Written by: Dr. Jacquie Jacob, University of Kentucky
When a hen lays an egg, the lower part of the oviduct, the vagina, folds inside out through the cloaca. This movement helps to minimize contact between fecal matter and the egg. Normally the vagina is brought back into the body cavity quickly. Prolapse is a condition in which the vagina fails to retract (and the term applies when greater lengths of the oviduct remain exposed).
Prolapse is common in overweight hens and small, early-laying pullets (young hens). The oviduct may be slow to retract when a hen is too fat. Prolapse can also be caused by injury to the vagina, which can occur when a large egg is laid. In houses that lack nests or that are brightly lit, when the vagina is exposed during laying, other hens may see the moist, red tissue in the vent area and peck at it. This causes the tissue to become inflamed, which prevents the vagina from retracting normally. When this happens, other birds in the flock may be attracted and peck the protruding organ, causing the whole oviduct and a portion of the small intestine to be pulled out. Hens with this extreme prolapse generally die from shock. It is unlikely that a bird with prolapse will recover, and it should be euthanized.
Refer to the article "Avian Reproductive System—Female" for more information about the organs of a hen's reproductive tract.