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Every birder is familiar with David Allen Sibley’s beautifully illustrated field guides. So one might expect that there’s little in the avian world that would surprise Sibley at this point. Not so, he explains in the introduction to his new book: “One of the themes that impressed me throughout my work on this book is that a bird’s experience is far richer, more complex, and more ‘thoughtful’ than I’d imagined.”

The engrossing tome consists of essays organized by bird type, and every page features at least one gorgeous, informative illustration. Sibley doesn’t necessarily expect readers to make their way from front to back. Each essay stands alone, but all are interconnected—providing a deeper understanding of avian evolution, instinct, and survival—and helpful cross-references suggest which page to flip to next. The excerpt below from the introduction gives a peek into the book’s rich contents. All birds have feathers, but Sibley’s captivating exploration gives readers a deeper appreciation for, and understanding of, what those seemingly simple outgrowths tell us about avian evolution, instinct, and survival.

Excerpted from What It's Like to Be a Bird, by David Allen Sibley. Copyright © 2020 by David Allen Sibley. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

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Birds are dinosaurs. Some dinosaurs grew feathers more than 160 million years ago and gave rise to true birds. The meteor impact 66 million years ago wiped out more than two-thirds of all terrestrial species on earth, including all dinosaurs and all but a few bird species. The general consensus is that there are about eleven thousand species of birds on earth today, and about eight hundred are found regularly in North America north of Mexico. These species are incredibly diverse, and a sampling of their remarkable adaptations and abilities are presented in this book.

Feathers

THE ­FUNCTION­ OF ­FEATHERS

When asked “What does a feather look like?” you probably think of an oval shape, a central shaft, and many barbs on each side (like the one shown here), but feathers are extremely diverse in structure and size. Similarly, when asked “What are feathers for?” you might think of flight or insulation, but feathers have adapted to serve a myriad of different functions. Feathers keep birds warm and dry, streamline the body, provide color and ornamentation, allow birds to fly, and more. Two of the key properties of feathers are very light weight and incredible strength.

-Feathers did not evolve from scales. The precursors of feathers were bristle-like and hollow, and gradually evolved more complex structures.

-The precise multibranched structure of feathers makes possible many of their remarkable properties.

-Feathers resist breakage because fibers run continuously from the tips of the smallest barbules to the base of the feather shaft.

-Feathers have evolved many different forms, even on an individual bird, specialized for each part of the bird’s body.

-Owls’ feathers have several adaptations to make them silent during movement.

-Bristle-like feathers around the mouth apparently function to protect the eyes.

Feathers as waterproofing

-Feathers are water resistant because of the precise spacing of the barbs; water can neither flow through nor stick to the surface.

-Water birds have feather barbs closer together, making it harder for water to penetrate, and also more and stiffer feathers than land birds.

-Feathers wrap around the underside of a swimming bird to create a waterproof shell.

-Cormorants’ body feathers have waterproof centers but get wet on the margins.

-Owls’ feathers are less water repellent than other birds’, which may be why so many owls seek sheltered roost sites.

Feathers as insulation

-Down from ducks and geese is still the most efficient insulation known, natural or synthetic.

-Feathers insulate birds from heat as well as cold.

Feathers for flight

-The large feathers of the wings and tail form a broad flat surface that makes flight possible

-Details of shape and structure ensure that wing feathers have the right combination of strength and flexibility.

Feathers as ornamentation

-Feathers have evolved a myriad of colors and patterns (see col- oration, below), and feathers also create three-dimensional shapes.

-The “ears” or “horns” of some owls are tufts of feathers, for display and for camouflage.

The crest of a jay or a cardinal is simply feathers, and can be raised or lowered at will.

-Highly modified feather tips in waxwings have a hard, smooth texture, just for decoration.

HOW­ MANY ­FEATHERS ­DOES­ A ­BIRD ­HAVE?

The number of feathers depends partly on the size of the bird, and also on how much it needs waterproofing.

-Small songbirds generally have about two thousand feathers, fewer in summer and more in winter. Larger birds like crows mostly have larger feathers, not more.

-Water birds have more feathers than land birds, especially on the parts of the body that are frequently in contact with water.

-A swan’s long neck is covered with a dense coat of feathers, with more than twenty thousand feathers on the neck alone.

FEATHER ­MAINTENANCE

Feathers are critical for a bird’s survival, so birds spend a lot of time on feather care. Preening is the most obvious and frequent feather-care activity, and involves using the bill (for body feathers) and claws (for head feathers). Preening effectively combs the feathers into place, cleans out any dirt or debris that might be stuck there, removes parasites like feather lice, and applies protective oil to the feathers. Many other activities are also related to feather care.

-Birds spend at least 10 percent of each day preening their feathers, and there is a routine to it that is similar in all species. Preening is so important that some details of bill shape have evolved specifically for that activity.

-A bird can’t preen its own head with its bill, so it must use its feet; some species get around this by preening each other.

-Birds bathe frequently, most likely because water helps to rejuvenate their feathers.

-Dust bathing is common in some species, though the reason is unclear.

-Sunning and anting are two behaviors that are often confused by observers, and neither is well understood; sunning likely has to do with feather maintenance, anting with food

-Vultures often spread their wings in the sun; here, too, the reason is unclear.

-The wing-spreading behavior of cormorants is still not fully explained, but probably helps dry the feathers after swimming.

GROWING NEW FEATHERS

Feathers wear out and have to be replaced periodically— generally once a year— and this process is called molt. Since feathers are so critical to survival, most birds have evolved a very orderly process to molt gradually without hindering their ability to fly or to keep warm and dry.

-Feathers grow from follicles in the skin, rolled up in a cylinder; the tip emerges first.

-The same feather follicle can grow feathers with entirely different colors and patterns at different times, triggered by hormones. Many species take advantage of feather replacement to change their color. They molt twice a year, once to acquire a drabber nonbreeding plumage, and again for a bright plumage for the spring-summer breeding season

-Once a feather is grown it is “dead,” like our hair, and can only be changed through wear, fading, or staining.

-Each feather only grows a few millimeters a day, so even small birds take at least six weeks to go through a complete molt. Large birds can take much longer.

-Faint dark and light bars indicate each day and night of growth in a feather

-Growing new feathers requires a lot of energy, and makes flying and keeping warm more difficult, so molt generally happens during a warm season and doesn’t overlap with any other demanding activities like nesting or migrating.

-When replacing wing feathers, most species use a gradual process so the bird can continue flying.

-Geese and ducks molt all of their flight feathers at the same time, becoming flightless for a few weeks in late summer, putting the birds at higher risk but for a shorter time.

-On rare occasions, a bird will molt all of its head feathers at once, with no apparent negative effects. 

What It’s Like to Be a Bird: From Flying to Nesting, Eating to Singing—What Birds Are Doing, and Why, by David Allen Sibley, Knopf, 368 pages, $35. Buy it online at .