 20 fields being surveyed six times. Numbers in parentheses represent the total number of individuals recorded. Asterisks indicate functional groups for which stubble height reduction had a significant effect on within-field distribution (*P < 0·05, **P < 0·01, ***P < 0·001).
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Spring seed densities were significantly lower than autumn seed densities on both treatment and control plots (t17 = 2·8, P= 0·01, t16 = 4·1, P= 0·001, respectively; Fig. 2). There was no significant difference in the overall level of depletion on treatment and control plots (t16 = −0·92, P= 0·37). In both treatment and control plots, the highest level of seed depletion was recorded in samples taken 30 m from the field edge, while the lowest level of seed depletion was recorded in samples taken 5 m from the field edge (Fig. 2). There were no significant differences in seed depletion levels between treatment and control plots at any of the distance bands measured (P > 0·4 in all cases).
Figure 2. Density of weed seeds (+ 1 SE) in samples collected from treatment and control plots in November 2003 (autumn) and March 2004 (spring). Mean density on each plot type and at each of three sampling locations (5 m, 30 m and 60 m from field edge).
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This study has shown that vegetation height manipulation can bring about differential spatial use of stubble fields by a range of bird species. The abundance of granivorous passerines and invertebrate feeders was higher in treatment plots, which had undergone stubble height reduction, while skylark and partridge abundances were higher in control plots.
The potential energetic gain from a patch is likely to be dictated by food abundance, detectability and accessibility and the costs associated with foraging in that patch. While there was no difference in seed abundance between plot types at the start of the winter, it is likely that both food detectability and accessibility were higher in the shorter vegetation of the treatment plots (Whittingham & Markland 2002; Butler & Gillings 2004). Thermo-energetic and mobility costs are also likely to have been lower for foragers in the shorter vegetation of treatment plots; taller or denser vegetation is likely to retain more moisture, particularly as a result of frost and dew in winter, and to be more difficult to move through (Dawson, Carey & van't Hof 1992; Brodman, Reyer & Baer 1997; Clarke et al. 1997).
The granivorous passerines recorded in this study are likely to rely on early detection of predators to flee to protective cover (Lima & Dill 1990; Whittingham & Evans 2004), often feeding near field edges and using surrounding hedgerows and trees as refuges (Robinson & Sutherland 1999). The level of visual obstruction offered by the vegetation within a foraging patch is therefore likely to have a far greater influence on their perception of predation risk than the degree of protection it offers. The results of aviary experiments (Whittingham et al. 2004; Butler et al. 2005) suggest that the preferential selection of treatment plots by granivorous passerines in this study occurred because they are likely to have associated both a lower predation risk and greater potential energetic gain with treatment plots.
Treatment plots are also likely to have offered greater potential energetic gain and lower predation risk to invertebrate feeders. While no measure of food abundance for these species was made, it is unlikely that it differed between the two patches when stubble height reduction took place, although invertebrates may have subsequently responded to the increased stubble residue on the surface of treatment plots (Hole et al. 2005) and any consequent changes in microclimate at soil level (McCracken, Foster & Kelly 1995). However, invertebrate prey detectability and accessibility are likely to have been greater in the shorter vegetation of treatment plots (Butler & Gillings 2004), and recent research has shown that the mobility and foraging efficiency of invertebrate feeders is lower in long vegetation than in short vegetation (Devereux et al. 2004). When threatened, most of the species included in this functional group fly to nearby bushes and trees until the danger has passed (Clement & Hathway 2000), so the influence of vegetation on visual obstruction is again likely to have a greater influence on perceived predation risk than its protective value.
The effect of stubble height reduction on the predation risk of individuals that leave their foraging site and flee to protective cover in the event of predator attack may be influenced by the size of the individual. For example, the difference in visual obstruction, and therefore perceived predation risk, between treatment and control plots is likely to have been much smaller for larger-bodied species such as pigeons and corvids. This limited effect of stubble height reduction on the perceived predation risk of pigeons and corvids, as well as their broader diet (Murton, Isaacson & Westwood 1966; Snow & Perrins 1998), may explain why there was no treatment effect on their within-field distribution in this experiment.
