Topic summary
Nestling
Extracted from the Wikipedia article Bird.
Excretory system
It is a common but not universal feature of altricialpasserine nestlings (born helpless, under constant parental care) that instead of excreting directly into the nest, they produce a fecal sac. This is a mucus-covered pouch that allows parents to either dispose of the waste outside the nest or to recycle the waste through their own digestive system.