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, and it helps parent birds clean up after their young. When a nestling eats food, it quickly produces a fecal sac, making 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, whose baby birds stay in the nest for a long time. Some parent birds even eat the fecal sacs of very young birds, while 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 and reduces the chance that predators will find them by smelling or seeing the waste. Scientists also find fecal sacs useful because they can tell what a baby bird has been eating and what harmful substances it may have been exposed to by studying the contents. When scientists see an adult bird carrying a fecal sac, it tells them that the birds are breeding, which is important information for bird censuses.