X-ray films of starlings in flight have shown that the furcula expands when the wings are pulled downward, and snaps back as they are raised. Acting like a spring, the furcula stores some of the energy generated by contraction of the breast muscles. Then it releases the energy during the upstroke as the furcula snaps back to the normal position. This, in turn, draws the shoulders toward the midline of the body.
Surprisingly, the furcula is not essential for flying, and there are a number of birds which can fly well, but have a reduced furcula, or none at all.