Bird handling

Today, while at work, I had to rescue(ok maybe the word rescue isn't the best word) a bird-it looked to be sort of like a sparrow, maybe a bit bigger, who was crashing into one of our large windows trying to get out of the building. There were open doors all around, and I was trying to get the bird to go out the window. Finally I got a broom and kinda got him cornered with that, and sort of held him down with it while picking him up from the window sill with my other hand. He tweeted as loud as he could, guess he thought I was gonna squash him or something. Anyway I took him outside and off he flew. Afterwards I thought maybe that wasn't the best way to handle him but it was all I had to work with at the time. If there is anyone here who knows about birds, my question is why do they crash into windows if there is an opening near by? Is it that they cannot see very well?