Card #27 is the only butterfly most people can name, and the only one that does anything like what birds do. Every autumn, monarchs across North America turn south and fly for weeks towards a winter home most of them have never seen. Late September is the middle of that journey, which makes this the best fortnight of the year to go and look.
Where they are this month
There are two migrations, split by the Rocky Mountains.
- East of the Rockies, which is most of the population, monarchs funnel south and west across the United States and into central Mexico. They spend the winter in dense clusters on oyamel fir trees in a few mountain forests. The colonies usually settle in around early November and hang there, mostly still, until late winter.
- West of the Rockies, monarchs head for groves of trees along the California coast, where they cluster in much smaller numbers through the winter.
In late September the northern edge of the range, from southern Canada down through the Great Lakes and New England, is emptying out. Butterflies from there are somewhere in the middle of the country, joined each week by monarchs starting from further south. Through October the stream tightens into Texas and crosses into Mexico. Timing varies from year to year with the weather, so treat any exact date with suspicion; a cold front pushes them on, a warm spell holds them in place.
Our own seasonal data says the same thing from the other direction. On card #27, iNaturalist records peak in August, with July and September close behind and October still far above spring. That autumn shoulder is the migration passing through the parts of the continent where people live. Monarchs also live in Australia and New Zealand, where they make much shorter seasonal movements and do not do the journey described here.
The journey in one paragraph
Monarchs that hatch in summer live only a few weeks, breed, and die. The generation that hatches in late summer is different: shorter days and cooler nights switch off its urge to breed, it stores fat instead, and it lives through the winter. That one generation makes the whole flight south and, in spring, the first leg back north, laying eggs on milkweed in the southern states before it dies. Two or three more short-lived generations then carry the population back up to Canada by early summer. No single butterfly completes the loop.
How to see it
Monarchs migrate in daylight and are choosy about conditions. The best chances:
- A warm, sunny afternoon with a light wind from the north. They ride the wind. With a strong headwind they stay put and feed.
- Coastlines, lake shores and peninsulas. Monarchs avoid crossing open water, so they pile up on the shore and wait for good conditions. Points of land that stick out into a lake or the sea are the classic spots, and often the same places birdwatchers use in autumn.
- Flowering fields. Migrating monarchs need fuel and stop wherever late flowers are thick: asters, goldenrod, ironweed, sunflowers, and sedum in gardens. A field of goldenrod in the sun on a September afternoon is the easiest sighting there is.
- Roosts at dusk. Towards evening monarchs gather on the sheltered side of trees, often the same tree year after year, and hang in clusters with their wings closed. From a distance a roost looks like a bunch of dead leaves. Look for the orange when the sun catches them in the morning before they warm up and leave.
Front desk note: a monarch resting with its wings closed shows the paler orange underside, which is what the butterfly wants the birds to see. Photograph it from the side and the identifier will still name it; the black veins and the white-spotted black border are on both surfaces.
Which monarch is which
Several orange butterflies fly in the same fields in autumn, and one of them is a deliberate copy.
| Butterfly | How to tell it | In the Field Guide |
|---|---|---|
| Monarch | Large, roughly the width of a hand. Bright orange with bold black veins and a black border dotted with white. Slow, gliding flight with wings held in a shallow V. | Card #27 |
| Viceroy | Noticeably smaller, with an extra black line curving across each hindwing. Faster, flatter flight. A mimic that gains protection by looking like a monarch. | Not in the guide yet |
| Queen | Southern. Darker, browner orange, with the black veins much less obvious on the upper side and more white spots scattered across the wings. | Not in the guide yet |
| Painted lady | Smaller, more mottled, with a pattern of black and white patches rather than clean veins. Also a migrant, on a different schedule. | Card #28 |
A male has a small black swelling on a vein in the middle of each hindwing and slightly thinner veins overall; a female has no spot and thicker veins.
The caterpillar is its own card, #178: fat, smooth, banded in yellow, black and white, with a pair of black filaments at each end, always on milkweed. The chrysalis is a jade-green case with a line of gold dots, which turns clear the day before the butterfly comes out. Monarchs go egg, caterpillar, chrysalis, adult, the full complete metamorphosis.
How to help
Most of what a garden can do is about milkweed and late flowers.
- Plant milkweed native to your region. It is the only plant the caterpillars eat, and different species suit different places. In warm regions where tropical milkweed stays green all year, it is usually advised to cut it back in autumn, because milkweed that never dies down can hold monarchs that should have left and lets a milkweed-borne parasite build up.
- Plant for autumn nectar. The migration runs on flowers. Asters, goldenrod, ironweed, coneflowers and sedum feed monarchs when most of the garden has finished. Let them flower rather than tidying them away in September.
- Skip the insecticides on and near those plants. A caterpillar on treated milkweed does not survive it, and a monarch feeding on treated flowers fares no better. This includes systemic products taken up by the plant.
- Leave the roost tree alone. If monarchs choose a tree in your garden for the night, keep lights off, keep the dog in, and let them go in the morning.
- Count them. A monarch record on iNaturalist with a date and location is a data point researchers use to follow the migration; the seasonal bars on our species pages are built from the same records.
Milkweed has other tenants. The large milkweed bug, card #112, sits in groups on the seed pods in late summer wearing the same warning colours for the same reason, and confuses people who are expecting a caterpillar. It is harmless to the plant in normal numbers and harmless to you.
If you find an orange-and-black butterfly and are not sure which one you have, a photo in the identifier gives a name and a safety read, and the butterflies section of the Field Guide has a page for each one with its seasonal pattern from iNaturalist records.
Questions
Where do monarch butterflies go in autumn?
Monarchs east of the Rocky Mountains fly south to a handful of high mountain forests in central Mexico, where they spend the winter clustered on fir trees. Monarchs west of the Rockies fly to groves along the California coast instead. Both groups leave in late summer and early autumn and travel through September and October; the Mexican colonies usually settle in around early November.
Is the monarch migration happening right now?
In late September, yes. Monarchs from the northern part of the range are already on the move, and the flow continues through October as butterflies from further south join in. On iNaturalist, monarch records peak in August and stay high through September and October, which is the migration passing through the most populated parts of North America.
How can I see the monarch migration?
Look on warm, sunny afternoons with a light wind from the north, especially along coastlines, lake shores and peninsulas, where monarchs bunch up before crossing open water. Late in the day, check the sheltered side of trees for roosts, where dozens or hundreds hang together like dead leaves. Flowering fields of asters, goldenrod and ironweed are the other reliable place, because migrating monarchs stop to feed.
How can I help monarch butterflies?
Plant milkweed native to your region, since it is the only plant monarch caterpillars can eat, and plant late-flowering nectar plants such as asters and goldenrod to fuel the autumn flight. Avoid insecticides on and around those plants. Recording the monarchs you see on iNaturalist or a migration-tracking project adds to the data that researchers use to follow the migration each year.
Do monarchs bite or sting?
No. Monarchs have no sting and cannot bite, and Peekabug's Field Guide lists them as safe to be around. They are distasteful to birds because the caterpillars store chemicals from milkweed, which is why both caterpillar and butterfly are so brightly coloured, but that only matters if you try to eat one.
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