Perched on a pylon in the fields. It was being harassed by a flock of White-breasted Wood-Swallows, who were unsurprisingly disturbed by the presence of this assassin in their midst
At first I thought it was a harrier, it was so large! It flew around the fields three or four times before heading south past the mountain. Features of callidus visible in this photo are "narrower moustaches, bigger white cheek patches..." as well as "on average bulkier and longer-winged..." (Ferguson-Lees & Christie). The streaking on the breast indicate this is a juvenile.