I think he is wanting to make sure he gets 'his share'. We have them too, we set our feeder so the heavy birds like him can't get to the seeds. They would clean it out really fast. They have to go to an open feeder or wait for me to feed the squirrels.
Years ago a cat that we let outside once in a while climbed a tree and about 5 or 6 Blue Jays started attacking her. That was the last time she climbed that tree. We were out there trying to get her to come down, they got her down for us.