San Juan Island conservation biologist Thor Hanson's delightful new book "Feathers" looks at the wondrous construction and functions of feathers. Hanson discusses his book Friday at Third Place Books in Lake Forest Park and Sunday at the Eagle Harbor Book Co. on Bainbridge Island.

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Everything in the natural world is fair game for study, San Juan Island conservation biologist Thor Hanson writes in his delightful second book, “Feathers” (Basic Books, 336 pp., $26.99).

His first, “The Impenetrable Forest: My Gorilla Years in Uganda,” won the 2008 USA Book News Award for nature writing. Feathers and gorillas might seem an odd pair of passions. But Hanson adds that ever since he saw vultures squabbling over a carcass in Kenya, the sight of their dark plumage, downy collar and bald heads (this last detail the perfect evolutionary solution for ease of cleaning themselves after feeding) made him want to write about feathers.

He then tells a story about jogging one day, hoping to discover what subject to tackle in a new book. The unmistakable scent of something decaying attracted him to a stand of trees where a bald eagle and four turkey vultures were eying a dead deer. One of the vultures flew off, dropping a flight feather, “long, dark and beautifully curved, lying there on the pavement like an open parenthesis.” Feathers and vultures, clearly, had chosen Hanson’s topic for him.

The result is a fascinating inquiry into one of those common things that are easy to overlook until someone shows what a miracle it is. Feathers protect birds from sun and rain, wind and cold; they display healthy males during breeding season but camouflage nesting females; they ward off bugs and bacteria; best of all, they enable flight.

The world’s roughly 400 billion birds sport from 1,000 (ruby-throated hummingbird) to more than 25,000 (tundra swan) feathers each. In most species, they outweigh the skeleton’s dry weight 2 to 1.

Feathers figure prominently in art (angels, for instance) and literature (Pegasos, Hermes’ sandals). They embellish hats and showgirl costumes, fill parkas, pillows and sleeping bags, serve as pens and inspiration for flying machines. In the fossil record, they’ve prompted many scholarly battles. Did birds descend from dinosaurs? Did feathered dinosaurs take flight by gliding from trees or running and flapping?

In his chapters on evolution, Hanson returns to 1861 when Archaeopteryx lithographica, “a delicate, crow-sized fossil that would change science forever,” was found in Bavaria. The first feathered dinosaur, it added fuel to fires recently lit by Charles Darwin’s book on evolution.

What preceded Archaeopteryx? Recently, more feathered dinosaur fossils have appeared in China’s Liaoning province, adding data to this patchy biological puzzle.

Hanson, whose apt analogies simplify complex concepts, explains that tracing the evolution of feathers with so many gaps is like a future archaeologist realizing that an iPhone developed from a bulky box and handset, devoted to the single function of sending sound over copper wire.

As part of examining how feathers insulate, Hanson visits Seattle’s Pacific Coast Feather Co. He explains feathers’ aerodynamics and man’s aspirations for flight, gets a lesson in tying flies for fishing and delves into feathers’ fabulous colors. While swallows sometimes use feathers as toys, club-winged manakins use them to make music.

Birds, the only animals with feathers today, wear these magic coats of stunning variety whose forms so perfectly fit their functions. Hanson’s book reveals much about that marvelous magic.