The Secret Overnight Hotels of Monarch Butterflies

The Secret Overnight Hotels of Monarch Butterflies

Imagine walking past an ordinary-looking tree at sunset.

At first, nothing seems unusual. Then you notice something hanging beneath a leaf. Then another. And another.

Look more closely and suddenly the branches seem to be covered in orange and black.

You have stumbled across one of the lesser-known spectacles of the monarch migration: an overnight monarch roost.

Every fall, millions of monarch butterflies begin moving south across North America. Eastern monarchs travel toward the mountains of central Mexico, with some individuals covering thousands of miles before reaching their winter destination.

Most of that journey is made individually.

But when daylight begins to disappear, the rules change.

Migrating monarchs descend from the sky, find sheltered trees and gather together for the night. These temporary monarch butterfly roosts can contain a few butterflies, hundreds of butterflies or, under the right conditions, thousands.

By morning, the “hotel” may be empty again.

Monarchs Don’t Migrate Like Birds

When we picture animal migration, we often imagine groups.

Geese form their familiar V-shaped formations. Caribou move in herds. Schools of fish travel together.

Monarch migration doesn’t work quite that way.

During the day, monarchs generally migrate individually rather than traveling together as a coordinated flock. They take advantage of favorable winds and rising columns of warm air called thermals to help carry them south.

But monarchs don’t continue flying through the night.

As evening approaches, migrating butterflies begin looking for somewhere to land.

And that’s when individual travelers can suddenly become a crowd.

They gather in trees and other vegetation, sometimes clustering closely together on branches and leaves.

Scientists call these gatherings roosts.

They are also sometimes called bivouacs.

We prefer “butterfly hotels.”

A Hotel That May Exist for Only One Night

One of the fascinating things about a monarch roost is how temporary it can be.

Some roosts contain only a handful of monarchs. Others contain hundreds or thousands.

A particular site might be occupied for a single evening and completely empty the following day. Other locations can host migrating monarchs for several nights, and some areas are used during migration in multiple years.

That doesn’t necessarily mean the same butterflies are staying there night after night.

Migration happens in waves.

One group may leave in the morning while another group arrives later.

Think of it less like a permanent colony and more like a roadside hotel along a busy migration highway.

The guests keep changing.

What Makes a Good Monarch Hotel?

Monarchs don’t simply land in the first tree they encounter.

Observations of migratory roosts suggest that certain characteristics can make a location particularly attractive.

Protection from wind appears to be important.

A sheltered group of trees can provide a calmer place to spend the night than an exposed branch in an open field.

Nearby nectar sources can also matter.

Migrating monarchs need energy. During fall migration, nectar-rich flowers allow them to build the fat reserves that will help sustain their long journey and, eventually, their winter stay.

A landscape containing late-blooming flowers near protected trees can therefore provide two important resources in one place: dinner and a bedroom.

Stream valleys, wooded areas and isolated stands of trees surrounded by otherwise open landscape can also attract roosting monarchs.

And sometimes monarchs return to the same general locations during multiple migration seasons.

Why Sleep Together?

This is where the story gets particularly interesting.

Scientists still have questions about exactly why monarchs form overnight roosts.

One hypothesis involves protection.

Butterflies are ectothermic, meaning their body temperature is heavily influenced by their surroundings. As temperatures fall, monarchs become less active.

That makes nighttime very different from daytime.

A monarch warmed by the sun can quickly fly away from danger. A cold monarch hanging nearly motionless from a branch has fewer options.

Gathering in groups may provide some protection from predators, although researchers continue to study roosting behavior and the factors behind it.

Shelter from weather is another obvious advantage of choosing the right location.

Whatever the complete explanation turns out to be, monarchs have been gathering in these temporary nighttime communities throughout their fall migration for generations.

The Morning Checkout Depends on the Weather

Sunrise doesn’t necessarily mean immediate departure.

A monarch needs warmth before it can fly effectively.

On a cool morning, a roost can remain remarkably still.

Butterflies may hang from branches with their wings closed while they wait for conditions to improve. As sunlight reaches the roost and temperatures rise, activity begins.

A few butterflies start moving.

Wings open.

Individuals begin leaving the branches.

Then, under favorable conditions, the night’s guests take to the air and continue south.

The tree that seemed covered in butterflies earlier that morning can eventually look completely ordinary again.

Until another wave arrives.

A Chain of Overnight Stops Across a Continent

Monarch roosts can appear throughout the migration corridor as butterflies move south across North America.

Eastern monarchs travel from areas of southern Canada and the United States toward their overwintering grounds in central Mexico. Along the way, temporary roosts have been reported across the Great Lakes region, the central United States, the Great Plains, the southern states and into Texas before the butterflies cross into Mexico.

Western monarch populations follow different routes, with some overwintering primarily along coastal California while others may move toward Mexico.

Where a roost forms on a particular night depends on a combination of migration timing, weather, available nectar and suitable shelter.

That means an overnight gathering can appear in surprisingly ordinary places.

A stand of trees beside farmland.

A wooded river corridor.

A park.

A shelterbelt.

A backyard.

Or a patch of trees that thousands of people pass every day without noticing anything unusual.

