Cassin's Vireo whacking a wooly bear caterpillar (Vireo cassinii)

I think the wooly bear caterpillars can sting, which could be why this Cassin's Vireo was whacking this one repeatedly and quite hard against the branch before trying to eat it. Sorry about the branches blurring this shot, but I wanted to show this behavior. I think I've posted a better shot of this bird a while back, maybe right after seeing it. It was one of three lifers I saw at the North Lake that day.