Taxidermy: An Unusual Twin Beaked Greyhen (Tetrao tetrix), circa 1948, a large full mount adult hen Black Grouse, with two beaks, stood upon moss covered faux rockwork, set against a wash painted back drop, enclosed within an ebonised framed four-glass table display case, 43.5cm by 19.5cm by 39cm, together with another smaller specimen stood upon moss covered faux rockwork (2)

Beak abnormalities occur during the animal's lifetime as a result of an encounter with a predator or a disease, or as a result of the choices the animal makes in its lifetime. Other abnormalities occur during the animal's development within the egg or the womb. Some abnormalities that occur during development produce deformed individuals. They can be caused by a variety of factors, including temperature, the mother's nutrition, genetic recombination, and environmental pollutants; however, across all species deformities are uncommon.