These engineers really soar

If you have any doubt that birds are master builders, try this: Get a bunch of thin twigs and grass and try making a nest yourself. And no fair using your hands – birds, remember, only use their beaks.

Douglas Causey of Harvard University's Museum of Comparative Zoology has asked young students to do just that. You can imagine the result. (You can also guess that Dr. Causey has an impish sense of humor – and you'd be right.)

But if you think making a simple nest is difficult, imagine building a more elaborate nest. Take, for example:

• The Megapode bird of New Guinea, north of Australia, which makes a 12-foot-high pile of vegetation. The bird is about a foot tall, so this is like a six-foot-tall person building a seven-story house.

• Or the South American ovenbird, which may take months to fashion one nest from clay or mud mixed with bits of straw, hair, and fibers. The tropical sun bakes the walls brick-hard.

• Or the bald eagle, which uses sticks, some two inches thick and several feet long, to make nests sturdy enough to support a human adult. They may look like a jumble of materials, but the sticks are usually placed in layers, beginning with a triangle, followed by more rotated, triangular layers.

Birds are capable of grand engineering feats. But are they engineers? Not in the way you might think. They didn't go to school and study architecture. They didn't even learn how to build by watching their parents or each other.

Just as birds know how to fly, they know how to build a nest without instructions or apprenticeship. It's a matter of instinct, scientists claim.

"They are 'hard-wired,' " Causey says, "sort of like robots." Birds craft their nests without consciously thinking about it.

How then did some species of birds develop such well-engineered, elaborate nests? Books have been written on the subject without providing a real answer, says Jeremiah Trimble, an assistant in the Harvard museum's bird department.

One possible explanation involves natural variation and evolution. If a particular bird happens to build a nest that is stronger or more predator-safe, that bird's offspring are more likely to survive and pass along this trait to succeeding generations.

Another possibility, Mr. Trimble says, is that when females choose mates based on the quality of the nests they build, this means the best nest builders are more likely to breed. Nest-building, therefore, may still be evolving, but so slowly that no one really detects any change. This makes nest-building one of the most difficult bird behaviors to understand.

It helps, therefore, that Causey and his Harvard colleagues have one of the largest nest collections in the world, with 25,000 specimens to study. Most were collected nearly 100 or more years ago (see accompanying story). His interest isn't limited to these specimens, though.

This winter, Causey traveled to Costa Rica to study a wren that makes a cup-shaped nest with a roof. The roof helps protect the nest, which is built along stream banks, from being damaged by trickling water and rain.

Causey didn't expect to come back with a nest, but when he happened upon a vacated one, it became the first addition to Harvard's collection in half a century.

Such finds occasionally occur much closer to home. In fact, presently he is keeping his binoculars trained on a red-tailed hawk that built a nest in a tree outside the museum on Harvard's campus. This species is not often found nesting in Cambridge, Mass, he says.

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