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Pesticides disrupt farmland bird food chains

December 17, 2001

Preliminary results are emerging from important new work on the indirect effects of agricultural pesticides on farmland birds in Britain. At the British Ecological Society`s Winter Meeting, at the University of Warwick on 18–20 December 2001, Tony Morris of the Royal Society for the Protection of Birds (RSPB) will present early evidence from a DEFRA funded project aimed at understanding the role of indirect effects from pesticides in the declining populations of a number of farmland bird species. The results of the work, which was instigated by DEFRA`s Pesticide Safety Directorate, will be important in ensuring that indirect effects are properly considered before agricultural pesticides are approved.

In the past, some pesticides are known to have had direct toxic impacts on birds, but these have been withdrawn and currently approved pesticides are unlikely to have such effects. However, work by the Game Conservancy Trust has already shown that for the grey partridge the use of herbicides at particular times of year reduced weed numbers. This in turn reduced the availability of insect food for grey partridge chicks, contributing to a dramatic decline in bird numbers. For other species such a causal link has been suspected but not demonstrated.

The DEFRA funded research, carried out by a consortium of leading research and environmental organisations1 will present its final results and recommendations in 2003. As part of this work, the RSPB has examined the effects of pesticide use on breeding yellowhammers and skylarks. Insect numbers and birds were recorded, as well as details of pesticide applications, on a number of mixed and arable farms in Oxfordshire, Berkshire, Lincolnshire, Suffolk and Norfolk.

Morris says: "Results from the RSPB research indicate that pesticide, especially insecticide, spraying in spring and summer reduces the abundance of key insects, which are an important part of farmland birds` diets."

The results so far suggest that crop spraying indirectly affects the birds by killing the insects they feed on, or by killing the weeds that the insects live on. Morris found that while looking for food, yellowhammers avoid fields sprayed with summer applications of insecticides, and that yellowhammer and skylark raise fewer chicks in nests that are in, or near, summer-sprayed fields. "It appears that the timing of pesticide application is critical. Most significant effects on breeding birds are associated with pesticide use in spring and summer, rather than cumulative use over the crop year," says Morris.

In common with many other European nations the UK Government`s Quality of life indicator shows that farmland bird numbers are declining far more quickly than birds from other habitat types. Populations of several farmland bird species have more than halved in the last 25 years, including the tree sparrow (down by 95%), corn bunting (down by 85%), grey partridge (down by 84%) and turtle dove (down by 70%). The Government has adopted a Public Service Agreement to reverse these declines by 2020, and it is hoped that a better understanding of indirect effects of pesticides will contribute to this objective.

British Ecological Society (BES)