Action: Leave overwinter stubbles
- Eighteen studies (including four reviews and one systematic review) investigated the effects of overwinter stubbles on farmland wildlife. Thirteen studies from Finland, Switzerland and the UK (six replicated trials, including two site comparisons, four reviews and a systematic review) found evidence that leaving overwinter stubbles provides some benefits to plants, insects, spiders, mammals and farmland birds. These benefits include higher densities of farmland birds in winter, increased grey partridge productivity, and increased cirl bunting population size (in combination with several other conservation measures) and territory density.
- One replicated site comparison study from the UK found evidence that leaving overwinter stubbles had inconsistent or no effects on farmland bird numbers. Three studies found only certain bird species showed positive associations with overwinter stubbles. Two replicated studies (of which one also randomized and controlled) found that only Eurasian skylark or both Eurasian skylark and Eurasian linnet benefited, out of a total 23 and 12 farmland bird species tested respectively. One study found that only grey partridge and tree sparrow showed positive population responses to areas with overwinter stubbles.
- Two studies from the UK (one randomized, one replicated and controlled) found that different farmland bird species benefited from different stubble heights. One replicated site comparison found mixed effects between different stubble management options on seed-eating bird abundance.
This intervention involves leaving crop stubbles in fields until at least February-March. These stubbles may provide an important food source for seed-eating birds over the winter (Campaign for the Farmed Environment 2011).
The availability and extent of overwinter stubbles may have an important influence on bird populations. One study from the UK (Gillings et al. 2005) showed that national population trends of 16 out of 26 bird species were positively influenced by the presence of overwinter stubbles. The same study also predicted that yellowhammer Emberiza citrinella and Eurasian skylark Alauda arvensis population declines would be lessened in areas with a high proportion of stubbles and that 10-year population stability/growth could be achieved by increasing the coverage of stubbles within 1 km squares to 15 ha or 20 ha respectively. Additionally, a 2008 literature review and analysis of the Environmental Stewardship scheme, particularly Entry Level Stewardship in the UK (Vickery et al. 2008), suggested that, for Eurasian skylark, approximately 0.1 km2 of stubble/km2 would be needed to prevent population declines. The authors also suggest that having these patches over 1 km apart would maximize winter use.
Gillings S., Newson S.E., Noble D.G. & Vickery J.A. (2005) Winter availability of cereal stubbles attracts farmland birds and positively influences breeding population trends. Proceedings of the Royal Society B, 272, 733-739.
Vickery J., Chamberlain D., Evans A., Ewing S., Boatman N., Pietravalle S., Norris K. & Butler S. (2008) Predicting the impact of future agricultural change and uptake of Entry Level Stewardship on farmland birds. British Trust for Ornithology, The Nunnery, Thetford.
Campaign for the Farmed Environment (2011) Guide to voluntary measures 2011 edition. Campaign for the Farmed Environment, Warwickshire.
Supporting evidence from individual studies
A 2000 literature review (Aebischer et al. 2000) found that the UK population of cirl bunting Emberiza cirlus increased from between 118 and 132 pairs in 1989 (Evans 1997) to 453 pairs in 1998 (Wotton et al. 2000) following a series of agri-environment schemes designed to provide overwinter stubbles, grass margins, and beneficially managed hedges and set-aside. Numbers on fields under specific agri-environment schemes increased by 70%, compared with a 2% increase on neighbouring land not under the scheme.
Evans A.D. (1997) Cirl Buntings in Britain. British Birds 90, 267-282.
Wotton S.R., Langston R.H.W., Gibbons D.W. & Pierce A.J. (2000) The status of the Cirl Bunting Emberiza cirlus in the UK and the Channel Islands in 1998. Bird Study, 47, 138-146.
