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

Animals can adapt to changes in feeding conditions by switching between for aging methods. Dabbling ducks use different foraging methods, including dab bling in deep water with the head and neck submerged, and grubbing in the m ud (or shallow water) where the eyes are above the surface, so the bird can visually monitor its environment while foraging. Deep foraging is consider ed to provide lower intake rates and to have high associated costs, such as predation risk, compared to shallow foraging. Ducks should thus prefer sha llow foraging and switch to deeper methods when feeding conditions deterior ate. We conducted a set of experiments with Mallard to assess the importanc e of intake rate as a cue to choose between patches associated with differe nt foraging methods, and evaluate the influence of food depletion on the de cision 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 le d to an increase in deep foraging, although the birds still preferred the s hallow 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 pa tch was empty. These results show that food depletion affects the choice be tween feeding patches hence foraging method. However the value of intake ra te is not the main cue for decision, rather the birds appear to choose betw een patches with different methods on account of their respective costs. (C ) 2000 Elsevier Science B.V. All rights reserved.