鳴禽類的骨骼或是空的，以便飛行，然而，其骨骼密度事實(shí)上比同大小的哺乳動(dòng)物要高，這樣可以使身體更加有力、堅(jiān)硬，滿足飛行的需要。

For earthbound creatures like us, flight just seems so fantastical. How do birds and bats and other flying beasties manage to get off and stay off the ground? Well, having wings obviously helps. And bird bones are hollow and seem delicate, which should help lighten the load.

Or so you’d think. But a new study shows that songbirds’ bones are actually pretty tough. In fact, they’re more dense than the bones of mammals of the same size. The results appear in the Proceedings of the Royal Society: Biological Sciences. [See http://bit.ly/aODM0G]

For centuries, biologists have known that bird bones are thin and hollow. Yet bird skeletons don’t actually weigh any less than the skeletons of similarly sized mammals. To sort out this seeming discrepancy, Elizabeth Dumont of the University of Massachusetts Amherst studied the skulls and limb bones of song birds, rodents and bats. And she found that, on average, bird bones are the densest, with bat bones coming in a close second.

That denseness makes Tweety’s thin little bones surprisingly strong and stiff, good structural features for flight. And thus for keeping those bones from winding up in the mouth of a hungry mammal. Sorry, Sylvester.

—Karen Hopkin