Where do birds sleep?

Where do birds sleep?

I am a bird lover and almost every day I take pictures of birds in my backyard. Some are migrating birds and some (many) others are local for my area. Today around 5:30 pm, I saw a downy woodpecker getting inside of a birdhouse, so I started searching online where they sleep. And I found out that they normally have roost holes in trees, where they spend the night. For my surprise, this woodpecker liked our birdhouse. This bird house was the nest for Chickadee family of 4 babies last spring (2012). Hereby is the picture that I took of him and some very interesting sites about where birds like to spend the night.

Downy woodpecker

http://www.wvdnr.gov/wildlife/magazine/Archive/06winter/AvianQuestions.pdf
http://birding.about.com/od/birdbehavior/a/Birds-At-Night.htm