Which bird can eat only when its head is upside down?

The flamingo is well-known for its habit of eating with its head upside-down. This behavior is largely due to the structure of its mouth and the logistics of lowering its long, graceful neck into the water Flamingos are filter feeders, using their tongues as a sieve to catch food. It will put its neck down under the water with its mouth essentially upside down, then close its mouth and force the water through comb-like extensions on its beak, using its tongue to push the water out while keeping all the food in.

Source: Dawn