Feather eating in Great Crested Grebes Podiceps cristatus: a unique solution to the problems of debris and gastric parasites in fish-eating birds

The occurrence of feathers in stomachs of 407 Great Crested Grebes Podiceps cristatus from Lake IJsselmeer, The Netherlands, is described. Only four of the 8718 identified stomach feathers originated from a species other than Great Crested Grebes. In all, 68% were breast and belly feathers and 19% were flank feathers. Flank feathers were positively selected from those becoming available during moult, and also occurred more in the ejected feather pellets than expected from the intake. This suggests that flank feathers, which are long and curved and have a black tip, may be especially suited for the formation of pellets. Feather ingestion was more than balanced by feather production. Feathers were most abundant in stomachs in autumn and were least numerous in winter. Most of this variation can be explained by seasonal differences in the intensity of body moult. In addition, the type of prey was related to the number of ingested feathers. When the diet consisted of smelt, which leave little indigestible matter, more feathers were eaten than with a diet of perch and pikeperch, which leave more fish debris. We suggest the hypothesis that ingested feathers, in the absence of other indigestible matter, contribute substance to the stomach content, enabling the formation of pellets that can be ejected. The habit of regularly ejecting the stomach contents minimizes the chance that any serious population of gastric parasites will build up in the upper part of the alimentary tract.