The nest pictured above was occupied by four bluebird nestlings for 17 days. Amazing how clean it is. We all know birds poop ALL the time — so how can this be?
Bluebird (and other) nestlings excrete their poop in sacks. The sacks are removed by the parents (yep, mom AND dad) and placed in a location far from the nest. Creation is amazing! Insects are not drawn into the nest, and predators are not led to the nest, by the presence of piled-up droppings.
In the photo, below, a parent bluebird is shown removing a poop sack from an active nest.