Birds are best thought of as the little feathered dinosaurs that they are, generally speaking a bird will eat anything it can fit down its tough and leathery gullet. Once I seen a purple martin fledgling, about as big as a cardinal fully grown but unable to fly, leave the nest and fall to the ground. A flock of about fifty grackles immediately lifted up off of a nearby lawn, landed all around it and commenced to peck it to death.
IIRC Kingbirds aren't known for robbing nests, but I'm sure they would if given the opportunity.
Hey Mike, managed to get the glasses on the bird bothering the swallow nest box. Best I can figure it's an ash throated flycatcher. Sibley says they are cavity nesters. Maybe it wants that box badly. So far, the swallows are winning.
ETA: nighthawks haven't made it up here yet. Wonder if ours go all the way down to Argentina too?
