A few days ago, I saw a couple of squirrels were having some kind of argument; they chased each around my yard & up a tree, which had a bird house on it that was home to a family of house sparrows. The squirrels weren’t interested in the chicks as far as I could see, only in chasing each other around & around the tree, but when the squirrel’s disagreement brought them close to the sparrow’s nest, the parents flew out of the bird house, & started dive-bombing the squirrels. The squirrels were startled, & quickly retreated. It was amazing how they wouldn’t stand up to an animal half their size & a fraction of their weight. The sparrows must have pretty sharp beaks.
Here is a video of a bird fighting with a squirrel:
& two squirrels fighting: