Why do birds do this?

So the other day, my mom was sitting in our living room and then she heard two thumps! So we went out side to make sure everything was all right and then she saw two birds laying on the ground below our window. One of them had his eyes shut and was not breathing, and the other bird was laying on the ground and was hardly breathing with his his eyes wide open and was panicking.

Any way the next say we went outside to look, and the bird that was panicking was gone. The other bird was dead. And it is still laying there. I feel so bad to leave it there, but we are going to bury it soon.

This is what the bird looks like.

My saying for the day is; Don't run into windows!!!!!