Northern harrier (Circus cyaneus) ?

Possibly the same bird as in the previous photo. I'm not sure of the identification of either photo.
This one was perfectly posed between me and the oak tree when I first saw it. It flew away less than a minute later.

I have tentatively changed the species of this bird at Dörthe's suggestion. I'm still not sure about this. I continue to see them from time to time, so perhaps I can get some better photos.