I spent an hour watching the rivalry play out at our bird feeder the other day.  There were black capped chickadees and red breasted nuthatches coming and going freely, getting their fill of seed prior to the cold snap moving in.  Suddenly they would disperse, and one of the red bellied woodpeckers would take over the feeder.  Their size and aggressiveness would send the smaller birds away.  But don’t rule them out…the chickadees and nuthatches would fly in quickly, grab their seed, and leave just as quickly.  Eventually the woodpeckers would move to the nearby trees, and the smaller birds would reign over the seeds.  It was fascinating to watch, and it seemed that everyone had their time at the feeder.