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