Backyard Nature: The Great Journey of the Monarchs

by Marjorie Rachlin

This morning there was an orange Monarch butterfly flitting around my garden, sipping nectar from the flower. Who would guess that soon that fragile butterfly will be off on a long trip to Mexico?

It’s an amazing saga – the butterflies make an 1800-mile trip south in the fall, then several generations make a slower return trip in the spring. Eastern Monarchs leave in late August through October to make the two-month journey to their wintering grounds, which are about 60 miles northwest of Mexico City. Groups of monarchs roosting have already been reported north of us, in New York and Minnesota.

They fly during the day, then drop down at sunset to roost in a tree or bush. I have seen them in Cape May, New Jersey in late September, flying low along the beaches near the shore during the day, then covering a small bush on the dunes with fluttering orange as night falls.

Those Monarchs were taking the flyway along the Atlantic coast. Others fly south to Mexico down the Mississippi River valley or along the Rocky Mountains. West coast Monarchs use a Pacific Ocean flyway to winter in Pacific Grove or Santa Cruz, California.

It’s estimated that in some years 60 million to a billion Monarchs arrive in Mexico by late October. They go to a pine-oak forest in the mountains sixty miles northwest of Mexico City. Here they roost near the top of the mountains at 10,000 feet in tall fir trees. They need to keep cool so that their metabolism will slow down and save energy. The trees buffer them from snow and wind, although they fly out for water during brief sunny periods. There are about 14 roosts in the area, which draws tourists from all over the world.

Back to Forest Hills

In March those same butterflies will fly north, stopping in Florida, Texas or Arkansas. There the old generation will feed, lay eggs and die.

Caterpillars from the new generation produce adults that fly several hundred miles further north, and repeat the process. By the time they reach us, in June, we are probably seeing the fourth generation. Some will go as far as southern Canada.

They Don’t Taste Good

When they get here, Monarchs nectar on many flowers, but they will lay eggs only on milkweed plants, the only food the caterpillars will eat. From those plants the caterpillars absorb cardiac glycosides, which are unpalatable to birds.

This chemical is passed on to the butterfly. The orange color of Monarchs is a warning to birds, and once a bird eats one, it stays away from the others. Viceroy butterflies, which look much like Monarchs, were long thought to be just a mimic, but recent research shows they are also unpalatable.

The caterpillar eats milkweed leaves steadily for about two weeks, then forms a lovely green chrysalis, where metamorphosis takes place. The adult butterfly emerges in ten days, dries its wings and begins life. Monarchs in our yards can live from 4 to 6 weeks in the summer, but the generation born in August may live 8 months, making the long journey south and resting over winter to start north again in the spring.

What’s the tagging about?

Scientists at MonarchWatch have set up a program of tagging the butterflies, in order to get better data on their journey and their lives. The program relies on citizen scientists as well as professionals and has been used by a number of school groups.

When the Monarch is caught, a thin tag, 1/4-inch wide, is placed on the underside of the hindwing of the butterfly before it is released. The butterflies do not seem to notice the tag, and it has no harmful effect. The tagger fills in a data sheet, showing weather, and when and where the butterfly was tagged, and sends it to MonarchWatch.

If this tag is found when the butterfly dies, the finder can send its number to MonarchWatch, which then notes how long that butterfly had been flying, where it was, etc. Gradually a better picture of Monarch routes and behavior is being gathered.

A few other butterflies in the world migrate short distances, but none makes the long, hard journey of the Monarchs. For more information, try “Monarch butterfly” on Wiki, or go to MonarchWatch.org. YouTube has a number of videos, including ones of the butterfly roosts in Mexico.