The dorsal elevators, principally the deltoideus major, can effect the recovery stroke by themselves, as they did in Archaeopteryx. The German anatomist Maxheinz J. Sy proved this when he cut the tendons of the supracoracoideus in living crows and pigeons (1936). Sy found that pigeons were capable of normal, sustained flight; the only capacity they lost was the ability to take off from level ground.https://repository.si.edu/bitstream/handle/10088/6524/VZ_93_Archaeopteryx.pdf?sequence=1&isAllowed=y
Furthermore, the supracoracoideus muscle,
and hence an ossified sternum, is not necessary to effect the
recovery stroke of the wing. Thus the main evidence for
Archaeopteryx having been a terrestrial, cursorial predator is
invalidated. There is nothing in the structure of the pectoral girdle of Archaeopteryx that would preclude its having been a powered flier.