How can seagulls digest fish bones?

Aquatic birds have three chambers in its stomach. The first, the proventriculus chamber, secretes gastric juice. The second, the gizzard, has two sets of muscles opposing each other, and this serves as a filter for bones, fur, feathers, and other non-edible items that come from its prey. The third, exclusive to aquatic birds, is filled with feathers and is used to filter out fish bones. The filtered items (bone, fur, feathers, etc.) are made into pellets, similar to owl pellets.