The migration of millions of monarch butterflies in North America is one of the most compelling wonders of nature, particularly irresistible for those who have stumbled across clusters of monarchs as they roost along the way or have seen clouds of them, with their stained-glass wings of black and orange.

Separate populations of monarchs — those west of the Rockies and those east of the Rockies– travel North America. Western monarchs spend their winters along the California coast. The most celebrated monarchs, east of the Rockies, winter on a dozen forested mountain tops in central Mexico about two miles above sea level. (See the triangle farthest south in the map above.)

From Mexico, as spring arrives, they head northward in a journey of generations. Members of the early generations have short life spans and don’t get all the way north. Each generation lays eggs and dies shortly thereafter, passing the mission onward to their progeny as they continue heading northward and then breed in the summer. The final generation-the fourth or fifth-is a super-generation, capable of living for nine months and flying the entire distance of up to 2,000 miles in the fall, back to the warmth of that exact spot in Mexico. Click here for more about the cycle.

The route and progress of monarchs is vividly illustrated above, thanks to a project called Journey North funded by the Annenberg Foundation, which makes the map each year using monarch sightings reported by thousands of “citizen scientists.” About half are school children, according to Journey North director Elizabeth Howard. You can participate here on the Annenberg Learner site. Over the past year some 9,100 sightings were reported to Journey North. About 31,000 citizen scientists take part in the program, which tracks, in addition to monarchs, robins, hummingbirds, gray whales and other species.

The colors indicate the range of dates of sightings in two-week clusters. A great burst of a new color like the pink dots that explode between May 24 and June 6 represents a surge of first Spring-generation monarchs born to carry on northward, Howard explained.

The monarchs have always been a bit of a mystery, less so recently as scientists have begun to understand what guides and sustains them. Recent research unlocked a monarch navigation system that uses both light and the Earth’s magnetic field for guidance. The monarchs lay their eggs on milkweed and the caterpillars feed only on milkweed. Recent studies led by the University of Georgia’s Andrew Davis demonstrated monarch larvae deprived of milkweed for even 24 hours may emerge with somewhat stunted wing-growth.

Unfortunately, the supply of milkweed in the United States has declined dramatically due to a number of factors, including crops made resistant to herbicide through genetic modification, which allow the farming industry to use more weed killer in the fields. Among the weeds killed: milkweed.

Thousands of Americans are now trying to help by planting milkweed along monarch migratory routes. People are also raising monarchs and releasing them. A sense of urgency pervades the effort. The number of monarchs overwintering in Mexico declined dramatically during the past decade. They occupied about 27 acres in 2003. By 2013, that territory dwindled to 1.65 acres. The decline seems to be accelerating: In 2010, some 201 million monarchs overwintered in Mexico; in 2011, 145 million; in 2012, 60 million; and in 2013, 33 million.

 Clarification: The original version of this story was fuzzy about the monarch generations. The story has been clarified.