Partridges and skylark adopt different strategies for avoiding predation compared with granivorous passerines and invertebrate feeders. Instead of retreating to cover, partridges often remain still and rely on crypsis to avoid predator detection (Madge & McGowan 2002). The usual raptor avoidance behaviour of skylark is also to crouch, often not breaking cover until the last minute (Donald 2004). While the shorter stubble on treatment plots may have provided less visual obstruction and allowed earlier predator detection, it is likely to have offered less protection to partridges and skylark once a predator had been detected. The greater abundance of partridges and skylark on control plots suggests they associated lower predation risk with these plots and were trading this off against the greater potential energetic gain on treatment plots. A number of other studies, across a range of taxa, have shown that the trade-off between potential energetic gain and predation risk is often more heavily weighted in favour of minimizing predation risk (Brown 1988; Nonacs & Dill 1990; Todd & Cowie 1990; Moody, Houston & McNamara 1996; Beck & Watts 1997; Butler et al. 2005).
The discussions above have centred on the effects of stubble height manipulation on forager behaviour. It is also possible that predator behaviour may have been influenced by treatment effects. Sparrowhawks are the main predators of adult farmland birds (Götmark & Post 1996). Quinn & Cresswell (2004) showed that sparrowhawks hunt according to the vulnerability of their prey, which is likely to be affected by vegetation structure, while studies of kestrels Falco tinnunculus L. have shown that foraging activity and hunting success are higher over less densely vegetated habitats (Shrubb 1980; Toland 1987). The effect of stubble height manipulation on hunting behaviour by farmland bird predators, both avian and terrestrial, needs further investigation to allow a greater understanding of how it influences actual predation risk for farmland birds.
There was no significant difference in the level of seed depletion between the two plot types. However, as stubble height manipulation may have influenced a number of other factors that influence seed depletion levels, besides foraging birds, it is not possible to draw any firm conclusions about seed depletion from this result. For instance, although most plants only set seed prior to the start of the winter (Grime, Hodgson & Hunt 1989), topping in late October may have suppressed some over-winter seed production. Furthermore, differences in microclimate between plots may have caused differential seed germination or rotting rates (Wright 1993). It is also possible that stubble height manipulation may have affected seed depletion rates by other seed predators, such as insects and small mammals (Telleria, Santos & Diaz 1994; Jacob & Brown 2000; Thorbek & Bilde 2004). Further research should therefore aim to investigate the effects of stubble height manipulation on other taxa that play an important role in stubble field dynamics.
synthesis and applications
We have shown that stubble height manipulation can influence foraging site selection by a range of farmland bird species, many of which are of current conservation concern. It appears that the value of wheat stubble fields to granivorous passerines and invertebrate feeders can be increased simply by lowering stubble height. However, wholesale reductions in height would be detrimental to other species, such as skylark and partridges, which have different foraging requirements and predator escape strategies. Increasing the structural heterogeneity of overwinter stubble is likely to increase its value as a foraging habitat, making it better suited to the foraging requirements and predator escape strategies of a greater diversity of species. Incorporating targeted management options into agri-environment schemes such as the British government's new Environmental Stewardship Scheme may represent the most cost-effective strategy to achieve this increased structural heterogeneity. In this experiment, farmers were paid £5 per hectare cut. Structural heterogeneity could also be achieved through rolling, discing or low intensity grazing, but further research is required to determine which approach is most suitable and also the most appropriate spatial scale for such management. We predict that increasing structural heterogeneity within other farmland habitats, such as grassland and cereal crops, will also increase the value of these habitats, as they too are utilized by a range of species with differing foraging requirements and predator escape strategies.