A titan on orange wings

How does the monarch migrate 2,500 miles?


If there is such a thing as visual clamor, that’s what monarch butterflies have been creating in my backyard for the past few weeks. They demand to be looked at. They’re so big, so graceful, so luminous, and so plentiful, that whenever I’ve gone outside with a book I’ve ended up watching the monarchs. On sunny afternoons there are dozens of them hovering and alighting on the purple buddleias (one bush is some fancy variety I planted, and two are just big weeds; but the butterflies don’t seem to care whether they’re drinking champagne or beer from the can). Their abundance this year made me suddenly curious to learn more. When I look at a monarch in my garden, what exactly am I seeing?

Not some fragile, ditzy little creature, that’s for sure. One of the first things you learn when you start asking about monarchs is that they migrate. The individuals who seem to be whimsically flitting around my garden are actually on their way to Mexico for the winter. How an insect has the strength to fly 2,500 miles is a mystery. How it navigates is even more mysterious. New Hampshire naturalist Davis Finch told me that while on a boat observing seabirds he has seen great flights of monarchs 20 to 30 miles offshore. “They don’t go in flocks, but they are all headed in the same direction. It’s very organized. They have the knowledge in their genes.” Scientists have posited that monarchs rely on sun angles, or visual cues such as coast- and ridge-lines, or an internal magnetic compass.

Once the monarchs reach the mountains of central Mexico, they cluster in pine and fir trees and hang out there for the winter. In the spring, they rouse themselves and fly northward again; once they reach the southern United States, they lay eggs on milkweed plants and die. When the eggs hatch, the caterpillars feed on the milkweed, ingesting nourishment and, equally important, milkweed toxins, which do a nasty job on a bird’s stomach: No bird who’s tasted a monarch would ever make the same mistake again. The monarch’s distinctive orange-and-black appearance is so effective at deterring predators that another butterfly mimics it: the viceroy, which is not poisonous, protects itself by looking almost identical to the monarch, like a timid teenager swaggering around in tough-guy clothing.


As the spring progresses, the new generation of monarchs continues to head north, breeding, pupating, and dying. The monarchs I’m seeing in my garden right now are probably the great-great-grandchildren of the ones I saw last fall. The monarchs’ yearly cycle is an Olympic medley relay in which each team member has a different specialty: flying north, hanging around breeding and eating, flying south.

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Talking with naturalists, I learned that there are butterfly watchers, just as there are bird watchers. The Massachusetts Butterfly Club listserv has almost daily observations of this fall’s migration: 60 monarchs feeding in a field in West Bridgewater; a stream of monarchs passing Nobska Light in Woods Hole; 300 monarchs at the tip of Cape Ann. You’ll also find grim notations on monarchs that didn’t make it — a pile of wings under a bush (the likely killer, a praying mantis); a young mockingbird at Plum Island who ate a monarch on the wing (his stomach will punish him). Mass Audubon’s Butterfly Atlas is another collection of data from butterfly watchers. It’s from this mosaic of reports that scientists can compile a detailed picture of the monarch’s life cycle, its migration routes, and whether its numbers are holding steady or decreasing.

For as tough as each individual monarch is, the species could be vulnerable if its environment changes too much. Just about every monarch living east of the Rockies spends the winter in a few mountain forests in Mexico. Logging poses a danger, even if the butterfly land is protected, because thinning of the buffer forest reduces protection from cold and storms. And all along the migration routes, a lack of milkweed would interfere with breeding — causing that generation of the monarch relay team to drop the baton, perhaps fatally.

From now on, when I look at a monarch in my garden, I will know that I am looking at genetics, evolution, ecology, scientific knowledge, and cosmic mystery — pretty much all of nature, in one of the most beautiful and efficient packages nature ever designed.

Joan Wickersham’s column appears regularly in the Globe. Her new book is “The News from Spain: Seven Variations on a Love Story.”