How do feathers in a grebe’s stomach help with digestion?

Grebes swallow feathers to aid in their digestive process. These feathers form a plug within the pyloric pocket of the stomach, which acts as an effective filter for material passing into the intestine.1

For grebes that consume invertebrates containing significant amounts of chitin, this mechanism helps them frequently regurgitate pellets. For species that primarily eat fish, the mass of feathers is maintained in the main portion of the stomach, where it is believed to hold fish bones in place until they are fully digested. Adults also feed these feathers to their young to establish this protective plug shortly after hatching.1