Sometimes the babies fall asleep. It is hard work to be alert to every sound and movement in and around the family nest. But the babies are alert until they have a full belly and then, like human beings, they doze off at every opportunity. I managed to photograph a family from the first egg laid to the last fully fledged robin had left the nest. Most birdwatchers have no idea about the eggs bird lay in nests, but assume, that they eventually hatch, the birds grow up and fly away.
The first bird to leave the nest happens to always be the first egg that was laid. The second egg becomes the second bird to leave the nest and the third day is the third bird to leave the nest. The fourth egg is the last bird to leave the nest. I was able to watch the reaction of the birds left in the nest when the first one flew away—they were not content but seemed to me to be anxious to leave—as if the time was growing short for those in the nest.