A 2001 paired site comparison study in south Devon, UK (Peach et al. 2001) found that the presence of areas of spring sown barley followed by overwinter stubbles was associated with an increase in the number of cirl bunting Emberiza cirlus. Along with a number of other Countryside Stewardship Scheme management options found to be important for cirl bunting, an increase in the area of spring sown barley followed by overwinter stubbles coincided with an increase in the number of cirl bunting pairs from 1997 to 1998. Six of seven Countryside Stewardship Scheme plots that had 6 m grass margins and were within 2.5 km of former cirl bunting territories gained birds, and there was a tendency for farms providing grass margins to also include spring sown barley (followed by overwinter stubbles). The association between grass margin uptake and overwinter stubble uptake leads the authors to suggest that overwinter stubbles (and spring sown barley) may have a positive influence on cirl bunting, although these results are not definitive. More generally, there were declines of 20% in cirl bunting numbers on land not participating in the Countryside Stewardship Scheme. Forty-one 2 x 2 km² squares containing both land within the Countryside Stewardship Scheme and non-Countryside Stewardship Scheme land were surveyed in 1992, 1998 and 1999. In each year, squares were surveyed for cirl bunting at least twice, the first time during mid-April to late May, and the second time between early June and the end of August.
A review (Evans et al. 2002) of two reports (Wilson et al. 2000, ADAS 2001) evaluating the effects of the Pilot Arable Stewardship Scheme in two regions (East Anglia and the West Midlands) in the UK from 1998 to 2001 found that overwinter stubbles benefited plants, bumblebees Bombus spp. and true bugs (Hemiptera), especially when followed by spring fallow. Stubbles also benefited ground beetles (Carabidae) and sawflies (Symphyta). The effects of the pilot scheme on plants, invertebrates and birds were monitored over three years, relative to control areas, or control farms. Only plants and invertebrates were measured within individual options. Overwinter stubbles were the most widely implemented options, with total areas of 974 and 2200 ha in East Anglia and West Midlands respectively.
Wilson S., Baylis M., Sherrott A. & Howe G. (2000) Arable Stewardship Project Officer Review. F. a. R. C. Agency report.
ADAS (2001) Ecological evaluation of the Arable Stewardship Pilot Scheme, 1998-2000. ADAS report.
A replicated study in the winters of 1997-1998 and 1998-1999 on 122 stubble fields on 32 farms in central England (Moorcroft et al. 2002) found that 13 bird species were found using stubble fields. Four species (Eurasian linnet Carduelis cannabina, Eurasian skylark Alauda arvensis, reed bunting Emberiza schoeniclus and corn bunting Miliaria calandria) were found more frequently on intensively-farmed barley Hordeum spp. stubbles than intensive or organic wheat Triticum spp., whilst woodpigeons Columba palumbus were found most frequently on organic wheat. Intensive barley stubbles had the highest cover of weeds (51% cover on intensive barley, 40% on intensive wheat, 28% on organic wheat). Weed seed densities in March were highest on undersown organic wheat stubble fields compared to intensive barley or wheat stubbles. Weed seed density decreased the least on undersown organic wheat stubbles between October and March compared to intensive barley or wheat stubbles (11% decline on undersown organic wheat stubbles, 23% decline on intensive wheat stubbles, 35% decline on intensive barley). Seventeen stubble fields contained organic wheat with the previous crop undersown with rye grass Lolium spp. and white clover Trifolium repens. Sixty-seven fields were managed for intensive wheat and 38 fields for intensive barley, both intensively-managed crops received inorganic fertilizer and pesticide applications. Each study field was either overwintering as stubble or entered into the first year of a set-aside scheme. Plants were surveyed in forty 20 x 20 cm quadrats in each field in October. Seed densities were recorded in 27 fields from 10 soil cores/field in October 1997 and March 1998. Birds were surveyed monthly on parallel transects.
A replicated study in the summers of 1999-2000 comparing ten different conservation measures on arable farms in the UK (Critchley et al. 2004) found that overwinter stubbles had high total plant cover, but not as many plant species as some other measures. Overwinter stubbles and spring fallows had relatively high total plant cover, and over 50% cover of grasses. Litter cover was higher while richness of annual plant species was lower in overwinter stubbles compared with spring fallows, probably due to cultivation in spring fallows. The average numbers of plant species in the different conservation habitats were: overwinter stubbles 4.2, wildlife seed mixtures 6.7, uncropped cultivated margins 6.3, undersown cereals 5.9, uncultivated margins 5.5, no-fertilizer conservation headlands 4.8, spring fallows 4.5, sown grass margins 4.4, conservation headlands 3.5, grass leys 3.1. Uncropped cultivated margins, wildlife seed mixtures and no-fertilizer conservation headlands appeared to be the best options for conservation of annual broadleaf plant communities. Plants were surveyed on 37 farms in East Anglia (dominated by arable farming) and 38 farms in the West Midlands (dominated by more mixed farming). The study included 294 habitat sites (defined as a single field, block of field or field margin strip). Vegetation was surveyed once in each site in June-August in 1999 or 2000, in thirty 0.25 m2 quadrats randomly placed in 50-100 m randomly located sampling zones in each habitat site. All vascular plant species rooted in each quadrat, bare ground or litter and plant cover were recorded.
