Black-shouldered Kite: Let it Be Known
Black-shouldered Kite, Elanus axillaris
This is Belle, the female of the local clutch. There is a second pair of Kites working on a nest in the same area.
Not something I've encountered before.
The raised tail and lowered wings are a good sign that she is claiming territory.
HANZAB explains it as territory defence and calling on the resident male to expel the intruding bird.