Fecal sac

A fecal sac (also spelled faecal sac) is a special mucus covering that surrounds the feces of some baby birds, called nestlings. This sac is usually white or clear with a darker end. It helps parent birds clean up after their young. When a nestling eats food, it quickly produces a fecal sac. This makes it easier for parents to remove the waste from the nest.

Not all birds make fecal sacs. They are most common in a group of birds called passerines and their relatives. These are birds whose baby birds stay in the nest for a long time. Some parent birds even eat the fecal sacs of very young birds. Others carry the sacs far away from the nest to throw them away. As baby birds get older and ready to leave the nest, or fledge, they usually stop making fecal sacs.

Cleaning up fecal material helps keep nests healthy. It also reduces the chance that predators will find the nest. Scientists find fecal sacs useful. By studying them, they can learn what a baby bird has been eating and if it has been exposed to harmful substances. When scientists see an adult bird carrying a fecal sac, it tells them that the birds are breeding. This is important information for bird censuses.