A replicated, randomized study from November 2003 to March 2004 in 205 cereal stubble fields under a range of management intensities in arable farmland in south Devon, UK (Defra 2004) found that barley stubbles following low-input herbicide were more beneficial for cirl bunting Emberiza cirlus than wheat or conventionally managed stubbles. The number of breeding cirl bunting territories the previous season and small field size (probably, as the authors point out, because cirl buntings prefer to forage near hedgerows and because smaller fields are less intensively managed) also correlated positively with population size. Overall, barley fields were generally preferred by seed-eating species. Low-input barley stubbles had significantly higher seed abundance and broadleaved weed cover (approximately four times greater). Fields where stubbles were grazed over winter led to significantly lower densities of seed-eating birds in general. The authors point out that seed-eating bird species’ preference for barley stubbles was independent of the positive correlation with broadleaved weed density and should be taken into account when planning prescriptions.
A replicated, randomized, controlled study from November-February in 2000-2001 and 2001-2002 in 20 arable farms in eastern Scotland (Parish & Sotherton 2004) found that, of 23 bird species recorded, only Eurasian skylark Alauda arvensis was significantly more abundant in fields with stubble left over winter than fields with wild bird cover crops or conventional crops. Stubble fields were those in which cereal and oilseed rape stubbles were left over winter. Six to 28 ha were sampled on each farm annually.
A replicated, controlled study in winter 2003-2004 on 20 wheat fields on 12 lowland farms in central England (Butler et al. 2005) found that seed-eating songbirds and invertebrate-feeding birds were more abundant on stubble fields cut to 6 cm, whereas Eurasian skylark Alauda arvensis and partridges (Phasianidae) were more abundant on fields with uncut stubble, approximately 14 cm tall (seed-eaters: 343 individuals seen on approximately 25 of 120 visits to cut fields vs 89 individuals on 15 visits to control fields, invertebrate-eaters: 623 birds on 17 visits vs 34 on five visits, skylark: 557 on 50 visits vs 814 on 80 visits, partridges: five on two visits vs 235 on 27 visits). Crows (Corvidae) and pigeons (Columbidae) showed no response to stubble cutting. Each field was split so that half was cut (late October 2003) to approximately 6 cm tall, with the other half left as a control.
A controlled trial from 2003 to 2004 in Jokioinen, southern Finland (Huusela-Veistola & Hyvanen 2006) found that uncultivated barley stubble had significantly more spiders (Araneae) than a control spring barley crop, but similar numbers of ground beetles (Carabidae) and unsown plant species. The stubble field had around 20 spiders/trap, compared to around five spiders/trap in the control plot. A 44 x 66 m plot of uncultivated spring barley stubble was established in 2004 (the barley sown and harvested in 2003), and compared with an equivalent plot of spring barley crop sown in 2004. Insects were sampled using a yellow sticky trap and three pitfall traps in the centre of each plot for a week in June, July and August 2004. Unsown plant species were counted in four 50 x 50 cm quadrats in each plot in late August 2004.
A replicated study in 1999 and 2003 on 256 arable and pastoral fields across 84 farms in East Anglia and the West Midlands, England (Stevens & Bradbury 2006) found that only two of twelve farmland bird species analysed were positively associated with the provision of overwinter stubble, set-aside or wildlife seed mixtures. The two species were Eurasian skylark Alauda arvensis (a field-nesting species) and Eurasian linnet Carduelis cannabina (a boundary-nesting species). The study did not distinguish between set-aside, wildlife seed mixtures or overwinter stubble, classing all as interventions to provide seeds for farmland birds.
