Birds had to relearn flight after meteor wiped out dinosaurs
This seems to say that forests were gone, so all tree dwelling birds died out and thus all modern birds are descended from ground dwelling birds.  Looking at the paper [1] it seems to only say 'reduced flight capacity' which makes more sense, considering we only know of 4 times flight has only evolved so this would be a big deal if birds had evolved it twice.1. https://www.cell.com/current-biology/fulltext/S0960-9822(18)...