Sarah,
 I witnessed this same thing last summer in my atlas block. A  Red-tailed 
Hawk flew too close to an Eastern Kingbirds nest territory, and they  flew out in 
pursuit. One of them harassed the hawk by landing on and  pecking at its 
back. There is an illustration in the Sibley behavior guide of  just such an 
incident.
 
Rick Sussman
Ashton,MD