Why Do Monarchs Form Roosts
During Fall Migration?


Monarch butterflies only migrate during the day. They come down at night and gather in clusters. A cluster of butterflies is called a roost or bivouac.

Monarchs migrate alone. They do not travel in flocks the way many birds do. Why do they come together at night and form roosts?

Some roosts have only a handful of butterflies. Some roosts have too many butterflies to count!

Most roosts last for only a night or two. That's one reason it's so exciting to discover one! In other places, these gatherings may last as long as two weeks.

Much of what is known about monarch roosts is based on the observations made by citizen scientists. People like the Viger family in Minnesota have made interesting discoveries in their backyards.

This map shows when and where people have discovered roosts during fall migration. Each fall, roosts suddenly begin to form in the north in mid-August. The data reveals the fall migration pathway to Mexico.

Where do monarchs form their roosts?

"I've been experimenting with finding roosts," says Mr. Tom Murphy of Minnesota. "I think the key is to find a nectar source. Even when migrating hard, monarchs need to stop for a sip."

What causes roosts to come and go?
An observer counted the number of monarchs that roosted in his yard each night. The graph below shows the pattern he noticed. What might cause the number of monarchs to change this way?

Here is what the observer noticed:

"The roost grew bigger and bigger when the wind was from the south. When the wind blew from the north the numbers dropped."

How long does one butterfly stay at a roost?
People often tag monarchs hoping to find out how long individual butterflies stay at a roost. The person who tagged the two butterflies pictured here wondered: For how many days will these same butterflies be here—and will I be able to find them?

How do the butterflies find the roost?
Few people have had the chance to watch a roost form. One observer described it this way:

"The air resembled a monarch airport, with monarchs coming in from every direction, floating through the air, checking out various trees, landing, then taking off again. It took from about 7:15 to 7:35 p.m. for them to cluster in the trees, just before dark."

Why do roosting monarchs flash their wings?
When a butterfly approaches a cluster, the roosting butterflies flash their wings. What does this behavior signal?

Why do monarchs form roosts?
Scientists still have many questions about roosting behavior. Dr. Lincoln Brower has studied monarchs for over 50 years. He knows that roosting must be critical for monarch survival. How would Dr. Brower search for answers?

"If I discovered a roost, I'd pull up a chair, grab a pair of binoculars and just sit and watch! I'd try to stay hour after hour, day after day — as long as the monarchs were there. People can contribute important observations by going with an open mind and documenting what they see."

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