Christan Long of Haast asks :-

When a seagull swallows a fish whole, what happens to the bones?

Christine Thomson, a physiologist at Massey University's Institute of Veterinary, Animal and Biomedical Sciences, responded.

Raw fish is the standard diet of piscivores – fish-eaters, such as seagulls. Small rodents and lagomorphs (hares, rabbits) are the preferred foods for carnivores – meat-eaters, such as raptors (birds of prey). But fish and small prey animals are not just made of meat; they also have scales or fur or feathers and bones, which are relatively indigestible. Birds cannot separate out the meat from the bones before eating (like humans), nor do they have teeth to grind the bones up (like dogs) so everything gets swallowed whole. What happens to the indigestible bits? Bird stomachs have a muscular compartment – the gizzard, and a glandular compartment, which secretes strong acid. Digestion in each stomach compartment depends on the type of bird.

In granivores – grain-eating birds, food is ground up in the gizzard and birds may deliberately swallow small stones to help with this grinding process. In piscivores (e.g. seagulls), the gizzard grinds the fish bones into smaller fragments. These fragments then pass into the glandular stomach where the acid dissolves most of the bones. The remaining bone fragments pass through the intestines and are excreted in the faeces.

Carnivorous birds (raptors) use the muscular gizzard to compact fur, feathers and some bones from their prey into pellets. The pellets are then forced back up the oesophagus/foodpipe and ejected via the mouth. You can sometimes see pellets lying on the ground when you walk through an owl or hawk’s territory. The structure of the digestive systems in animals and birds vary and this reflects the type of food they eat. By studying the anatomy of the digestive system you can work out whether an animal’s preferred diet is sushi, vegetables or a bit of both.