Each newborn defecates within seconds of feeding. Four baby birds defecating forty times a day is a big mess. Although nestlings don’t come with diapers, Mother Nature understands the health risks of a nest littered with droppings and created the next best thing.
Each of the approximate one hundred sixty daily droppings comes neatly enclosed in a white, translucent sac known as a fecal sac. This eliminates a nest painted white from droppings; but now the parents have to get rid of the bouncing sacs. They are able to rid the nest of the bagged poop, conveniently deposited directly into their mouths right after feeding, before they leave to find more food. Since the fecal sacs are devoid of bacteria, the adult robins simply eat them.