Today, a hummingbird flew into a window at my house. When I went to find it, I found a hummingbird, but it was alive, sitting on the cement. Nearby was a dead one. He sat there for quite while. What do you make of this?
Two possibilities come to mind.
First, perhaps the dead one was the other's sibling or young and it was trying to figure out why it hand "landed" on the ground?
Second, perhaps the one alive was the one that hit your window but was stunned and the second one hit your window earlier and died?
I would tend to go with the first one.