JOHN YANG: Monarch butterflies make one of the most extraordinary migrations in the natural world, often traveling thousands of miles across North America.

Now scientists are using new tracking technology to get a greater understanding of those journeys.

Ali Rogin spoke with Dan Fagin.

He's a Pulitzer Prize winning journalist who teaches science journalism at New York University and is writing a book about monarch butterflies.

ALI ROGIN: Thank you so much for joining us, Dan.

I have to be honest, I wasn't aware of the complicated and long journeys that these monarch butterflies take when they migrate.

So tell us about their migration patterns.

DAN FAGIN, New York University: Yeah, it's kind of amazing what they do.

They live one way during the warm weather months when those of us in the U.S.

are more familiar with them, but when the weather gets cooler, they change.

Instead of just living, you know, a few weeks, they live the whole winter.

At least if they're fortunate enough to survive, they live the whole winter and they undertake this amazing migration.

For monarchs in the eastern U.S.

they go to Mexico.

And for monarchs in the west of the Rockies, they go to the California coast.

And they go there because they are very weather sensitive.

They can't survive if it's too cold or if it's too hot.

So they have found these havens where they can safely overwinter, and that's what they do every year.

ALI ROGIN: And you've written about these new tiny sensors that scientists have created to be able to track individual butterflies.

How does this work?

DAN FAGIN: It's really an amazing development.

I mean, people have been using paper tags and sticker tags on monarch butterflies for a very long time.

80 years, you can see where the monarch started and where the monarch ended, but you have no idea where they went and detect, which is sort of like trying to read a book with only reading the first and the last page.

But now, thanks to this new technology developed by a startup company called Cellular Tracking Technologies, they create radio tags and they've now figured out how to miniaturize these radio tags so they're small enough that even a butterfly can handle it.

And so now for the very first time, after all these years of trying to understand the migration of monarch butterflies, now we really know exactly where they're going.

And it's just quite fascinating.

ALI ROGIN: Yeah.

To build on your book analogy, now that we know what's happening in the middle of the story, what have we learned?

DAN FAGIN: We knew that monarchs are affected by weather, but we didn't really realize until we have these new tracks just how far off course they get blown when the wind is blowing the wrong way or it's a rainstorm and they can be blown hundreds of miles off course.

Yet the amazing thing is their navigational adaptations.

They have two different compasses, one that works when the sun is shining and one that works when it's not.

And those compasses are so accurate that even after being blown far off course, many of them anyway, can recover and head back down in the correct southwesterly direction all the way to Mexico.

Another thing that really wasn't well understood is we kind of assumed that almost all these monarchs went to the same small area in Mexico, and also some specific areas in California.

But now that we have these tags, we can see that they're also going lots of other places, too.

And it suggests that maybe the monarch is going to be more resilient to climate change and all these other problems that it's facing than we had feared.

ALI ROGIN: Monarch populations have been declining.

What are some of the factors that are leading to that?

And why should we be concerned about what's happening?

DAN FAGIN: Monarchs need to find just the right narrow temperature range to survive in the winter, and also in the summer, too.

And climate change is changing temperatures, it's changing their habitat, making it harder for them to find the nectar plants that they need.

The second thing you asked me was why we should care, and that's a really interesting question, because what monarchs really are is just gorgeous.

They're beautiful and they do fascinating things.

And it would be just incredibly sad to lose the migration, which is just such an amazing natural phenomenon, really, not duplicating anywhere else in the natural world.

ALI ROGIN: And truly, the fact that they're gorgeous is quite enough.

Dan Fagin, writing a book about this.

Thank you so much.

DAN FAGIN: It's a pleasure to be with you.