My wife and I were out, I said" look at that hawk." It was flying erratically over a field of wildflowers. As it came closer it looked really odd. it passed 15' in front of us and off popped its hitchhiker, it had a bird riding on it's back. We both saw it distinctly. It was a black and white bird, too big to be a chickadee, more like woodpecker size. I have seen birds harassing hawks before, but never riding ones back for a hundred yards.
Pretty cool! I had a similar experience, caught these a few weeks back. There were four Eastern King Birds harassing this hawk. I had never seen birds actually ride a hawk either, only dart in and out taking jabs, but these birds would land on her back and pluck feathers for up to 5-7 seconds. Only difference from your story, is that this bird never showed a single sign that the birds were bugging her, not a flinch.
Red Wing blackbirds harass the heck out the hawks around our place. Crows and Starlings to a lesser degree. Never seen one hitch a ride though.
They’ll ride the hawks to distract them from their nests. Some birds are finishing raising their 2nd broods for the year. Cool sighting.