Here is a close up video of a mother Robin feeding her babies and collecting her babies poop immediately after.

Every time a nestling is fed, they will lift up their rear end and excrete a white sac, which is collected in the beak of the parent bird. This ensures that their parent that brought the food will still be there to carry away the fecal sac. The fecal sac is a strong “bag” made of thick mucus that the parent birds can carry the feces away in.

To put this in perspective, many articles refer to the baby robin’s fecal sac as a diaper. The sac provides the parent bird with a clean way of removing the feces from the nest. Other than the obvious reason why they would want to keep their nest clean, they are also preventing predator detection by keeping their nest free of poop. The nest might be less noticeable to predators that are usually drawn to the sight and smell of feces.