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?
If they are the nighthawks that fly up in the sky, say "peeeent!" and then dive and zoom they are common nighthawks and they go clear south of the rain forest. If they mostly stay lower, are a bit slower and make a trilling sound, they are lesser nighthawks and only go as far as Guatemala/Honduras. Lesser nighthawks are desert birds, we get both on account of San Antonio is so hot and dry in summer.
Easy solution to the next box problem, put up another one. The flycatcher will probably win, and when it does it will fill up the whole nest box cavity with nesting material and then build a nest on top of that by the entrance. Weird, but that's what they do.