A small randomized site-comparison study in winter 2004-2005 in central England (Whittingham et al. 2006) found that seed-eating songbirds and invertebrate-feeding birds were found at higher densities on sections of fields where stubble had been cut short (404 seed-eating birds and 244 invertebrate-feeding birds recorded on uncut stubble plots vs 77 and seven on cut stubble). Eurasian skylark Alauda arvenis, partridges (Phasianidae), pigeons Columba spp., and meadow pipit Anthus pratensis were found at higher densities in areas of uncut stubble (241 skylark, 100 partridges, 37 pigeons and 81 meadow pipit on uncut plots vs 27, 7, 12 and 9 on cut plots). In addition, skylarks and invertebrate-feeders were found at higher densities on scarified (i.e. lightly tilled) sections of fields than control (unscarified) sections (339 skylarks and 1,371 invertebrate feeders on scarified plots vs 241 and 251 on controls). The stubble on one half of each field was cut in the winter of 2004-2005 (late December-early February) before the fields were surveyed between December 2004 and March 2005.
A 2007 review of published and unpublished literature (Fisher et al. 2007) found experimental evidence of benefits of overwinter stubble to plants (one study: (Critchley et al. 2004)) and use of overwinter stubbles by brown hares Lepus europaeus (one correlative study not included here). This review assessed the evidence for wider benefits of UK agri-environment prescriptions aimed at conserving wild birds on arable land.
A 2007 systematic review identified five papers investigating the effect of overwinter stubble provision on farmland bird densities in the UK (Roberts & Pullin 2007). There were significantly higher densities of farmland birds in winter on fields with stubbles than on conventionally managed fields. In particular, there were greater densities of seed-eating songbirds and crows (Corvidae) on fields with stubbles than on control fields. The meta-analysis included experiments conducted between 1992 and 2002 from three controlled trials, one time series, and one site comparison study.
A replicated before-and-after study in 1989-2005 on 28 selected arable fields in the western Swiss Plateau (Bisang et al. 2009) found that populations of two hornwort species (Anthoceros agrestis and Phaeoceros carolinianus, the latter rare) declined between surveys carried out before and after introduction of the Swiss agri-environment scheme in 1999. An index of hornwort abundance was greater during an initial survey in 1989-1995 than in a repeat survey of the same sites in 2005-2007. Hornwort abundance was strongly affected by the availability of stubble fields. The proportion of stubble fields left unmanaged after harvest was found to decrease between the survey periods. The scheme appeared suboptimal for conserving hornwort taxa because it did not support the maintenance of autumn or winter stubble fields (which, in turn, declined as a result of soil conservation measures introduced in 2005). Selected fields (on average 1-2 ha) were surveyed every September-October, and observations of hornwort occurrences (of A. agrestis and P. carolinianus gametophytes, based on 20 minutes search by two people) were used to calculate an abundance index. Crop type and management were also recorded.
A 2009 literature review of agri-environment schemes in England (Natural England 2009) found a 146% increase in cirl bunting Emberiza cirlus territory density on land under a Countryside Stewardship scheme ‘special project’, which (amongst other interventions) increased the amount of weedy overwinter stubbles in the target area between 1992 and 2003. In addition, the national population increased from 319 to nearly 700 pairs over the same period (Wotton et al. 2000, Peach et al. 2002, Wotton et al. 2004). Generally, the review found high densities of seed-eating songbirds and Eurasian skylark Alauda arvensis on stubbles and wild bird seed or cover mix, compared to other land uses, and a survey in the winter of 2007-2008 found the highest densities of skylark on stubble fields, compared with other agri-environment scheme options (Field et al. in press). The review also stated that overwinter stubbles deliver benefits for brown hare Lepus europaeus, but did not provide further details.
Wotton S.R., Langston R.H.W., Gibbons D.W. & Pierce A.J. (2000) The status of the Cirl Bunting Emberiza cirlus in the UK and the Channel Islands in 1998. Bird Study, 47, 138-146.
Wotton S., Rylands K., Grice P., Smallshire S. & Gregory R. (2004) The status of the Cirl Bunting in Britain and the Channel Islands in 2003. British Birds, 97, 376-384.
Field R.H., Morris A.J., Grice P.V. & Cooke A. (In press) Winter use of seed-bearing crops by birds within the English Environmental Stewardship Scheme. Ibis.
A replicated site comparison in 2005 and 2008 of agricultural land across England in 2005 and 2008 (Davey et al. 2010a) (same study as (Davey et al. 2010b)) found that four of eight regions had at least two farmland bird species that showed positive responses to wild bird cover and overwinter stubble fields. Across all 15 species thought to benefit from these interventions, only one region (the northwest) showed significantly more positive responses than would be expected by chance. Some species responded positively in some regions and negatively in others. There were 2,046, 1 km2 lowland study plots, surveyed in 2005 and 2008.
