I remembered an amusing encounter long ago and finally made the effort to get it out of my database:
This provides another theory about why they do it: to tease RTHAs.
Sally Nelson, Creswell, OR *Query1* DATE NOTES MEMO 6/1/2002 ON YOUNG PINE LEADER; DID HAWK CALL-MEMO AFTER SEVERAL HAWK CALLS BEGAN SERIES OF SHAK-SHAK CALLS. TWO RED-TAILS GLIDED IN SAME HEIGHT AS JAY IN TREE AND SEMI-CIRCLED HIM ABOUT 15' AWAY. HE FLEW OFF "LAUGHING"