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.