In Tree Swallows, the last egg laid may not hatch for 1-2 days after the others, since the female starts incubation after laying the penultimate (next-to-last) egg. That is why it is important not to remove unhatched eggs too early. In areas where temperatures are high, development may start before incubation officially begins.
Anyway, I’m guessing the developmental differential is due to asynchronous hatching. I suppose it could also just be a runt that is not getting as much food, as I have seen similar variations in size with bluebird nestlings.
Unfortunately I don’t know the exact age of these nestlings.
Nature is NOT putting up boxes for the birds to nest in. Putting up a box is interfering with nature. With that intervention comes responsibility.
Nicholas A. Zbiciak, Bluebird_L, 2000