You see the new baby birds, but can you also find a little white sac? This is called a fecal sac, and it's just like a disposable diaper for birds! Baby robins produce their poop in fecal sacs, encased in strong membranes that don't leak. The parents pick up the sacs in their beak and carry them away from the nest. Why are fecal sacs important? Baby robins are in their nest for about 13 days. They poop just about every time they gulp down some food. Let's see—that's 13 days x 4 babies x 356 insects and worms on average each day. That's a LOT of poop! Fecal sacs help keep their nest clean.