Bird bones are specifically adapted to facilitate flight primarily because they are hollow. This adaptation reduces the overall weight of the skeleton without sacrificing strength. Although bird bones do contain some compact bone, the hollowness (a feature known as pneumaticity) allows birds to achieve the essential balance between strength and lightness required for flying.
Additionally, some bones are still reinforced with a network of internal struts that provide structural integrity. In contrast, while birds do have some cartilage in certain areas (such as the flexible joints), their primary adaptations for flight are predominantly seen in the bone structure itself. Thus, the correct answer to how bird bones are adapted to help them fly is that they are hollow.
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