This is Fresh Air.

I'm Dave Davies in for Terry Gross, who's off this week.

Did you know that when some migratory birds prepare for flights that can take them thousands

of miles, their intestines and digestive organs shrink while their heart, lung, and leg muscles

can double in size?

That's just one of the amazing facts you can learn from our guest, Scott Wyden Saul.

He spent decades studying migratory birds, reporting on and writing about them, and doing

field work in tracking and conservation efforts.

The scale of bird migration is staggering, involving billions of birds, and the diversity

of the species mating, nesting, and flying habits is awe-inspiring.

Wyden Saul's latest book, A World on the Wing, The Global Odyssey of Migratory Birds,

is now out in paperback.

Wyden Saul writes about what he calls this majestic global pageant and about the threats

the animals face.

One study found that since 1970, roughly 30 percent of North American birds have disappeared,

more than three billion of them.

Scott Wyden Saul has written 30 previous books, and his articles have appeared in Audubon,

National Wildlife, and other publications.

He co-founded Project Snowstorm, which tracks and studies snowy owls, and is a founder of

the Critical Connections Project, which is tracking the migration of birds that breed on

national parklands in Alaska.

I spoke to him in March of last year when a world on the wing was first published.

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