Ardea
Official journal of the Netherlands Ornithologists' Union

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Swennen C. (1968) Nest protection of Eiderducks and Shovelers by means of faeces. ARDEA 56 (3-4): 248-258
During field work on the island of Vlieland, it was noticed that disturbed breeding Anatidae, Common Eiders in particular, soil their nests with ill-smelling faeces before fleeing in a hurry. The alarmed ducks returned to their nests after a short while and eggs hatched in a normal way. This phenomenon of contaminating the nest in alarm situations was not found in other species of birds (Table 1). The question arises as to the significance of this behaviour in ducks, since the strong odour of the faeces released over the eggs makes them easy to find, even in thick vegetation. The chance of contaminated clutches to be left undisturbed by egg-predators seems small, unless the released faeces have a strong repellent effect. Experimentally it was determined that nest faeces from Common Eiders (Somateria m. mollissima) and Shovelers (Anas clypeata), even in small amounts, make food otherwise usually highly preferred, distasteful to hungry ferrets and rats. The behaviour of the predators observed in the experiments suggests primarily an olfactory response. Circumstantial evidence is presented, that nestfaeces from eiders also have this anti-predator effect under natural conditions and that it is effective against more predators than used in the experiments. Faeces from non-breeding Common Eiders do not have this effect. It was also found that excrements of Pheasants, House Sparrows and Black-headed Gulls collected during the breeding season were not discriminated against by rats and ferrets when added to the food. Contrary to many other Anatidae, Common Eiders generally do not show distraction behaviour when disturbed during incubation, as long as the eggs have not hatched. Since Common Eiders in general release more faeces over the eggs than other ducks in alarm situations, it is suggested that this difference is related to the lack of distraction behaviour.


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