We have a camera set up that takes pictures of the hummingbird nest every few minutes and transmits them to the Internet. Yesterday, we noticed one baby missing from the nest. After verifying with a few more photos that the baby was missing, we started looking for it under the nest. We found it a few feet away and scooped it up and put it back. After a while, it settled in next to its sibling and we were relieved when the mother returned and fed them both.
They should fledge in about a week, and we will be nervously watching and hoping it doesn't fall out again.
We think the mother has got the message. Today, a windy day, she sat on the nest on top of the chicks.