Monarchs on the Move: You can Contribute! by Kent McFarland
Monarchs are on the move southward and the story of this massive migration is truly amazing. To follow their annual flight, let’s begin their story in the Transvolcanic Mountains of central Mexico.
Not far from Mexico City there are 13 known sites in the mountains that contain what is believed to be the entire population of Monarchs from east of the Rocky Mountains. These are small peaks ranging from 7,800 to 11,800 feet in elevation and covered with Oyamel Fir (Abies religiosa), a species closely related to the balsam fir found on the mountain tops here in northeastern North America. In 1984, a study found that there may have been as many as 60 overwintering sites that can be used by the butterflies, but commercial and illegal logging has now destroyed many them. A site may contain up to 4 million monarchs per acre and cover as little as one-tenth up to 8 acres of fir forest.
The butterflies arrive from the north in November to late December and hang out on the trees metabolizing fat reserves that they have built up during migration. Remarkably, they actually gain weight on migration and arrive on the wintering grounds with fat reserves for the winter, unlike songbirds, which require huge fat stores to burn on migration.
The overwintering sites begin to break up in March and early April and they migrate to the Gulf Coast of the southeastern US where females arrive just as the milkweed is sprouting from the ground. They lay eggs on the fresh plants and then most die. One or two generations of Monarchs are raised in the south before it is too hot and dry for milkweed to persist. The young and fresh adults continue the northward migration laying eggs along the way.
Finally, they arrive in New England at the end of May and early June just as the milkweed begins to sprout. This generation mates, lays eggs and dies. Monarchs may raise 2 or 3 generations in the north. Each female Monarch can lay about 400 eggs on average. With each generation the population grows larger and larger and larger, if they conditions are right.
But, why is one year good for Monarchs but not another? Scientists are just beginning to understand what may cause boom and bust cycles in Monarch populations. A continent-wide study called the Monarch Larval Monitoring Project was developed by researchers at the University of Minnesota to collect long-term data on reproduction in their milkweed habitats. The goal of the project is to better understand how and why monarch populations vary each year and in each region.
Beginning in mid-August the amount of daylight signals a physiologic change in Monarchs causing them to migrate to the wintering sites in Mexico. Incredibly, the late summer and fall adults in have never seen Mexico. They are 5 or 6 generations beyond those that wintered there. Yet, somehow they are guided back to these small sites thousands of miles away.
The winter generation lives up to 8 months while the successive spring and summer generations are lucky to live 5 weeks. It takes up to 6 generations of spring and summer Monarchs to produce the final “super-Monarch” that migrates to Mexico in the Fall and then back to the southern United States in the Spring.
How do we know that New England Monarchs actually make it to Mexico? Many of us have been trying to find out by tagging adults during fall migration in cooperation with Monarch Watch, a non-profit organization at the University of Kansas. Using small tags like tiny bumper stickers with unique identification numbers on them, volunteers capture and place them on the Monarch’s wings in the fall. With over 10 million butterflies out there the odds of a recapture are very poor. Here in Vermont we have had a few lucky folks. There have been 16 Monarchs tagged in Vermont and found in Mexico! Maybe you could be a lucky tagger. Anyone can do it. Just visit Monarch Watch for more details.
You can also watch Monarchs move southward on the internet as people like you report sightings to Journey North. Whether you find eggs or caterpillars, see them nectaring or actively migration southward, you can add your sightings to the database to help get a picture of the migration across the continent.
Monarchs are on the move! Let all of us at NatureShare know how the migration is going in your region.