Lori Hayes, age 10, of Tacoma, Wash., for her question:

DO BIRDS SWEAT?

Birds have no sweat glands and therefore cannot cool their bodies by perspiring. When birds are in flight, excess heat is dispersed by the passage of air through the air sacs. When they are at rest, they rid themselves of heat by panting.

Many bones of adult birds are hollow rather than filled with marrow and are connected to a system of air sacs dispersed through the body.

The bodies of birds are highly modified for efficiency in flight. The digital and wrist bones of the forelimbs are extensively fused to form a rigid support for the large flight feathers of the wing. Fusing bones for strength and lightness is also found in the skull and pelvic girdle.

The sternum, or breast bone, of most birds is relatively huge and bears a central ridge, or keel, known as the carina. The sternum and carina support some of the major muscles used in flying.

Toothlessness lightens the weight of the skull, another adaptation for flying.