Eastern Kingbirds, songbirds in the flycatcher family, have been known to be extremely aggressive toward Red-tailed Hawks. The Kingbirds' normally hidden red "crown" is exposed when the bird is agitated.
The Kingbird will literally cling onto the back of a flying hawk...
...and peck viciously at the hawk's head and neck.
The Red-tail keeps one eyelid shut as it desperately tries to shake off the aggressive and persistent Kingbird!