An adult mistle thrush stands over its nest with something white and squishy dangling from its beak. The songbird is holding a package filled with its baby’s poop! The bundle, called a fecal sac, is basically the avian version of a disposable diaper.
Baby birds are born completely helpless, says Michael Murphy. He’s a retired wildlife biologist who studied birds at Portland State University in Oregon for more than 40 years. For several days after hatching, nestlings are blind and featherless. They’re unable to do much except eat and poop—which they do almost constantly. When a baby thrush does its business, it sticks its rear in the air so a parent can pluck out the fecal sac.
The sac is made of a thin, slimy tissue called a mucous membrane. The membrane prevents feces from leaking out and makes the package easy to carry. To get rid of the sac, the parents often do something humans would find disgusting: They eat it! Adult birds work so hard catching bugs to feed their babies that they barely have time to feed themselves. Baby birds don’t digest and absorb all the nutrients in their food. So fecal sacs still contain plenty of leftovers for the adults.
If the parents don’t snack on a sac, they’ll discard the poop by dropping it a short distance from the nest. The easily disposable fecal sacs allow parents to keep their nests tidy. “If five baby birds were pooping the whole time in the nest, it would be really foul,” says Murphy.