I realized what it was, scrambled to get my camera out of the bag, and then just pointed towards the object that was closest to where the sound was coming from and started clicking. This is the first shot I got:
And then one of the birds flew by overhead:
Although the light was dim, I liked the story, the action captured in the first photo, and the classic silhouette of a Peregrine Falcon in the second.
Were these falcons in a territorial dispute?