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Comparisons of trends in breeding abundance in fed and control areas revealed no significant, positive effects of supplementary winter feeding in either Bird Aid or WFFB, indicating that resources provided in the relevant quantities and contexts is unlikely to reverse population declines. However, this result masks important associations between winter feeding and population trends in fed areas. More positive trends were observed for yellowhammer, robin and dunnock in WFFB central blocks, where more food was provided, although the effects were not sufficiently large to produce increases. Additional effects of supplementary feeding were revealed post hoc when relationships between trends in abundance and the level of use of winter food were examined. In Bird Aid, there was a significant positive association between food use and population trends for yellowhammers, but not for tree sparrows (non-significant) or corn buntings (confounded patterns). In WFFB, up to five species (depending on the controls included) declined less steeply with higher food use: chaffinch, dunnock, yellowhammer, house sparrow and reed bunting. The results provide evidence for winter resource limitation in these populations. They suggest that supplementing winter food can produce landscape-scale, positive effects, but that effective supply to enough individuals is critical: increasing general availability may not increase the seed that target species actually consume. Nevertheless, the results constitute the first experimental evidence that appropriate enhancements of food availability, such as through agri-environment measures providing seed resources when they are most needed during the winter, have realistic potential to halt, or at least to slow, population declines at the landscape scale.
The latter scenarios are among the potential explanations for trends in abundance in fed areas not being more positive than those in control areas, despite the widespread use of feeding sites by declining species. Possible methodological explanations also included the duration of the experiments (perhaps too short for effects to be seen), the quantity of food supplied (potentially too small genuinely to benefit the flocks attracted, especially after depletion by competitors), the quality of food supplied (perhaps nutritionally inadequate, although energy is probably the key requirement), sample sizes (perhaps providing insufficient statistical power) and abundance monitoring methods (perhaps too coarse to detect small changes in abundance). Biologically, the populations concerned might only be limited by winter food in harsher conditions than in the study winters [maximum and minimum (respectively) January daily temperatures (°C), annual means for 2001–04: Durham 7·0 (range 5·7–7·6), 1·5 (–0·1 to 2·4); Cambridge 7·8 (6·7–8·9), 2·1 (1·2–2·7); Lyneham 7·3 (5·9–8·5), 1·8 (0·7–2·8); Anon 2001–2004, see Fig. 1 for locations]. Wintering and breeding populations might also have been insufficiently linked for a change to be detectable, if within-winter movements encompassed large areas (but see Siriwardena et al. 2006) or dispersal from wintering to breeding locations was too extensive. In addition, impacts of predators such as sparrowhawks Accipiter nisus L. attracted to feeding sites could have overridden positive effects of feeding (e.g. Bro et al. 2004). However, although fed areas supplied more resources than controls, neither experiment established a clear dichotomy, in practice, between zero and significant food supplementation for individual species; this was accounted for in analyses incorporating site-use.
Many species’ use of feeding sites ranged from zero to regular large flocks (Siriwardena et al. 2006), while use by competitors, including gamebirds, deer and rats, also varied. Variation in local habitat features and local bird abundance probably also affected the propensity of each species to use each feeding site, despite every effort to standardize site context (Siriwardena & Stevens 2004). Food supplementation to each species was not therefore consistent across sites. Despite this, there was a fourfold difference in the quantity of food available to the whole granivorous community, across 1·5 times the land area (6 km2 vs. 4 km2; Fig. 2), between WFFB central and outer blocks. Radio-tracking results suggest that most individuals’ regular winter foraging movements occur within such areas (Calladine, Robertson & Wernham 2006; Siriwardena et al. 2006). Central blocks therefore probably supplied more food to local populations of each species, which seems to have changed the average yellowhammer trend from decline to stability (Table 3, Fig. 5c). Similarly positive effects occurred for dunnock and robin (Fig. 5a,b).
Data quantifying feeding site use provided a surrogate for species-specific quantities of food consumed in both experiments. Most significant relationships between site-use and population trend suggested that heavier food use resulted in more positive trends. Yellowhammers, which have recently declined rapidly (Baillie et al. 2005), responded particularly strongly (Table 4, Fig. 7). There were also significant positive effects of site-use on population trends for goldfinch, house sparrow, reed bunting and chaffinch, but not for greenfinch or tree sparrow, while apparent effects for corn bunting were inconsistent (Table 4). The differences between species may reflect differences in ecology and population status. For example, yellowhammers and house sparrows are still declining in many areas (Baillie et al. 2005), suggesting ongoing shortfalls in winter food resources. At the farm scale, house sparrow breeding abundance can respond to supplementary winter food (Hole et al. 2002); this study suggests similar patterns at the landscape scale. Reed buntings are now stable nationally, having disappeared from many farmland areas in the 1980s (Peach, Siriwardena & Gregory 1999), but a lack of winter food could be limiting re-colonization. Weaker effects might be expected for species with stable or increasing national populations because they are less likely to suffer from winter food shortages, which could explain the tree sparrow and greenfinch results. Further, there was a significant effect on chaffinch trends, but it was small and harder to detect (Fig. 7d, Table 4). The corn bunting results reflect confounded relationships between trend and each of site-use and absolute abundance (Fig. 6b): site-use was high where abundance was high and these populations increased. At low abundance, greater site-use was associated with steeper declines, perhaps because insufficient food was provided to help smaller, declining populations. Note, however, that the Bird Aid site-use measure may have been less accurate because only two counts were made per winter, compared to 20 or more in WFFB.
