Heers et al. 2021 discuss the origin of flight using hatchling birds

Short one today
as Heers et al. 2021 miss the one key trait in the evolution of flapping flight in bird origins: a locked-down elongate coracoid necessary for left-right simultaneous flapping. This morphology is derived from the primitive disc-like coracoid sliding on the sternal rim, originally used during quadrupedal locomotion to extend each stride (Fig. 1). Disc-like coracoids are retained in bipedal theropods like Tyrannosaurus, Khaan, and Haplocheirus.

By contrast
in Velociraptor, Archaeopteryx and living birds the coracoid is no longer mobile and disc-like, but narrows and becomes immobile. These are flapping bipeds. This fact was overlooked by Heers et al. who looked a baby birds.

From the Heers et al. abstract:
“Although extinct theropods are most often compared to adult birds, studies show that developing birds can uniquely address certain challenges and provide powerful insights into the evolution of avian flight: unlike adults, immature birds have rudimentary, somewhat “dinosaur-like” flight apparatuses and can reveal relationships between form, function, performance, and behavior during flightless to flight-capable transitions. Here, we focus on the musculoskeletal apparatus and use CT scans coupled with a three-dimensional musculoskeletal modeling approach to analyze how ontogenetic changes in skeletal anatomy influence muscle size, leverage, orientation, and corresponding function during the development of flight in a precocial ground bird (Alectoris chukar).”

Unfortunately, even baby birds have a locked-down elongate coracoid. So the transition to flapping must be found in the fossil record, not the ontogeny of chicks. Heers et al. needed an outgroup and a convergent set of taxa.

Pterosaurs,
starting with Cosesaurus (Figs. 3, 4) had bird-like immobile coracoids.

Bats
(Fig. 5) don’t have coracoids. but elongate locked-down clavicles are analogous.

References
Heers AM, Varghese SL, Hatier LK and Cabrera JJ 2021. Multiple Functional Solutions During Flightless to Flight-Capable Transitions

Locked down coracoid discussion