Action: Reduce grazing intensity
- Nine studies from the USA and the UK, one replicated and controlled, found increases in populations of some species on fields with reduced grazing, or increased use of such fields by birds. Three of the studies used multiple interventions at once. Five studies from Europe, four replicated and controlled, found that some or all species were no more numerous on fields with reduced grazing, compared to intensively-grazed fields. One paired sites study from the UK found that black grouse Tetrao tetrix populations increased at reduced grazing sites (and declined elsewhere), but that large areas of reduced grazing had lower densities of female grouse.
- A before-and-after study from the USA found that the number of species on plots with reduced grazing increased over time. A replicated, controlled study from four countries in Europe found no differences in the number of species on sites with low-intensity or high-intensity grazing.
- One replicated trial in the UK found that some bird groups preferred grassland short in winter (grazing effect simulated by mowing), and others preferred it long (unmown to simulate removal of livestock). Frequency and timing of the simulated grazing did not alter this preference.
Overgrazing is responsible for the degradation of habitats across the world, being especially damaging in arid environments, where the removal of vegetation can quickly lead to soil erosion. Reducing grazing intensity may reduce the damage to vegetation and can also help reduce disturbance to birds and accidental loss of nests.
Supporting evidence from individual studies
A small 1967 study in Maryland, USA (Burger & Linduska 1967), investigated the impact of limiting livestock grazing, as well as other interventions, on northern bobwhites Colinus virginianus and found that the population on the farm increased from five to 38 coveys in eight years. This study is described in ‘Plant new hedges’.
A before-and-after study in an 8,357 ha grassland site under rest-rotation grazing since 1967 in Montana, USA (Mundinger 1976), found that the number of wildfowl nesting on the site, the species richness and the number of broods produced all increased between 1970 and 1973-4 (190 pairs of seven species producing 127 broods in 1970 vs. 270 pairs of 12 species producing 191 broods in 1974). The grazing regime involved five areas of the site being grazed at different times each year to allow the vegetation to recover. The highest densities of wildfowl were found in areas that had been rested in the previous year.
A before-and-after study in Gloucestershire, England, (Owen 1977), found that the proportion of geese on a grassland site using a specifically managed 130 ha area increased from 33% in the winter of 1970-1971 to 87% by 1975-1976, following a reduction in grazing intensity over this period. Starting in 1970, stock were sequentially removed from three sections of the area: the first was ungrazed from the 30th September, the second from the 31st October and the third from the 30th November. A fourth area was not grazed at all. Other interventions are discussed in ‘Increase crop diversity’ and ‘Undersow spring cereals’.
A randomised, replicated and controlled study in spring and summer 1995-6 on 12 fields in Sussex, England (Wakeham-Dawson et al. 1998), found that Eurasian skylark Alauda arvensis densities were significantly higher on fields grazed at lower intensities (4.4-14.3 birds/km2 for six lightly-grazed fields vs. 1.3-2.4 birds/km2 on six intensely-grazed fields). The density of carrion crows Corvus corone and rooks C. frugilegus did not vary between treatments. Intensively-grazed fields were managed to keep the sward under 10 cm long, less intensively managed fields had a 15-25 cm sward. This study is also described in ‘Undersow spring cereals’, ‘Revert arable land to permanent grassland’, ‘Habitat restoration and creation’ and ‘Provide or maintain set aide areas in farmland’.
A paired sites study on moorland in 1996-2000 in northern England (Calladine et al. 2002) found that the number of displaying black grouse Tetrao tetrix males increased by an average of 5% each year at 10 sites where levels of sheep grazing were reduced, compared with average declines of 2% each year at ten control sites. Changes were most positive in the first years after grazing reduction. The proportion of females with chicks was also significantly higher at treatment sites (average of 54%) than at control sites (32%). However, there were declines in female densities at sites where restricted grazing areas exceeded approximately 1 km2. Grazing was reduced to below 1.1 sheep/ha in summer and 0.5 sheep/ha in winter for at between one and five years on treatment sites. Densities were two or three times higher on control sites.
A before-and-after study of grazing marshes in east England from 1993-2003 (Smart & Coutts 2004) found that the number of northern lapwing Vanellus vanellus and wildfowl increased and vegetation communities changed following a reduction in grazing intensity and improved footdrain management in 1996. This study is discussed in ‘Raise water levels in ditches or grassland’
A randomised, replicated, controlled trial on four farms in southwest England in 2003-6 (Defra 2007) found that 12, 50 ´ 10 m plots of permanent pasture managed as conventional silage but without autumn/winter grazing did not attract more foraging birds than 12 control plots, managed identically but with autumn and winter grazing. Plots were fertilised and cut twice in May and July. This study is also discussed in ‘Reduce management intensity on permanent grassland’, ‘Reduce pesticide or herbicide use generally’, ‘Raise mowing height on grasslands’, ‘Undersow spring cereals’ and ‘Plant wild bird seed or cover mixture’ .
A controlled replicated trial in the UK (Vale & Fraser 2007) found that the response of bird populations to the removal of grazing from upland improved grassland between late May and July varied between functional groups of birds and depended on the time of year. Plots with seasonal removal of grazing had the greatest number of birds of songbird species between May and July (126 birds compared to 60 in control plots), and between July and September (312 birds compared to 169 in control plots), but numbers were similar to those in control plots between October and January (13 and 11, respectively). Between July and September, there were more birds of invertebrate-feeding species on plots with seasonal removal of grazing (105 birds, compared to 41 on control plots), but between October and January there were more birds on continuously grazed plots (5,833 birds, compared to 1,458 on plots with seasonal removal of grazing). At all times of year, crows were more abundant on continuously grazed plots. Bird numbers and species were recorded in plots with and without seasonal removal of grazing for silage making (10 replicates).