A large replicated site comparison study in 2005 and 2008 of lowland farmland in England (Davey et al. 2010b) (same study as (Davey et al. 2010a)) found that three years after the 2005 introduction of the Countryside Stewardship schemes and Entry Level Stewardship schemes, there was no consistent association between the provision of stubbles and farmland bird numbers. Grey partridge Perdix perdix and tree sparrow Passer montanus were the only two species that showed more positive population change (population increases or smaller decreases relative to other plots) from 2005 to 2008 in the 9 km² and 25 km² areas immediately surrounding plots planted with stubble than in the area surrounding plots without stubbles. The effect of stubbles was small, however, with tree sparrow numbers increasing by 0.05 at the 9 km² scale for every 0.07 km² of stubble and by 0.07 at the 25 km scale for every 0.14 km² of stubble. The 2,046, 1 km² lowland plots were surveyed in both 2005 and 2008 and classified as arable, pastoral or mixed farmland. Eighty-four percent of plots included some area managed according to Entry Level Stewardship or the Countryside Stewardship Scheme. In both survey years, two surveys were conducted along a 2 km pre-selected transect route through each 1 km² square.
A replicated site comparison study from 2004 to 2008 in England (Ewald et al. 2010) found that the proportion of young grey partridges Perdix perdix in the population (ratio of young to old grey partridge) was positively associated with the amount of sites left as overwinter stubble. However, when stubbles were used in conjunction with other interventions, the results were mixed. In conjunction with small field sizes and reduced chemical inputs, stubbles were weakly positively correlated with year-on-year changes in partridge density but negatively related to brood size. In conjunction with undersowing spring cereals, stubbles were negatively associated with year on year changes (in 2006-2007), overwinter survival (2004-2005, 2005-2006 and generally).
A replicated site comparison study between November 2007 and February 2008 of 75 fields in East Anglia and the West Midlands, England (Field et al. 2010a) (same study as (Field et al. 2010b)) found no differences in the number of seed-eating birds or Eurasian skylark Alauda arvensis on Environmental Stewardship stubbles and non-Environmental Stewardship stubbles. There was also no significant difference in the number of seed-eating birds on stubbles managed under Higher Level Stewardship of the Environmental Stewardship scheme (18 birds/ha) than in fields managed under Entry Level Stewardship (8.5 birds/ha). Skylarks, however, were found to be more numerous on Higher Level Stewardship fields (9.3 birds/ha) than Entry Level Stewardship fields (1.2 birds/ha). Entry Level Stewardship stubbles had no post-harvest herbicide and no cultivation until mid-February, Higher Level Stewardship stubbles had the basic Entry Level Stewardship requirements plus reduced herbicide use and cereal crop management prior to the overwinter stubbles. Non-Environmental Stewardship stubbles were rotational stubbles without restrictions on herbicide or cultivation practices. Seed-eating birds were surveyed on two visits to each site between 1 November 2007 and 29 February 2008.
A replicated site comparison study in winter 2007-2008 on farms in East Anglia and the West Midlands, England (Field et al. 2010b) (same study as (Field et al. 2010a)) found more seed-eating farmland songbirds on overwinter stubbles managed under Entry Level Stewardship than on non-stewardship stubbles in the West Midlands (average 6.0 birds/ha on Entry Level Stewardship vs 2.5 birds/ha on conventionally managed stubble). This difference was not significant for farms in East Anglia (3.5 birds/ha on Entry Level Stewardship stubble vs 0.7 birds/ha on conventionally managed stubble fields). Overwinter stubble fields in stewardship schemes have restrictions on herbicide use and cultivation times. The group of birds analysed included tree sparrow Passer montanus and corn bunting Emberiza calandra, but not grey partridge Perdix perdix. More of these birds used overwinter stubbles on Higher Level Stewardship farms than on Entry Level Stewardship farms. There were 5 birds/ha compared to 2 birds/ha on average, on stubble fields on Higher Level Stewardship and Entry Level Stewardship farms respectively. The survey was carried out in winter 2007-2008 on 27 farms with Higher Level Stewardship, 13 farms with Entry Level Stewardship and 14 with no environmental stewardship.
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