I leaned against the concrete wall of a downtown bank building, waiting for a friend to get off work. I used to work in the Loop too, in an office next to the bank. Then as now, the cement plaza in front of the building was lined with 24 locust trees planted in raised boxes as though they were petunias or some other dinky flower. I knew that as soon as the trees dropped their leaves maintenance workers from the bank would rush out and hang Christmas lights on their bare branches. But now the afternoon was warm, and that kind of resistance against winter’s darkness seemed distant.

As I watched the office workers rushing by, I noticed a monarch butterfly floating south on Clark Street slightly above the flow of traffic. The bright orange insect bounced up and down as it flew toward me, buffeted by winds too subtle for me to feel.

I’m no lepidopterist, but I knew enough to recognize a monarch on a mission. This fragile fugitive was headed for Mexico. It would travel 2,000 miles just as I saw it, bobbing up and ducking down, but always moving forward at its own fluttery pace. The delicacy of its appearance disguised an unshakable driving instinct that would not let it quit until it reached its goal.

“No obstacle will stop migrants,” entomologist Torben Larsen wrote last year in Natural History magazine. “Once I stayed on the ninth floor of a hotel…during a migration. All the butterflies flew over the roof of the hotel, none making the slight detour that would have allowed them to bypass the building. Even when a migrating butterfly is trapped on a porch, it appears to be trying to batter down the house, and will persist in its chosen course rather than retreat a few yards or deviate from it. At the risk of being anthropomorphic, I find migrating butterflies really do seem to have a ‘gleam in their eyes.'”

Illinois’ painted ladies and dwarf sulfurs also fly south to spend the winter, but they only go as far as the gulf states. No other American butterfly comes close to duplicating the travel phenomenon monarchs carry off each year. The orange and black beauty that sipped nectar in Chicago’s backyard gardens in late August flies down to Louisiana in the fall, taking nourishment from asters, goldenrods, or whatever’s in bloom as it goes. Then it crosses the gulf, heading toward forests filled with tall fir trees somewhere in a mountain range in the state of Michoacan.

A biosynthetic form of magnetite that helps it follow the earth’s magnetic fields is embedded in this creature that weighs little more than a maple leaf. And that’s not all that’s crammed inside its tiny body–it has the ability to identify a member of the Asclepias plant family so it knows where to lay its eggs, the information that tells it not to fly when it rains, tips on how to find a mate, and instructions on how to store cardenolides, a kind of poison that discourages birds and other predators from making monarchs into meals.

After arriving in Mexico these butterflies live a quiet life clustered together on tall conifers in the cool, moist terrain. Larsen describes standing in one forest and being surrounded by 65 million brilliant butterflies. During this time the monarchs are not sexually or reproductively active, and they try not to range far afield when looking for nectar so they can conserve their lipids, the butterfly’s version of fat.

Around the time of the spring equinox the butterflies begin to mate. Then, flying together, they travel rapidly back to the southern United States, arriving in late March and early April. There they lay eggs on milkweed, establishing the first new generation of monarchs, and soon afterward they die. As the weather warms, the young generation moves northward until monarchs have repopulated all of the U.S. and southern Canada east of the Rockies. (There’s also a western range of monarchs in California, which appears to be a spin-off of the eastern division; they roost in winter in imported Australian eucalyptus trees and have their own routine going that doesn’t overlap with that of midwestern monarchs.)

Like most butterflies, most monarchs live in their adult stage for only a couple of weeks. Over the course of the growing season four or five successive generations hatch, live, breed, and die. Only the last-born generation migrates. So not a single butterfly making its way from Chicago to Memphis, across Mississippi, down to the Atchafalaya Bay on the coast of Louisiana, across the gulf (sometimes stopping to rest on offshore drilling rigs), over 200 miles of Mexican villages and hills, and into the Michoacan mountains has ever made the trip before. In fact, the last member of an individual monarch’s family to chance the journey was his or her long-dead great-great grandparent. There’s no chance that any butterfly learned something about migration from an ancestor or personal experience. It’s born with everything it needs to know already encoded in its genes.

The monarch was traditionally an animal of the plains, limited by the range of the milkweeds that thrived in prairies. But because milkweeds are now all over the eastern United States, its range has actually increased since European settlement. Monarchs are one of the five most common butterflies in the Chicago region, according to the Nature Conservancy’s volunteer monitoring program. They have joined the lucky club of animals like robins, squirrels, and deer that have assimilated comfortably in America’s suburbs.

Monarchs are not in danger of extinction, but the phenomenon of monarch migration is in jeopardy. When an animal species migrates, any stop along the way can lead to its destruction. The monarch does well while in the states, but it now faces disaster in Mexico. In the past decade population pressure has pushed villages up the slopes of the mountains where no one lived before. Such development requires wood, and trees where monarchs once roosted have been cut down. Even when roosting trees are protected, nearby clear-cuts sometimes decrease the amount of shelter from wind and cold, causing the monarchs’ lipid supply to diminish to the point where some can’t survive the long journey back to the States. Air pollution weakens trees, which then become prone to insect infestations; this inspires the government to spray Dipel to kill off the invading bugs, which sometimes affects the butterflies.

Back in downtown Chicago I continued to watch the monarch progress toward me. It veered away from the traffic and dipped down over the sidewalk, fluttering at about the height of passersby’s heads. If I spotted someone who wasn’t in a hurry I planned to point it out to them. With its fantastic Orange-Crush color, the monarch looked exotic against the gray of the buildings.

The butterfly swooped farther down toward the pavement, pushed by a breeze or a whim, I couldn’t tell. A man walking faster than the monarch was flying caught up with it. He didn’t notice it, but he did look straight at me. I smiled at him. The butterfly was right at his feet. I was about to point it out, maybe tell him something about where it was going, when his left foot, armored in a Johnston & Murphy wing tip, stepped on top of it. He smiled back at me, nodded, and walked on.