Nick Brickle reports on a project to investigate the effect of agricultural intensification on corn buntings Miliaria calandra. The main aim was to test the hypothesis that breeding success was affected by low chick-food availability, caused in part by the use of pesticides. Data from a study site on the South Downs in Sussex showed that corn buntings collected invertebrates for their chicks in areas that received relatively few insecticide and herbicide applications. These areas contained more invertebrates of the types commonly fed to chicks than more heavily sprayed areas. When the abundance of chick-food invertebrates was low close to a nest, breeding success was low. In these conditions chicks were underweight, parents foraged further from the nest and nest failure rate was high. When chick-food invertebrates near the nest were more abundant, breeding success was higher. The relationship between pesticides and food availability, and the relationship between food availability and breeding success, links the corn bunting to other species of farmland birds where pesticide use has been shown to affect breeding success, such as the grey partridge. Similar relationships between pesticides, food and breeding success may occur in some other farmland bird species such as skylarks and yellowhammers.
Corn buntings Miliaria calandra are fairly nondescript birds of open farmland. Once common across much of lowland Britain, they have declined sharply in numbers since the mid-1970s, both in Britain(1) and over much of their European range(2). They are now considered to be one of the 36 species of greatest conservation concern(3).
As their name suggests, corn buntings are closely associated with arable farming, seldom being found away from tilled land and cereal fields in particular. Their decline has been shared by many other farmland species, leading to the hypothesis that changing agricultural practices over the period of the decline have been harmful(4). On tilled land one of the main changes has been a dramatic increase in the use of pesticides(5).
The decline of the corn bunting was first thought to be due to reduced over-winter survival, possibly caused by the decreased availability of stubble fields(6). However, many of the invertebrates fed to nestling corn buntings(7), such as caterpillars (of butterflies, moths and sawflies), grasshoppers, harvest-men and ground beetles, have declined in abundance on farmland(8). This has led to the hypothesis that a general reduction in food availability could have led to a drop in breeding success(9).
Data from a study site on the South Downs in Sussex showed that corn buntings collected invertebrates for their chicks in areas that received relatively few insecticide and herbicide applications. These areas contained more invertebrates of the types commonly fed to chicks than more heavily sprayed areas. When the abundance of chick-food invertebrates was low close to a nest, breeding success was low. In these conditions chicks were underweight, parents foraged further from the nest and nest failure rate was high. When chick-food invertebrates near the nest were more abundant, breeding success was higher.
Parents collecting food for their chicks visited a range of habitats including grassy margins, set-aside and unimproved grassland. Nearly half of their foraging trips were made to cereal crops and it was in these habitats that the actions of pesticides appeared to reduce the availability of key groups of invertebrates. When the availability of other habitats was low a higher proportion of the foraging trips were made to the crops. The relationship between pesticides and food availability, and the relationship between food availability and breeding success, links the corn bunting to other species of farmland birds where pesticide use has been shown to affect breeding success, such as the grey partridge(10). Similar relationships between pesticides, food and breeding success may occur in some other farmland bird species such as skylarks and yellowhammers(11).
What steps could be taken to halt and reverse the decline of corn buntings? Sympathetic management should aim to increase the availability of invertebrates within the arable farming landscape. One approach is to increase abundance outside crops by providing invertebrate rich areas elsewhere. Such areas could include tussocky-grass field margins(12) and 'beetle-banks'(13), both now components of the piloted Arable Stewardship Scheme as well as included in an arable tier of some Environmentally Sensitive Area Schemes. A second approach would be a more environmentally sensitive use of pesticides on crops.
Organically managed fields can support higher levels of invertebrates compared with equivalent conventionally managed fields(14). Alternatively, on conventional farms the use of selectively-sprayed headlands, with reduced input of broad-spectrum herbicides and insecticides, can result in a higher density of caterpillars, sawflies and beetles around the edge of fields(15). Such areas are then readily used by corn buntings. Grassy field margins and beetle banks, as well as providing invertebrate rich habitat in their own right (see above), can increase the density of some invertebrates, such as predatory beetles, in the crops themselves. Including such measures in an integrated crop management regime is likely to result in benefits for farmland birds like the corn bunting and other wildlife.
