Fecal sac

Mucous membrane produced by bird nestlings

A fecal sac is a mucous membrane, generally white or clear with a dark end, that surrounds the feces of some species of nestling birds. It allows parent birds to more easily remove fecal material from the nest. The nestling usually produces a fecal sac within seconds of being fed; if not, a waiting adult may prod around the youngster's cloaca to stimulate excretion. Young birds of some species adopt specific postures or engage in specific behaviors to signal that they are producing fecal sacs. For example, nestling curve-billed thrashers raise their posteriors in the air, while young cactus wrens shake their bodies. Other species deposit the sacs on the rim of the nest, where they are likely to be seen by parent birds.

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