Background information and definitions
Conventional ploughing uses a mould-board plough, cultivating to a depth of around 20 cm. This can damage soil structure and potentially reduce the abundance of soil invertebrates, a food source for many farmland birds. This intervention includes various methods to reduce the depth or intensity of ploughing, such as layered cultivation, non-inversion tillage and conservation tillage. It also includes stopping tillage altogether, sometimes called 'no till'.
Supporting evidence from individual studies
A replicated, controlled study from May-July in 1982-1984 in nine experimental sites and three control sites in cropland in Iowa, USA (Basore et al. 1986), found that farmland bird species richness and nesting density and success were higher in fields without tillage. In total, 12 species were found nesting in the non-tillage fields with an average density of 36 nests/100 ha whereas only three species with an average of 4 nests/100 ha nested in tilled fields. Nest success was greatest in fields with corn residue (48% nestling survival rate). Three no-tillage treatments (corn planted in corn residue: 125 ha); corn planted in sod residue: 117 ha); and soybeans planted in corn residue: 113) and one control treatment (corn planted in tilled fields: 129 ha) were surveyed each year. Discovered nests were monitored every 2-3 days.Study and other actions tested
A replicated, controlled, site comparison study from 1991-1993 in ten reduced tillage, ten organic and ten conventional agricultural fields in North Dakota, USA (Lokemoen & Beiser 1997), found that more farmland birds nested on reduced-tillage than conventional fields (1 nest/10 ha vs. 0.5 nests/10 ha). Minimum tillage fields also possessed a significantly greater diversity of nesting species (2 species/field vs. 1). In spring, bird densities in minimum tillage fallow fields were higher than those in organic fallow, minimum tillage sunflower and wheat fields and all conventional fields. There were no differences in bird abundance between treatments in other seasons but fallow fields (across treatments) exhibited the highest densities in summer (1-2 individuals/ha). There were no significant differences in nest loss or daily survival rate between treatments.Study and other actions tested
A replicated, controlled study from June-July in 1996-1997 in 37 conservation tillage, 40 organic, 38 conventional and 31 wild (control) sites in both upland and wetland areas of crop farms (75% wheat) in Saskatchewan, Canada (Shutler et al. 2000), found that bird diversity and abundance were highest overall in wild areas compared to farmed areas, highest in conservation tillage farms in upland areas and in organic farms in wetland areas. In upland areas, of 37 species recorded, one was more abundant on farms, four more abundant in wild areas while the rest showed no distinct preference. Conservation tillage wetlands had similar bird communities to conventional wetland farms. Clusters of four treatments were located within a 25 km radius of one another. Fixed-radius (100 m) point-count surveys were used to survey twice per year.Study and other actions tested
A 2004 review of the effects of non-inversion tillage (NIT) on farmland bird abundance across the world, with special reference to the UK and Europe (Cunningham et al. 2004) found that the evidence for positive bird responses to NIT is inconclusive. Four studies in North America found higher bird density, diversity and nest productivity on NIT fields and another found greater bird diversity in summer on NIT fields (but not in autumn, winter or spring). Three studies found that Eurasian skylarks Alauda arvensis, gamebirds and seed-eating songbirds are more abundant on NIT fields. However, one study found that NIT fields act as ecological ‘traps’ when nests are destroyed by mechanical weeding. The authors point out that this type of weed control is not common in Europe.
A review of the effects of conservation tillage relative to conventional ploughing (Holland 2004) found mixed effects for birds. One study showed no effect on five bird species in the context of organic farming. Another showed a higher number and diversity of birds on conservation tillage fields in Spain.Study and other actions tested
A replicated, controlled study in the winters of 2000-2003 in 63 experimental and 58 control winter wheat and barley fields in Oxfordshire, Leicestershire and Shropshire, UK (Cunningham et al. 2005), found that Eurasian skylarks Alauda arvensis, seed-eating songbirds and gamebirds occupied a significantly higher proportion of fields managed through non-inversion tillage than conventionally ploughed fields in late winter (January-March). Species richness of seed-eating songbirds was also higher on non-inversion tillage fields (five species vs. one on conservation tillage fields). No birds showed any preference for field type in early winter (October to December), and crows, pigeons and insect-eaters showed no preference across the study period. Field size ranged from 1.6 to 22.3 ha, with similar numbers of non-inversion tillage and conventionally ploughed farms censused each year.Study and other actions tested
A replicated controlled paired site study from October to March 2003-6 in 12 pairs of winter wheat fields in Dióskál, Hungary (Field et al. 2007), found that the preference of some farmland birds for conservation tillage fields over adjacent ploughed fields (P) decreased over the study period. In one farm (with eight field pairs), Eurasian skylarks Alauda arvensis and seed-eating songbirds (mostly European goldfinches Carduelis carduelis) were more abundant on conservation tillage fields in the first winter (2003-4), whilst starlings Sturna vulgaris and skylarks were more abundant on conservation tillage fields over the second and third winters respectively. In a second farm, with four fields, skylarks and crows were more abundant on conservation tillage fields in the first winter only. The number of days with ground snow cover increased over the three years. The authors suggest such abnormal weather may have confounded the results.Study and other actions tested
A small replicated, randomised, controlled study from April-July 2005 in two experimental and two control fields of winter wheat in Rutland, England (Field et al. 2007), found that Eurasian skylark Alauda arvensis nest density was higher in fields managed through conservation tillage than control fields that were ploughed, with 24 of 32 nests found in conservation tillage fields. Average laying date was also significantly earlier on conservation tillage fields by 25 days. The authors suggest the effect was due to conservation tillage fields containing more crop residue than control fields (32% residue compared to none). Foraging distance of provisioning adult skylarks was 50% lower on conservation tillage fields (48 m vs. 93 m). However, nest success and nestling size were similar in both field types. Control fields were sown with winter wheat after mould-board ploughing, while conservation tillage fields were direct drilled into oil-seed rape residue after light rotary harrow.Study and other actions tested
A replicated, controlled study in the winters of 2006-8 in four (2006-7) and two (2007-8) fields of winter oilseed rape on a single farm in Cambridgeshire, UK (Dillon et al. 2009), found that bird densities were similar between non-inversion tillage and control fields. Neither individual species nor groups of species responded to differences in crop establishment. However, the Farmland Bird Index (which included omnivores, carnivores, insect-eating birds and seed-eating species) was significantly higher on control fields. The authors point out that the overall densities on both treatments were still relatively low compared to other interventions (such as wild bird seed and over-winter cereal stubble). Two surveys were made in each field each month between September-March across the whole field area.Study and other actions tested
A replicated, controlled study from April-June in 2006-2007 in 48 conservation tillage, 31 organic and 63 conventional winter barley and wheat fields in Seine-et-Marne, France (Ondine et al. 2009), found that that species differed in their responses to management. Two species were more abundant in conservation tillage fields than conventional fields, whilst seven were more abundant on conservation tillage fields than on organic. One species was more abundant on conventional fields and five on organic, compared to conservation tillage. Specialist species were least abundant on conservation tillage fields, whilst insect-eating birds were more abundant. The authors point out that conservation tillage fields were more intensely managed than conventional fields and experienced much disturbance.Study and other actions tested