Western kingbirds are known to be bullies. I have seen them attack raptors many times their size, including bald eagles. This past June, I was photographing an osprey nest and as one of them circled around, a kingbird came out of nowhere. The little yellow and gray bird was less than pleased that the raptor was near its territory and made sure to give the osprey an escort away. It happened so fast it caught me off guard and I only got two shots of the action but it was fun to see.
