The bones of birds maximize stiffness and strength relative to weight by increasing density.

“The skeletons of birds are universally described as lightweight as a
result of selection for minimizing the energy required
for flight. From a functional perspective, the
weight (mass) of an animal relative to its lift-generating surfaces is a
determinant of the metabolic cost of flight. The
evolution of birds has been characterized by many weight-saving
that are reflected in bone shape, many of which
strengthen and stiffen the skeleton. Although largely unstudied in
the material properties of bone tissue can also
contribute to bone strength and stiffness. In this study, I calculated
density of the cranium, humerus and femur in
passerine birds, rodents and bats by measuring bone mass and volume
using helium
displacement. I found that, on average, these bones
are densest in birds, followed closely by bats. As bone density
so do bone stiffness and strength. Both of these
optimization criteria are used in the design of strong and stiff, but
manmade airframes. By analogy, increased bone
density in birds and bats may reflect adaptations for maximizing bone
and stiffness while minimizing bone mass and
volume. These data suggest that both bone shape and the material
properties of
bone tissue have played important roles in the
evolution of flight. They also reconcile the conundrum of how bird
can appear to be thin and delicate, yet contribute
just as much to total body mass as do the skeletons of terrestrial
mammals” (Dumont 2010)

Last Updated August 18, 2016