PATH USE IN CRANES - A FIELD-TEST OF OPTIMAL FORAGING PREDICTIONS

The marginal value theorem states that foragers should leave patches w hen the instantaneous capture rate has fallen to the average capture r ate for the habitat. This predicts that patch residence time should in crease with increasing patch quality and decrease with increasing habi tat quality. These and other predictions from prescient, Bayesian and fixed-time models were tested using observations on 14 radio-tagged fr ee-living common cranes, Grus grus, foraging in cereal farmland. Crane s behaved as Bayesian foragers. Their intake rates on leaving patches changed with patch and habitat quality. The behaviour of cranes was co nsistent with marginal value theorem predictions only in patches where energy return was lower than required to meet daily food requirements . In contrast, birds left richer patches earlier than expected and at higher intake rates than poor patches; In addition, cranes stayed long er in larger flocks. These results suggest that cranes changed their f oraging rules according to their expected energy balance.