Birds have lightweight bones, many of which are hollow. They also have fewer bones than reptiles and mammals do. For added strength, the hollow bones are supported by internal struts (much like the struts that support the hollow wings of planes).

Believe it or not a rabbit's bones weigh about the same as a chicken's bones! Though a bird's bones are lightweight, the total skeleton weight is the same as a mammal of the same weight. The size of an animal, whether bird or mammal, determines the relative weight of its bones. Large animals, such as ostriches and elephants, have bones that weigh proportionately more, because they need more support to counter the effects of gravity.

A rabbit skull weighs a lot more than a skull of a bird of the same size, such as a chicken. A rabbit and a chicken both weigh about two pounds, yet the rabbit's skull weighs 12 grams, while the chicken's skull weighs only 4 grams.