Meandering in my neighborhood park, I notice a bird flapping around a squirrel that is clinging to a tree trunk . The bird is about the same size as the squirrel, maybe a little smaller, so at first it seems like an unlikely cross-species game of tag. The bird swoops around the squirrel, who darts around the trunk. The bird settles on a branch of the same tree, and the two stare at each other, unmoving. Then the dance repeats. I have seen squirrels chase one another–sometimes it feels like play, sometimes like aggressive territoriality–but this has an altogether different feel. There is a sense of urgency here, of desperation. And suddenly I realize that the bird is a falcon. It wants to eat the squirrel.