Why do grebes eat feathers?

14 Oct 2017 - 11:23 -- Eduardo de Juana

Grebes (order Podicipediformes) are unique among birds in the habit of eating their own feathers and later casting them in pellets. Why they do this has been the subject of controversial speculation for centuries. Suggestions include: 1) protection of the digestive tract; 2) enabling pellet formation; 3) guarding against or reducing parasites; 4) aiding digestion. Observations of the Black-necked or Eared Grebe (Podiceps nigricollis) in western North America suggest that the last of these may be the best explanation#R. Feather-eating may assist these birds to exploit abundant tiny aquatic invertebrates, such as brine shrimp, that can be hard to digest. Grebe stomachs are adapted for chemical, not mechanical, digestion and have three consecutive chambers: a large and highly glandular proventriculus; a relatively thin-walled gizzard with an undifferentiated lining; and a final small accessory chamber, the pyloric pouch. The ingested feathers form two distinct balls, one in the gizzard and the other in the pyloric pouch. The gizzard ball, relatively large, is used to retain food until can be fully liquified, and that of the pyloric pouch serves to prevent undigested or indigestible items from entering the intestine. Some of the gizzard ball is probably regurgitated nightly but the pyloric plug is expelled irregularly. This process allows Black-necked Grebes to achieve very high assimilation efficiency: over 87% for the adult brine shrimp and alkali flies that comprise their near-exclusive diet during the non-breeding season. Regarding the possible origin of this habit, it is interesting to note that flamingos and grebes are nowadays recognized as sister groups, despite their great morphological differences, both being successful in exploiting tiny aquatic invertebrates in hypersaline environments.

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Black-necked Grebe (Podiceps nigricollis)

Taxonomy:

Podiceps nigricollis

C. L. Brehm, 1831, Germany.

Sometimes placed in genus Dytes. Often considered to include †P. andinus as a race. Also closely related to P. juninensis, P...

Descriptive notes: 28–34 cm; 265–450 g. Distinctive in breeding plumage, the only grebe combining black head and neck, and yellow ‘ear-tufts’; bill looks slightly upcurved at tip; crown feathers erectile, usually forming peaked...
figure, drawing, illustration of Black-necked Grebe (Podiceps nigricollis)