1. Pesticides applied to seeds before sowing may present a high risk to seed-eating birds. Some of these chemicals are highly toxic but are avoided by captive birds in laboratory tests. However, evidence for birds avoiding them in the field is lacking. The objective of this study is to investigate avoidance and other factors affecting risk of poisoning, using the model of wood-pigeons Columba palumbus L. feeding on winter-drilled cereal seed treated with the organophosphorus insecticide fonofos.
2. Wood-pigeons feeding on arable fields in Cambridgeshire, UK, were counted once or twice per week over four winters from 1992 to 1996. In autumn, newly drilled cereal fields are shown to be preferred to alternative crops such as stubble and ploughed fields and root crops. In spring, newly drilled cereal fields are sometimes preferred to older cereal fields, stubble, ploughed fields and drilled onions.
3. Fewer fields drilled with fonofos-treated cereal seed were used by wood-pigeons than untreated (without fonofos) fields for the first week after drilling only. The extent of the avoidance reaction was related to the concentration of fonofos on the seed left exposed on field surfaces.
4. The concentration of fonofos on newly drilled seed exposed on field surfaces (mean of 241 mg kg–1) was much lower than expected from the approved application rate (1080 mg kg–1). Residues decayed linearly at a rate of 2% per day after drilling and varied widely between fields.
5. The density of seed left exposed after drilling also varied widely between fields, being greater on headlands (field edges) than the main field and declining at a rate of 2% per day after drilling.
6. Of 61 wood-pigeons shot within the study area over three winters, 26 had been feeding on cereal seed and of these, six contained fonofos residues at levels similar to poisoned birds submitted to the MAFF Wildlife Incident Investigation Scheme.
7. In conclusion, fonofos-treated cereal seed may be partially avoided in the wild. Avoidance fails to prevent poisoning under some (rare) conditions, possibly when seed density, concentrations of fonofos and levels of hunger are all above average. The implications for risk assessment are discussed.