ARI SHAPIRO, HOST:

As millions of monarch butterflies make their way down the east coast to Mexico for the winter, butterfly scientists want people in their flight path to report monarch sightings. Doing so can help these scientists answer critical questions as the butterflies' environment changes. Georgia Public Broadcasting's Sofi Gratas has more.

SOFI GRATAS, BYLINE: For centuries, eastern monarchs have been migrating thousands of miles south to nestle in volcanic mountain forests over the winter. Seeing this annual migration is what caused Susan Meyers to fall in love with monarchs when she visited a sanctuary in Mexico.

SUSAN MEYERS: There are butterflies flying all around you, and you can actually hear them. It's like soft rain.

GRATAS: Meyers is now a longtime volunteer with Monarchs Across Georgia. Unfortunately, over the years, she's played witness to another phenomenon.

MEYERS: We could be losing this migration pattern.

GRATAS: That's because many monarchs are skipping their migration and staying in the southern United States. Scientists think they know at least one reason why - an overplanting of non-native milkweed. It attracts the butterflies to stay and play. But what skipping migration means for the future of the species still isn't totally clear. So scientists are relying on some citizen science to help find answers.

Tell me a little bit about what we're asking people to do.

MEYERS: Right. So we're asking people in the southeast to be looking for adult monarchs first. And if you see adult monarchs, what are they doing?

GRATAS: Meyers says to pay attention. Are there monarchs resting in trees or drinking nectar from flowers? Different behaviors act as signals for scientists. For example, mating is a red flag because it uses up energy meant for migration. And as Sonia Altizer with the University of Georgia's Project Monarch Health explains, mating during the winter also poses a different risk.

SONIA ALTIZER: The winter breeding behaviors are accelerating the transmission of this debilitating parasite that infects monarchs.

GRATAS: This parasite, called ophryocystis elektroscirrha, or OE for short, causes deformities in the monarchs. They can get sick and die. Altizer has found that almost all winter-breeding monarchs in places like Texas and Florida have this parasite. At the same time, Altizer says that in the South...

ALTIZER: At least some monarchs in a few places are doing what we would call coastal overwintering, that they're staying in a non-reproductive overwintering state all winter long like we would see them doing in Mexico.

GRATAS: In the spring, these healthy monarchs will migrate back north. Altizer says they could be the future of the migrating monarch species, so learning more about them and tracking them with help from people on the ground is key. Monarch sightings can be reported online at journeynorth.org through March.

For NPR News, I'm Sofi Gratas in Macon, Ga.

(SOUNDBITE OF KACEY MUSGRAVES SONG, "BUTTERFLIES") Transcript provided by NPR, Copyright NPR.