In the past couple of days, I have watched flying birds. It is striking how those that travel in flocks follow different rules when it comes to synchronizing their wings. Small birds seem to ignore synchronization, big birds often adhere to it.
Smaller birds may fly in a flock, but any matching of wings seems to be merely by chance. Incidentally, these are Bohemian Waxwings. Notice that there is no hint of a crest on the head, which is collapsed when flying (see A crest would be a drag).
These are Trumpeter Swans. Five of them are flying together and matching their wings.