Measuring the risk of predation in an energy currency: field experiments with foraging blue tits, Parus caeruleus

A technique for measuring the ‘risk of predation’, in an energy currency, is described. Experiments on free-living blue tits determined the extent to which feeding in a risky site was compensated by making food easier to obtain. The birds were trained to use operant feeding devices which were placed at different distances from the edge of woodland tree cover. The reward schedules at the feeders were controlled by a computer, which adjusted the number of hops required to obtain a food reward in relation to the extent to which each feeder was used. When the feeders were used equally the reward schedules stabilized and were recorded. There was little difference in the final reward schedules of feeders placed under the cover of trees, but the frequency of reinforcement required to persuade the birds to feed in the open rose dramatically at relatively short distances (1·5m) from cover.