Correction Appended

LAWRENCE, Kan. — Pinching a bright orange butterfly in one hand and an adhesive tag the size of a baby’s thumbnail in the other, the entomologist bent down so his audience could watch the big moment.

“You want to lay it right on this cell here, the one shaped like a mitten,” the scientist, Orley R. Taylor, told the group, a dozen small-game hunters, average age about 7 and each armed with a net. “If you pinch it for about three seconds, the tag will stay on for the life of the butterfly, which could be as long as nine months.”

Dr. Taylor, who runs the Monarch Watch project at the University of Kansas, is using the tags to follow one of the great wonders of the natural world: the annual migration of monarch butterflies between Mexico and the United States and Canada.

The northward migration this spring was the biggest in many years, raising hopes of butterfly enthusiasts throughout North America. But a drought in the Dakotas and Minnesota meant that not nearly as many butterflies started the return trip. And without the late-summer hurricanes that normally soak the Texas prairies and sprout the nectar-heavy wildflowers where the monarchs refuel, many are presumably finding that leg of the journey a death march. Dr. Taylor has already halved his prediction for the size of the winter roosts in central Mexico, to 14 acres from 30.