To be completely fair to the birds, I think a lot of this comes from the fact that what matters for flying efficiency gets better as you size up thanks to the

To be completely fair to the birds, I think a lot of this comes from the fact that what matters for flying efficiency gets better as you size up thanks to the square-cube law: the drag is reduced and the engine efficiency increases. You could not scale down a plane to the size of a bird and match their efficiency. On the other hand you also couldn’t scale a bird to the size of a plane so I guess we can call it even!