Why Do Monarchs Form Overnight 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 a 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. In other places, these gatherings may last as long as two weeks.

How do the butterflies find the roost? Few people have 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. When one butterfly approached a cluster, the roosting butterflies flashed their wings. It took from about 7:15 to 7:35 p.m. for them to cluster in the trees, just before dark."

Where do monarchs form their roosts? Based on observer reports, trees that provide shelter from the wind are an important factor in roost-site selection. Having a source of nectar nearby also appears to be important.

"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."

One observer counted the number of monarchs in a roost each night. What can graphing the data show?

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

Why do monarchs roost? One hypothesis is that roosting behavior is an anti-predator strategy. Cool temperatures paralyze monarchs, making them vulnerable to predators. A roost provides safety in numbers. When overnight temperatures are warm, monarchs may not aggregate as tightly or roost at all. Perhaps monarchs shift to roosting behavior when cold overnight temperatures make them vulnerable.

Much of what is known about monarch roosts is based on observations contributed by citizen scientists. The roost map shows where there are large concentrations of monarchs. Week by week, it reveals the fall migration pathways to Mexico, and the pace of the migration.

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."