The black bird is a Crow, and it is mobbing the Great Blue Heron. Mobbing is a behavior where birds harass predators to get them to leave the area. It is not unusual to see quite a few birds chasing a hawk, owl, or eagle, swooping and diving at it. Birds will also mob cats, dogs, snakes, and other predators. Sometimes they harass other animals, such as this Great Blue Heron, if they get too close to an active nest.