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Regurgitation is the expulsion of material from the pharynx, or esophagus, usually characterized by the presence of undigested food or blood.
Regurgitation is used by a number of species to feed their young. This is typically in circumstances where the young are at a fixed location and a parent must forage or hunt for food, especially under circumstances where the carriage of small prey would be subject to robbing by other predators or the whole prey is larger than can be carried to a den or nest. Some bird species also occasionally regurgitate pellets of indigestible matter such as bones and feathers.
It is in most animals a normal and voluntary process unlike the complex vomiting reflex in response to toxins. Honey is produced by a process of regurgitation by honey bees, which is stored in the beehive as a primary food source.
In humans it can be voluntary or involuntary, the latter being due to a small number of disorders. Regurgitation of a person's meals following ingestion is known as rumination syndrome, a rarely diagnosed eating disorder. It may be a symptom of gastroesophageal reflux disease (GERD).
A subset of people, most current or former bulimics, are able to regurgitate without using any external stimulation or drug, by means of muscle control. Practitioners of yoga have also been known to do this. Professional regurgitators perfect the ability to such a degree as being able to exploit it as entertainment.
Birds often employ regurgitation to feed their young. In the Blue-footed Booby, the chicks feed on regurgitated fish from their parents. Only one chick can feed at a time using this method, so there is always the emergence of a brood hierarchy. The dominant chick, who is usually older, is almost always fed before the subordinate, younger chick, and so in times of food hardship, siblicide may occur, where the dominant chick kills its younger sibling in order to sequester all his parents' resources.
Regurgitation as a method of feeding is also seen in the Red-collared Widowbird.
Pollenia sp. blowing bubbles (video, 3m 1s)