Further benefits could be gained if the use of broad-spectrum insecticides on crops was reduced in favour of more specific chemicals that are less harmful to non-target insects. The use of such commonly sprayed pyrethroid insecticides (eg. deltamethrin, cypermethrin, lambda-cyhalothrin) could be discouraged, either through legislation, or perhaps more practically, through incentive schemes such as Arable Stewardship and the Environmentally Sensitive Area schemes.
The decline of the corn bunting appears to be continuing unchecked. Their future, together with that of many other farmland species, depends on making changes to current intensive farming practice. If more of the measures mentioned here are implemented, or incentives made more widely available, then the declines of corn buntings and other species could be halted and even reversed.
Comments
Dr Nicholas Aebischer of the Game Conservancy said: "This study shows clearly that condition during the breeding season can contribute to the corn bunting's decline in some parts of the UK. The variety of field types needed for nesting and feeding during the year highlights the importance of having a mixture of arable and pasture for livestock within a farm."
Jonathan Curtoys of the Royal Society for the Protection of Birds (RSPB) has called for more environmentally-friendly policies to be adopted. He said: "The government should increase spending on agri-environment schemes such as the Organic Aid Scheme and Arable Stewardship Scheme which reward farmers for wildlife-friendly farming and more selective use of pesticides. A tax should also be levied on the more harmful pesticides and the use of broad-spectrum pesticides on crops should be phased out. These chemicals kill target species and a wide range of associated wildlife."
References
1. Hagemeijer, W.J.M. and Blair, M.J., The EBCC Atlas of European Birds: their distribution and abundance, Poyser, London, 1997.
2. Tucker, G.M. and Heath, M.F., Birds in Europe: their conservation status, BirdLife Conservation Series No. 3, BirdLife International, Cambridge, 1994.
3. Gibbons, D.W., Avery, M.I., et. al., Bird species of conservation concern in the United Kingdom, Channel Islands and Isle of Man, revising the Red Data List, RSPB Conservation Review, 1996, 10, 7-18.
4. Fuller, R.J., Gregory, R.D., et. al., Population declines and range contractions among lowland farmland birds in Britain, Conservation Biology, 1995, 9, 1425-1441.
5. Campbell, L.H., Avery, M.I., et. al., A review of the indirect effects of pesticides on birds, JNCC, Peterborough, 1997.
6. Donald, P.F., The corn bunting Miliaria calandra in Britain. In: The ecology and conservation of corn buntings Miliaria calandra (eds. P.F. Donald and N.J. Aebischer), pp 11-26, JNCC, Peterborough, 1997.
7. Brickle, N.W. and Harper, D.G.C. (in press) Diet of nestling corn buntings Miliaria calandra in southern England examined by compositional analysis of faeces.
8. Op. cit. 5.
9. Op. cit. 6.
10. Potts, G.R., The Partridge: Pesticides, Predation and Conservation, Collins, London, 1986.
11. Op. cit. 5.
12. Aebischer, N.J. and Blake, K.A., Field margins as habitat for game. In: British Crop Protection Council Monograph No. 58: Field margins: integrating agriculture and conservation, pp 95-103, Farnham 1994.
13. Thomas, M.B., Wratten, S.D. and Sotherton, N.W., Creation of 'island' habitats in farmland to manipulate populations of beneficial arthropods: predator densities and emigration, Journal of Applied Ecology, 1991, 28:906-917.
14. Moreby, S.J., Aebischer, N.J., et. al., A comparison of the flora and arthropod fauna of organically and conventionally grown winter wheat in southern England, Annals of Applied Biology, 1994, 125:13-27.
15. Sotherton, N.W., Conservation headlands: a practical combination of intensive cereal farming and conservation. In: The Ecology of Temperate Cereal Fields (eds. L.G. Firbank, N. Carter, J.F. Darbyshire and G.R. Potts), 305-331, Blackwell, Oxford, 1991.
This work was carried by Nick Brickle of the University of Sussex. It was organised by The Game Conservancy Trust and funded by English Nature and the RSPB.
[This article first appeared in Pesticides News No. 43, March 1999, page 17]
Sources:
http://www.pan-uk.org/pestnews/Issue/pn43/pn43p17.htm
http://www.jstor.org/pss/2655923
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