Bird bones may be hollow, but they are also heavy

ScienceDaily (2010-03-23) -- For centuries biologists have known that bird bones are hollow, and even elementary school children know that bird skeletons are lightweight to offset the high energy cost of flying. Nevertheless, many people are surprised to learn that bird skeletons do not actually weigh any less than the skeletons of similarly sized mammals. In other words, the skeleton of a two-ounce songbird weighs just as much as the skeleton of a two-ounce rodent.

http://www.sciencedaily.com/releases/2010/03/100322112103.htm