On weekdays a construction road was in regular use by traffic passing to and from an electricity sub-station being built. Farm vehicles used the road at irregular intervals every day. They fed close to banks and they often allowed human approach to as close as 50 m before taking to the air. Only on weekends when traffic on the road was at a minimum did they feed on the south slurry-treated grassland or winter barley near the road. Although Eurasian wigeon preferred feeding on the fertilized slurry-treated grassland, an intolerance of disturbance led them to feed on the less disturbed, but nutritionally poorer unimproved grassland.
Disturbance thus can mask food preferences.
Disturbance and the nutritional value of grassland can be identified as the two main factors controlling distribution and availability of grazing for brant and Eurasian wigeon.