It is a common belief that the skeletons of birds are “lightweight,” but when examined more closely, what is meant by this is unclear. In order to be considered lightweight, bird bones must be light relative to something else. Looking at their thin and delicate appearance, it might be logical to assume that bird bones are light relative to non-flying animals of a similar size, however, this is not the case. Weight for weight, bird skeletons take up the same body mass percentage as the skeletons of equivalently sized mammals.
Some birds have pneumatized (air-filled) spaces within their largest bones, which can reduce skeletal mass by between 8 and 13 %. This aids with flying, however it is restricted to only some bird species and so it cannot be critical for flight.
The bones of flying birds are thinner than those of other animals. In order to be both thin and maintain the strength required to cope with the huge physical loads that are applied to them in flight, bird bones are denser than those of equivalently sized flightless animals. Bird bones are thin, but they are far from as delicate as they appear.
Birds tend to be small animals, which does aid with flying, and they stand out from flightless animals of a similar size in that they have a relatively large surface area to volume ratio. Even when standing with wings folded, birds have a much larger surface area for their mass than other animals. With wings extended, this increases dramatically, creating lift and enabling control.
Large bones require large support structures, that is, musculature to move them and blood vessels to supply them with nutrients. Although bird bones themselves are denser and, proportionally, take up a normal percentage of body weight, their small relative volume means they require less in the way of support. In this way, the skeletons of birds enable them to have a smaller total body mass relative to their surface area. Bird skeletons are not lightweight, but because of their unique structure, bird bodies can be.Edit Summary