Grebes eat a lot of small feathers, their own shed feathers and plumes floating on the water. They feed them to their chicks too. Feathers may be the first thing that the chicks eat.
The thought is that the feathers protect the birds’ stomachs from pointy fish bones—until the bones are soft enough to go through the rest of the way. The feathers may provide some health benefits too. Ron Dudly writes a little more about it on his photography site and the Wilson Journal of Ornithology has more yet.
Joseph R Jehl, The Wilson Journal of Ornithology 129(3):446-458. 2017