Bill sweeping in spoonbills Platalea : no evidence for an effective suction force at the tip

We analysed the hypothesis of Weihs and Katzir (1994) that feeding spoonbills use their broad bills by sweeping it through the water to shed a vortex. This would result in a hydrodynamic suction on the bottom for catching prey immediately or during the next sweep besides providing some extra benefits. The basic assumptions appear to be erroneous. (1) A spoonbill does not mainly feed on small, benthic invertebrates, but mainly on nekton such as fish and shrimps. (2) The inner surface of the upper mandible of a spoonbill is not concave but convex. (3) During feeding, a spoonbill does not keep its bill tip close to the bottom independent of the water depth. The outcomes of the tests supporting the hypothesis do not hold and the suggested benefits are dubious. Therefore, we reject the hypothesis and the claim that the bill of a spoonbill is used as a hydrofoil. The discussed paper tried to give an explanation of the behaviour of a bird with the help of hydrodynamic formulae. The fundamental mistake in the paper is that the use of the formulae is based on wrong assumptions concerning the food of the bird, its feeding behaviour as well as its anatomical details. The tests with a bird kept in captivity suggesting a proof of the rightness of the hypothesis lack a proper connection to the situation in the wild.