Why is this rook always hanging out with jackdaws instead of other rooks?

Since last year I've been seeing this rook around the fields in my town, and it's recently started coming to my garden. I've never seen any other rooks around it, only jackdaws. I know it's not strange for rooks and jackdaws to flock together, but I've always found it weird that a social bird like a rook would never be around its own kind at all. Just now it came into my garden with two jackdaws, and they ate for a few minutes and then they all flew off together. I'm not concerned or anything, it seems alright, but I'd like to know why it would choose to be around only jackdaws and not other rooks. Sorry about the blinds in the photo, I didn't want to scare it off by moving them.