Although insectivorous and generalist species might be less dependent on seeds in winter, dunnock population trends were strongly affected by winter feeding (Table 4, Fig. 7b). Reduced survival is the probable mechanism for this species’ decline (Siriwardena, Baillie & Wilson 1998; Baillie et al. 2005) and dunnocks used feeding sites heavily (Siriwardena et al. 2006), so positive responses to increased winter resources are plausible. There was also a significant relationship for great tit but this showed more negative trends with greater site-use (Table 4, Fig. 7a,b). It is possible that great tits used supplementary food where food resources in hedgerows, woodland, scrub and gardens were scarce (at higher breeding abundance: Fig. 7a), but not enough was available to prevent declines. With an increasing national population (Baillie et al. 2005), declines at high local abundance are more likely to reflect density-dependent stabilization than a conservation problem.
Overall, despite the various potential sources of noise in the comparisons conducted, there was some evidence of positive responses to winter food use for seven of the 13 species considered across both experiments and an eighth (robin) responded positively to food quantity alone (Tables 3 and 4). A further positive association was found for corn bunting, but an apparently stronger negative association was also found (Table 4). All species for which no positive effect was identified are currently increasing nationally (Baillie et al. 2005), so winter food may not currently be limiting these species. Yellowhammer and house sparrow were among the declining species that appeared to be limited by winter food, although effects varied with breeding density in WFFB study areas. Yellowhammers declined, irrespective of local population density, unless feeding site use was high, but intermediate or high food patch occurred most at higher breeding densities (Fig. 7g). High food use and more positive trends occurred at relatively low densities for house sparrow (Fig. 7f). These local populations may be declining, with insufficient winter food resources, while larger concentrations persist, supporting Hole et al.'s (2002) suggestion that smaller local populations are vulnerable to extinction.
There are two caveats to the interpretation of the site-use and population trend results. First, the analyses were, strictly, correlational. However, absolute abundance was controlled for, so positive results did not merely show greater site-use and healthier populations at higher densities. More positive trends could still have arisen from high local breeding success in the previous summer causing high winter abundance, and thus high site-use, but this assumes considerable spatial variation in breeding success and contradicts relevant existing evidence on population limitation (Siriwardena et al. 1998, 2000; Peach, Siriwardena & Gregory 1999; Hole et al. 2002). Moreover, differences in trend between WFFB central and outer blocks (Fig. 5) are very unlikely to reflect variation in breeding success.
The second caveat is that increased local breeding density could reflect re-distribution in response to winter food availability. This could be tested by comparing breeding density within and just outside heavily used fed areas. One recent study suggested that yellowhammers preferentially settle to breed adjacent to seed-rich late-winter stubble or set-aside (Whittingham et al. 2005), but failed to account for these habitats’ value for breeding. Ongoing research is investigating the extent of winter-to-breeding-season movements of resident granivores and effects of feeding on body condition, over-winter survival and abundance. This should clarify the relative importance of population growth and re-distribution in this context.
Winter food provision features in several agri-environment measures that aim to restore farmland bird populations. This study shows that such resource enhancement can slow or stop landscape-scale declines. The lack of demonstrable differences between experimental and control areas shows, however, that seed delivery mechanisms must be effective: the timing and context of seed provision must fill critical resource gaps for target species. Winter food resource prescriptions, such as within the English Environmental Stewardship (ES) scheme, include sown bird seed mixtures, over-wintered cereal and root-crop stubbles and unharvested conservation headlands. Such options, particularly given sympathetic chemical management, can produce large quantities of seed and field observations show that birds use such habitats intensively. These prescriptions are therefore likely to provide appropriate resources, but they have to be taken up sufficiently frequently by farmers. For example, more than 10 ha km2 of average-quality stubble (or a smaller area managed sympathetically) is likely to be needed to stop local skylark Alauda arvensis declines (Gillings et al. 2005), but this is considerably more than current levels of relevant ES option uptake will generate (Butler, Vickery & Norris 2007).
Resource dilution and obscuring vegetation (Butler et al. 2005) will make search times longer in open field habitats, even seed-rich ones, so a given quantity of seed might provide less benefit than in superabundant patches. However, interference competition from other birds will probably also be lower, as will seed depletion by rats and deer and disturbance from predator attacks (because flocks are less concentrated). It is unknown, therefore, whether resources in seed patches or open fields provide greater net benefits. Nevertheless, feeding sites were used most heavily in late winter (Siriwardena et al. 2006), when ‘natural’ food availability is probably lowest (Evans, Vickery & Shrubb 2004); open-field agri-environment options tend not to cover this period well. In particular, ‘over-wintered’ stubbles in ES can be ploughed in mid-February, unless part of a more stringent option specifying that they subsequently become a set-aside. Sowing, cutting and cultivation dates for wild bird seed mixtures are also not stipulated (Defra 2005), although doing so could increase their value in late winter. Late winter food availability may therefore still be low: research is needed into the benefits of agri-environment prescriptions in practice and possible improvements in their effectiveness in supplying late winter seed. Precise quantities of seed required for particular population outcomes are also unknown. Nevertheless, if they supplied seed effectively throughout the winter, our results indicate that agri-environment prescriptions could stop and, perhaps, reverse some farmland bird declines and hence contribute significantly to meeting UK government conservation targets (Vickery et al. 2004).