What kind of bird is nesting in my garage?
From your description, I would have to say that it is probably a barn swallow. These birds love to use all kinds of man-made objects such as house eves, the under side of bridges, and the occasional box. Beware these birds like to build colonies, if you leave them alone and in you garage you could find it filled with hundreds of these small birds. Not only that but once the birds have laid eggs they will become extremely territorial and attack anyone who comes into your garage. Needless to say if you have kids they wont be able to go anywhere near your garage. You should take steps to remove the nest as quickly as possible before you take the risk on injuring eggs or baby birds, no one wants that on there conscience.
