Keel

The keel (carina sterni) is proportionally largest in powerful fliers like swifts and hummingbirds and absent in ratites (ostriches, emus), which have a flat sternum. The depth of the keel reflects the mass of the pectoralis and supracoracoideus muscles. In archaeological contexts, keel depth on subfossil bones can indicate whether an extinct bird was a strong flier or had become flightless.