Laura Erickson's For the Birds

(Recording of an American Robin)

Everybody knows what a bird is–birds are among the very first animals that babies and toddlers learn to recognize. As a group, birds are far more uniform than mammals–differences between a robin and a duck and an ostrich and a penguin are minor compared to the differences between a bat and a whale and a wolf and a human being.

The feature above all others that determines whether an animal is a bird is its feathers. Feathers are the strongest material known for their size and weight. Every bird has feathers, and no other animal has these wonderful and specialized outgrowths of skin.

Birds have a unique skeleton, too–many of their bones are fused together, and to lighten their load even more, many of their bones are hollow. But birds don’t sacrifice strength for weight–bird bones must support huge pectoral muscles and a powerful pelvic girdle at the same time that they’re light enough to be airborne. To accomplish this, many of the bones have internal struts, much like the struts inside airplane wings. And the ribs are elegantly long, flat, thin, and jointed. Each rib overlaps its neighbor, reinforcing its own strength and that of the adjoining rib. Even the paper-thin bones of a songbird’s skull are delicately but powerfully reinforced. There is beauty in such functional form, inspiring Elliot Coues, an early American ornithologist, to call the avian skull a “poem in bone.”

The tail bone of birds is extremely reduced. Even a scissor-tailed flycatcher’s long tail feathers are attached to just a tiny stump of a bone called the pygostyle.

To further lighten the load, birds have neither teeth nor the need for a heavy jaw bone. Their food is mashed up nearer their center of gravity, in their gizzard. And the light- weight, streamlined beak also replaces a mammals’s fleshy nose.

A bird’s respiratory system is equally wondrous. Not only are its lungs extremely efficient, permitting a double-tide of fresh air over the lung surface with each breath, it also makes use of those hollow bones, which are filled with air sacs. And its breathing movements are synchronized with its flight movements. Each breath pulls in enough oxygen to keep a bird speeding along even in the thin atmosphere of high altitudes.

A bird’s digestive system is extremely reduced, compared to a mammal’s. Birds have no urinary bladder at all–wastes filtered by the kidneys are mixed with those of the intestines and emptied frequently. And although the intestine is shorter in birds than mammals, it’s also more efficient–birds derive more usable calories from a given food item than mammals do. And even in the food they select to fuel their engines, birds conserve weight, choosing items rich in calories like seeds, fruits, insects, fish, and rodents. Birds virtually never eat bulky, low calorie items like leaves and grass.

Finally, birds are the only class of vertebrates in which absolutely no species give birth to live young. All birds produce eggs, which they deposit as quickly as they are formed. Females have only one ovary, and in both sexes, the reproductive organs wither away to virtually nothing during the non-breeding season when they would be nothing but excess baggage. All in all, if birds have a motto, it would seem to be, “Aw, lighten up.”

(Recording of a Robin)

This is Laura Erickson, and this program has been “For the Birds.”