Shortly after being fed, most songbird nestlings produce what is called a fecal sac – a mucous membrane (usually white with a dark end) that contains their excrement. This sac enables easy removal of the chicks’ waste by the parents. In an effort to keep its young healthy and to reduce the chances of predators finding the nest, the parent takes the fecal sac in its beak, flies away and discards it some distance from the nest.
Some nestlings, including those of Warbling Vireos, are extremely cooperative when it comes to assisting their parents in keeping the nest clean. Just prior to producing a fecal sac they position themselves with their rear ends up in the air (see photo), where their parents can easily procure the fecal sacs when they appear.
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