Baby birds can stay in nests for many weeks before fledging (learning to fly). The Eastern bluebird takes 3 weeks to fledge, while larger birds like crows take 5 weeks. During that time, the nestlings are confined to a very small space that must be kept clean to prevent infection.
Birds do not urinate and defecate separately, instead they excrete all their waste in one action. Nestlings of some species excrete over the side of the nest, however for others this can attract predators and put the offspring at risk. In order to enable the parents to remove the waste and dispose of it elsewhere, the nestlings of many bird species defecate into fecal sacs. These membranous sacs collect the waste in a tidy parcel that the adult can easily collect and carry away.
The nestlings of different species of birds use different methods to draw attention to the sacs so that the parents notice them. Some have specialized shivering or presenting behaviors, while others deposit the sacs on the rim of the nest.Edit Summary
“The removal of feces of nesting birds provides sanitation and lessens the attractiveness of the nest to predators (Welty and Baptista 1988), but the presence of a mucosal sac around the feces allows parents the option of carrying them away” (McGowan KJ. 1995:50)
A Test of Whether Economy or Nutrition Determines Fecal Sac Ingestion in Nesting CorvidsThe Condor, 97(1): 50-56February 1, 1995
“The manner of defecation of the thrasher and wren was strikingly different. In both cases, the parents remove the feces when present, after they have fed the young. Thus, it is essential that the parents be aware of the presence of fecal sacs in the nest…The thrasher’s cup-shaped nest is deep, and when defecating, the nestling raises its posterior as high as possible, making the fecal sac readily visible from above. This action was invariably displayed on the flat tabletop in the absence of a nest rim. When the nestling is large enough, it can defecate onto the nest rim, making the fecal sac conspicuous without the parent’s having to observe the defecation. Towards the end of its “nestling” period, the hand-raised thrasher frequently defecated between feedings. Body-shaking did not accompany defecation in the thrasher as it did in the wren.” (Ricklefs 1966:49)