A replicated, controlled trial in four European countries (UK, Germany, France and Italy) from 2002-4 (Wallis De Vries et al. 2007) found that numbers of birds and bird species were not different between fields under low-intensity grazing, compared to intensively-grazed fields. Birds were counted every two weeks in early morning, from May to October in 2002-4, with a 7 minute observation period and a walking transect. Exact grazing regimes differed between countries.
A randomised, replicated trial of different winter cutting regimes, designed to simulate grazing intensity on grasslands in Oxfordshire, England (Whittingham & Devereus 2008), found that different groups of birds prefer different treatments. Foraging song thrushes Turdus philomenus and common starlings Sturnus vulgaris, crows and Eurasian kestrels Falco tinnunculus preferred mown (grazed) plots to unmown (ungrazed) plots. Grey herons Ardea cinerea and meadow pipits Anthus pratensis preferred unmown plots to plots that were mown once or twice. For gamebirds, wood pigeons and hedgerow species, there was no significant difference in numbers between the different mowing regimes. Seventeen grass fields (average size 5 ha) were used in the experiment, with two treatments (mown once vs. unmown) or four treatments (unmown, mown once at two different times or mown twice) in each. Winter mowing simulates the effects of grazing or cutting for silage. Grass height did not differ between the 14 replicate plots mown once in November/December, once in January or twice during winter, so one winter cut or grazing period was sufficient to create the habitat advantage for bird groups that prefer short grass.
A 2009 literature review of agri-environment schemes in England (Natural England 2009) describes a case study of a farm on Exmoor, Devon, which found that three species increased on the farm from 1993-2003, following a reduction in grazing intensity on moorland areas (Eurasian skylark Alauda arvensis increased from none to 13 birds; Eurasian linnet Carduelis cannabina from none to nine birds; common stonechat Saxicola torquata from none to one territory). One species (meadow pipit Anthus pratensis) showed little change (nine birds vs. eight) and another (northern wheatear Oenanthe oenanthe) declined slightly, from one territory to none. This review also examines several other interventions, discussed in the relevant sections.
A review of UK experiments on the effects of agri-environment measures on livestock farms in the UK (Buckingham et al. 2010) found two replicated controlled trials that reduced grazing pressure (fewer cattle, cattle removed from July onwards, or both) over two to four years. One also reduced fertiliser input from 150 to 50 kg N/ha. Reduced grazing significantly increased the number of foraging skylarks Alauda arvensis on the trial fields in both studies. Birds that eat only seeds - European goldfinch Carduelis carduelis and linnet Carduelis cannabina - preferred plots with cattle removed in July. These studies formed part of a Defra-funded project (BD1454) for which no reference is given in the review. The study including low fertiliser input used eight replicates, the other used 14. The review assessed results from four experimental projects (one incomplete at the time of the review) in the UK. This study also discusses other interventions, described in the relevant sections.
- Burger G.V. & Linduska J.P. (1967) Habitat Management Related to Bobwhite Populations at Remington Farms. The Journal of Wildlife Management, 31, 1-12
- Mundinger J.G. (1976) Waterfowl response to rest-rotation grazing. The Journal of Wildlife Management, 40, 60-68
- Owen M. (1977) The role of wildfowl refuges on agricultural land in lessening the conflict between farmers and geese in Britain. Biological Conservation, 11, 209-222
- Wakeham-Dawson A., Szoszkiewicz K., Stern K. & Aebischer N.J. (1998) Breeding skylarks Alauda arvensis on Environmentally Sensitive Area arable reversion grass in southern England: survey-based and experimental determination of density. Journal of Applied Ecology, 35, 635-648
- Calladine J., Baines D. & Warren P. (2002) Effects of reduced grazing on population density and breeding success of black grouse in northern England. Journal of Applied Ecology, 39, 772-780
- Smart M. & Coutts K. (2004) Footdrain management to enhance habitat for breeding waders on lowland wet grassland at Buckenham and Cantley Marshes, Mid-Yare RSPB Reserve, Norfolk, England. Conservation Evidence, 1, 16-19
- Defra (2007) Potential for enhancing biodiversity on intensive livestock farms (PEBIL). Defra report.
- Vale J.E. & Fraser M.D. (2007) High Value Grassland: Providing Biodiversity, a Clean Environment and Premium Products. British Grassland Society Occasional Symposium No.38. Pages 333-336 in: Effect of sward type and management on diversity of upland birds. British Grassland Society (BGS), Reading.
- Wallis De Vries M.F., Parkinson A.E., Dulphy J.P., Sayer M. & Diana E. (2007) Effects of livestock breed and grazing intensity on biodiversity and production in grazing systems. 4. Effects on animal diversity. Grass and Forage Science, 62, 185-197
- Whittingham M.J. & Devereux C.L. (2008) Changing grass height alters foraging site selection by wintering farmland birds. Basic and Applied Ecology, 9, 779-788
- Natural England (2009) Agri-environment schemes in England 2009 A review of results and effectiveness. Natural England report.
- Buckingham D.L, Atkinson P.W., Peel S. & Peach W. (2010) New conservation measures for birds on grassland and livestock farms. Proceedings of the Lowland Farmland Birds III: delivering solutions in an uncertain world, 60.