The furcula ("little fork" in Latin) or wishbone is a forked bone found in birds and some other dinosaurs, and is formed by the fusion of the two clavicles. In birds, its primary function is in the strengthening of the thoracic skeleton to withstand the rigors of flight.
The furcula works as a strut between a bird's shoulders, and articulates to each of the bird's scapulae. In conjunction with the coracoid and the scapula, it forms a unique structure called the triosseal canal, which houses a strong tendon that connects the supracoracoideus muscles to the humerus. This system is responsible for lifting the wings during the recovery stroke.
As the thorax is compressed by the flight muscles during downstroke, the upper ends of the furcula spread apart, expanding by as much as 50% of its resting width, and then contracts. X-ray films of starlings in flight have shown that in addition to strengthening the thorax, the furcula acts like a spring in the pectoral girdle during flight. It expands when the wings are pulled downward and snaps back as they are raised. Acting like a spring, the furcula is able to store some of the energy generated by contraction in the breast muscles, expanding the shoulders laterally, and then releasing the energy during upstroke as the furcula snaps back to the normal position. This, in turn, draws the shoulders toward the midline of the body. While the starling has a moderately large and strong furcula for a bird of its size, there are many species where the furcula is completely absent, for instance Australian scrubbirds and bush birds, some toucans and South American barbets, some owls, some parrots, turacos from Africa and mesites of Madagascar. These birds are still fully capable of flying. They also have close relatives where the furcula is vestigal, reduced to a thin strap of ossified ligament, seemingly purposeless. Other species have evolved the furcula in the opposite direction, where it has increased in size and become too stiff or massive to act as a spring. In strong flyers like cranes and falcons, the arms of the furcula are large, hollow and quite rigid.
In other animals
Seeing the occurrence in diplodocid dinosaurs of interclavicles, Tschopp and Mateus (2013)  proposed that furcula is a transformed and divided interclavicle, rather than a fused clavicle.
Superstitions surrounding divination by means of a goose's wishbone go back to at least the Late Medieval Period. Johannes Hartlieb in 1455 recorded the divination of weather by means of a goose's wishbone, "When the goose has been eaten on St. Martin's Day or Night, the oldest and most sagacious keeps the breast-bone and allowing it to dry until the morning examines it all around, in front, behind and in the middle. Thereby they divine whether the winter will be severe or mild, dry or wet, and are so confident in their prediction that they will wager their goods and chattels on its accuracy.", and of a military officer: "This valiant man, this Christian Captain drew forth out of his doublet that heretical object of superstition, the goose-bone, and showed me that after Candlemas an exceeding severe frost should occur, and could not fail." The Captain also said, "Teutonic knights in Prussia waged all their wars by the goose-bone; and as the goose-bone predicted so did they order their two campaigns, one in summer and one in winter." The custom of two persons pulling on the bone with the one receiving the larger part making a wish develops in the early 17th century. At that time, the name of the bone was a merrythought. The name wishbone in reference to this custom is recorded from 1860.
- Gill, Frank B. (2007). Ornithology. New York: W. H. Freeman and Company. pp. 134–136. ISBN 0-7167-4983-1.
- Proctor, Noble S.; Lynch, Patrick J. (October 1998). Manual of Ornithology: Avian Structure and Function. Yale University Press. p. 214. ISBN 0-300-07619-3.
- The Inner Bird: Anatomy and Evolution, by Gary W. Kaiser
- Currie, Philip J.; Padian, Kevin (October 1997). Encyclopedia of Dinosaurs. Academic Press. pp. 530–535. ISBN 0-12-226810-5.
- Carpenter, Kenneth (July 2005). The Carnivorous Dinosaurs. Indiana University Press. pp. 247–255. ISBN 0-253-34539-1.
- Tykoski, Ronald S.; et al. (September 2002). "A Furcula in the Coelophysid Theropod Syntarsis". Journal of Vertebrate Paleontology. 22 (3): 728–733. doi:10.1671/0272-4634(2002)022[0728:afitct]2.0.co;2.
- Tschopp, E., & Mateus O. (2013). "Clavicles, interclavicles, gastralia, and sternal ribs in sauropod dinosaurs: new reports from Diplodocidae and their morphological, functional and evolutionary implications". Journal of Anatomy. 222: 321–340. doi:10.1111/joa.12012.
- Edward A. Armstrong, The Folklore of Birds (1970), cited after Davis, Marcia. "Wishbone myth has long history". Knoxville News Sentinel. Retrieved 27 September 2012.