Pectoral Girdle

Wishbones and Breastbones

Flight musculature in the early reptilian birds is poorly understood because they lack large bones to which strong flight muscles could have been attached. In modern birds, the pectoral and supracoracoideus muscles that power the wings are anchored to a large bony keel along the midline of the sternum.

The fossil record suggests that part of that keel may have developed from an extension of the hypocleideum or tail of the wishbone. There is a particularly intriguing similarity between the keel of Cathayornis (Enantiornithes) and that of the modern day Hoatzin.

In most birds the keel extends the full length of the breastbone so that the flight muscles can be as long as possible. Pelicans and frigatebirds are both strong fliers but their keel is rather small and concentrated near the front of the breastbone. In both birds, the wishbone and breastbone are fused together and the keel appears to be an extension of the hypocleideum as much as a ridge on the breastbone.

In both of those birds, the bones are greatly inflated and may play an important role in heat regulation. Excess heat generated by the flight muscles is radiated into the hollow core of the bones and passed out of the body with the exhaled air. In Turkey Vultures, the large and inflated wishbones have numerous openings for connection to the air sacs. Hawks also have well-developed wishbones but owls are among the birds that may lack one. In some owls the wishbone is little more than a loop of ossified ligament connecting the clavicles.

Other parts of the Pectoral Girdle

Further information on the wings and shoulder bones of birds can be found in most ornithological texts and on other websites.