The selection of stubble fields by wintering granivorous birds reflects vegetation cover and food abundance

Abstract

Summary 1.Fields left fallow after harvest (i.e. stubble fields) support high wintering densities of many species of granivorous bird. We examined correlates of use by eight such species of different types of intensively managed wheat and barley stubble fields, organic wheat fields and set-aside fields on mixed lowland farmland in central England. Field occupancy was studied in relation to the physical characteristics of fields and seed abundance. 2. Higher seed abundance was associated with greater occupancy by linnet Carduelis cannabina, grey partridge Perdix perdix, chaffinch Fringilla coelebs, yellowhammer Emberiza citrinella, reed bunting Emberiza schoeniclus and corn bunting Miliaria calandra. Larger areas of bare earth within stubble fields were associated with greater occupancy by linnet, yellowhammer, reed bunting and corn bunting, but lower occupancy by woodpigeon Columba palumbus. 3. On conventional intensively farmed sites, seed abundance and area of bare earth were significantly greater on barley stubbles than on wheat stubbles. 4. Seed numbers fell throughout the winter in all stubble types, although reductions were greatest on intensive barley stubbles, intermediate on intensive wheat stubble and lowest on undersown organic wheat stubbles. 5. Within fields occupied by linnets, areas used for feeding had significantly greater quantities of seeds known to be important in their diet. Feeding areas also had a greater area of bare earth than randomly selected 'non-feeding areas'. 6. Linnets and reed buntings were rarely found on fields where densities of weed seeds important in their diets fell below 250 seeds m −2 . In autumn, yellowhammers and grey partridges rarely fed on fields where cereal grain density was below 50 m −2 . However, in spring, both species fed on these fields irrespective of grain density, perhaps indicating a switch to other food sources. 7. We suggest that land managers wishing to maximize the value of overwinter stubble fields for granivorous birds locate such fields where there is a substantial natural regeneration of weed flora and where previous cropping (e.g. barley) is likely to offer a sparse stubble with substantial areas of bare ground.

Counting birds on farmland habitats in winter

“…Meadow Pipit has also exhibited a population decline and has been afforded Medium BTO Alert. The population declines of many granivorous farmland birds have been linked with declines in survival outside the breeding season, probably caused by a lack of food in winter (Siriwardena et al 2000, Moorcroft et al 2002. There is, therefore, a real need to gain a better understanding of the winter ecology, habitat preferences and foraging behaviour of many of these species.…”

“…The first method uses counts from the perimeter only ('perimeter counts'; Gillings et al 2005, Tucker 1992); the second involves walking parallel transects across the field sufficiently close together to ensure that everything is flushed and counted ('whole-area search'; Moorcroft et al 2002, Wilson et al 1996. For small-scale studies the more accurate whole-area search method is preferable but at larger scales, e.g.…”

Biotic interactions, ecological knowledge and agriculture

“…Others have demonstrated the role of weeds in sustaining biodiversity. For example, in Europe weeds found in stubble after grain harvest greatly affected the number of birds using the field, notably linnets (Carduelis cannabina; Moorcroft et al 2002), and loss of weeds is thought be one of a number of causes in the overall decline of birds in farmland . Work in the UK also revealed that arable weeds support high insect diversity reinforcing the need to balance weed control and biodiversity conservation .…”

Seed food preferences of granivorous farmland passerines

“…These contrasts may, however, be explained by differing abundance and availability of grain and weed seeds on different cereal stubbles. Barley crops, for example, tend to have fewer herbicide applications than wheat crops, the structure of their stubbles is more open to foraging birds, and barley tends to shed more grain at harvest than wheat (Donald et al 2001, Moorcroft et al 2002.…”

“…Seed-rich habitats such as over-winter stubbles and fodder crops have become scarce in modern farming landscapes, being replaced by autumn-sown cereals, silage and maize (Fuller 2000). Similarly, increasingly efficient harvesting, storage and seed cleaning, as well as intensive herbicide regimes, have reduced the abundance of grain and weed seed food on farms during the winter (Wildey et al 1999, Brickle & Harper 2000, Moorcroft et al 2002, Robinson & Sutherland 2002. Where seed-rich habitats remain on farmland, as on rotational set-aside fields, they are heavily selected by seed-eating birds in winter (Wilson et al 1996, Buckingham et al 1999, Hancock & Wilson 2003.…”