One of the admirable, but dangerous, aspects of the Tree Swallow personality is that they will attack you. With great tenacity.
I was walking on the trails by this nest box, minding my own business. I was on the trail, not trying to get a closer look or hassling them in any way. I just passed them and suddenly I heard chirps and wing flaps, felt the small gusts of wind as they swooped down on me, beaks positioned to peck.
They are protecting their young, of course, but they can't be hatched. It's too early. Maybe they are just practicing. Maybe they hate me!