"Fish gotta swim, birds gotta fly…"
I hate to say it, but Jerome Kern and Oscar Hammerstein got it wrong in the latter part of their lyric for the Show Boat tune, "Can't Help Lovin' Dat Man." Of course, they probably weren't all that familiar with nuthatches.
If you're a nuthatch, flying is optional…as is gravity.
Should a nuthatch wish to get from one portion of a forked tree trunk to another, it could hippity-hop down one and ratchety-climb back up the other. Or it could just keep its wings tucked, fling itself in the intended general direction, and trust the rest to luck and levitation.
Nope…birds don't always gotta fly! Not if you're a nuthatch!