All well and good. But why does that happen in modern birds when it didn't happen in ancient birds?
Remember, birds are very, very ancient. They used to come in two varieties ~ those with breastbones, and those without breastbones. At the time of the great destruction of the dinosaurs, the birds without breastbrones appear to have all been destroyed.
Recent findings suggest that all modern birds are descended from one small branch of birds with breastbones ~ the waders.
We, on the other hand, find our origins among mammals with attitudes.