Foraging methods can affect patch choice: an experimental study in Mallard (Anas platyrhynchos)

Animals can adapt to changes in feeding conditions by switching between foraging methods. Dabbling ducks use different foraging methods, including dabbling in deep water with the head and neck submerged, and grubbing in the mud (or shallow water) where the eyes are above the surface, so the bird can visually monitor its environment while foraging. Deep foraging is considered to provide lower intake rates and to have high associated costs, such as predation risk, compared to shallow foraging. Ducks should thus prefer shallow foraging and switch to deeper methods when feeding conditions deteriorate. We conducted a set of experiments with Mallard to assess the importance of intake rate as a cue to choose between patches associated with different foraging methods, and evaluate the influence of food depletion on the decision to switch between methods. When 50 g of wheat were presented in two patches, one at a depth of 5 cm and one at 35 cm, most of the foraging was in the shallow area. Reducing food abundance to 10 g in the shallow area led to an increase in deep foraging, although the birds still preferred the shallow area at the beginning of the tests despite the fact that it did not provide a higher intake rate. This area was used until complete depletion, and birds did not turn to deep foraging before ensuring that the shallow patch was empty. These results show that food depletion affects the choice between feeding patches hence foraging method. However the value of intake rate is not the main cue for decision, rather the birds appear to choose between patches with different methods on account of their respective costs.