Then, for one evening, it becomes a stopping point in one of North America’s great animal migrations.

How Do Monarchs Know Where to Stop?

This question isn’t completely settled either.

Researchers and observers have identified several characteristics associated with roost locations, including wind protection, nearby nectar and landscape features.

But monarchs don’t have a reservation system directing them toward tonight’s available maple.

There is no parent monarch leading the group south, either.

The migratory generation making the journey to Mexico has never made the trip before.

Yet they navigate south, locate the general region used by generations before them and repeatedly find the resources they need along the way.

Exactly how monarchs make all of these decisions remains one of the remarkable parts of their migration.

Observations suggest that monarchs may locate potential roosting habitat visually. Once butterflies begin gathering, additional monarchs arriving in the area can join them.

By dusk, what began with a few individuals can become a tree filled with butterflies.

Roosts Can Help Us Follow the Migration

These overnight gatherings aren’t only beautiful.

They’re useful.

For years, community scientists have reported monarch roosts to migration-monitoring projects such as Journey North.

Plot those observations on a map and something begins to appear.

The migration itself.

As roost reports move south across North America, researchers and the public can watch the progression of fall migration.

Scientists have even analyzed citizen-reported roost locations to better understand the major flyways monarchs use on their journey toward Mexico.

In other words, a tree filled with sleeping butterflies isn’t merely an interesting sight.

It can also be a data point.

That connects perfectly with something we talked about last week: the increasingly sophisticated ways researchers are tracking individual monarchs.

Tiny transmitters and modern tracking technology can help scientists follow particular butterflies.

Roost reports offer another perspective.

Instead of asking, “Where did this butterfly go?”

They can help answer, “Where is the migration moving?”

Two very different methods can help scientists understand different pieces of the same enormous journey.

How to Recognize a Monarch Roost

Seeing several monarchs in a tree during the afternoon doesn’t automatically mean you’ve found a migratory roost.

Butterflies land for many reasons.

Timing and behavior matter.

Roosting monarchs typically begin gathering toward evening and settle together for the night. If you find multiple monarchs clustered in the same area around dusk—or discover a group still gathered together early the next morning—you may have found one.

Look carefully.

A large roost isn’t always obvious immediately.

With their wings closed, monarchs can blend surprisingly well into leaves and branches. What first looks like a handful may turn into dozens once your eyes know what they’re looking for.

And if you’re lucky enough to see butterflies arriving before dark, you may get to watch the roost form.

If You Find One, Don’t Disturb It

A monarch roost is something to observe, not interact with.

There is no need to shake branches or encourage the butterflies to fly for a photograph.

Give them space.

Take photographs from a respectful distance.

Note the location, date and approximate number of butterflies.

You can also record details such as nearby flowering plants, weather conditions, temperature and wind direction.

Those observations can be reported to citizen-science projects that track monarch migration.

Your accidental discovery could become part of a much larger scientific picture.

The Most Amazing Part May Be What Happens Tomorrow

There’s something almost unbelievable about a monarch roost.

Hundreds of butterflies can converge on a group of trees as daylight fades.

For a few hours, they share the same tiny patch of the continent.

Then morning comes.

The temperature rises.

One by one, they leave.

And they continue south.

The next evening, somewhere farther along the migration route, the process begins again.

Another tree.

Another overnight stop.

Another group of monarchs gathering before dark.

All along a journey that eventually leads many eastern monarchs to the mountain forests of central Mexico.

We tend to think of migration as the spectacular part of the monarch story.

But migration isn’t only about flying.

Sometimes it’s about knowing when to stop.

Look Up This Fall

At The Butterfly Connection, we’re fascinated by the details that make butterflies much more complicated than they first appear.

Their wings tell stories.

Their anatomy tells stories.

And their behavior tells stories.

A monarch hanging quietly from a tree at dusk might look like it’s doing absolutely nothing.

In reality, you’re looking at an animal in the middle of one of the most extraordinary migrations on Earth.

It has already traveled.

It has farther to go.

For tonight, though, it has checked in.

And tomorrow, it flies south again.

Frequently Asked Questions

Where do monarch butterflies sleep during migration?

Migrating monarchs commonly spend the night in trees or other sheltered vegetation. Multiple monarchs can gather together in temporary groups called roosts.

How many monarchs can be in a roost?

The number varies dramatically. Some roosts contain only a handful of butterflies, while others can contain hundreds or even thousands.

Do monarch butterflies migrate at night?

Monarch migration occurs during daylight. As evening approaches, migrating monarchs stop flying and settle into overnight roosts.

Do monarchs use the same roost every year?

Some locations host monarch roosts in multiple years, but the presence of a roost at a particular location can vary. Weather, wind, nectar availability and the path of that year’s migration can all influence where monarchs gather.

What should I do if I find a monarch roost?

Observe it without disturbing the butterflies. Photographs, approximate counts, weather information and details about nearby nectar plants can make the observation useful. Roost sightings can also be submitted to Journey North’s monarch migration project.

Why do monarch butterflies gather together at night?

Scientists are still studying the behavior. Protection from predators is one proposed explanation, particularly because cooler nighttime temperatures make monarchs less mobile. Sheltered roost locations can also protect butterflies from wind and weather.