Monarch Butterflies: Great Migration

It's one of nature's great migrations and also one of nature's great unsolved mysteries. Millions of monarch butterflies journey to the same place every year, some traveling more than 3,000 miles to get there, but it's also a place where none of these butterflies has ever been before. However, before we could fully appreciate their herculean travel odyssey, we had to first discover where it was they were going. These distinctive orange and black butterflies, the monarchs, are seen over large swaths of the United States, but every winter they disappear and we had no idea where they go until 1975. And then we discovered they were vacationing in Mexico. Right here, this is one of the reserves, Rosario. It is magical here. In central Mexico, some 60 miles west of Mexico City, forests of tall oyamel fir trees, pines, and oaks provide the resort of choice for most vacationing monarchs. Butterflies from across the eastern United States and Canada congregate in 14 different small reserves nestled high in the mountains. The forests provide shelter from storms and high winds, but primarily the trees offer warmth. Butterflies cling to trunks and branches in numbers so great, trees disappear under a blanket of wings. On occasion, as many as 24 million monarchs have been clustered in a single acre of reserve. Standing in the reserves, it can seem as if you're viewing an endless sea of And the numbers are impressive, ranging from 70 to 100 million in recent years, but that's a fraction of the estimated 1 billion monarchs wintering in Mexico in the 1990s. Storms, extreme temperatures, habitat loss, pesticides have all played a part in their decline, and yet they persist. This great insect migration is even more impressive and mysterious when you realize we have no idea how they know where to go. The monarchs here in Mexico are called the Methuselah generation, living eight to nine months, or the equivalent of about 600 human years. Born in the United States and Canada, none have ever been here before, but somehow they manage to fly to Mexico, survive the winter, and start the trip back north. They won't complete the journey. They will mate, give birth, and die. The next generation will revert to the typical monarch life expectancy of about five weeks, as will the following generation and the one after that. It is that third generation, after the Methuselahs, that will finally arrive at the birthplace of their great-great-grandparents. It will be yet another generation of monarchs, the Methuselahs, who return to these forests next year. And again, they will navigate their way south using some unknown internal genetic GPS system. But perhaps years ago, the local Mexicans had the best explanation for the mystery of the migration. Back when we in North America were still wondering where the butterflies went in the winter, the Mexicans were wondering where they came from. And because the monarch's arrival was close to the time the locals celebrated the Day of the Dead, many came to believe the butterflies were the spirits of departed ancestors.