It is that time of year when Monarch butterflies are starting their long migration south to Mexico. During the next three months, millions of Monarchs will travel 2500-3000 miles across Canada and the United States headed for warmer weather in Mexico.
The Monarchs (Danaus plexippus) fly during the day, typically traveling alone. They do not migrate in flocks like many birds. But in the early evening, they will rest in trees. Sometimes they cluster in small groups, and sometimes they form large clusters. These clusters of Monarchs are called roosts. Most roosts last for only a night or two but sometimes these gatherings may last as long as two weeks.
Why do monarchs roost? According to Journey North, “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.”
Where do Monarchs roost? Roosts are more likely found in certain habitats but are not consistently found in the same place. They vary from year to year. Roosts can often be found near nectar sources, in trees that are downwind, and near a major flyway. Flyways are typically near valley streams or depressions that provide a cool moist environment. (https://journeynorth.org/tm/monarch/FallRoosts.html)
One of the best ways to follow the fall migration is to track where they are forming roosts. Journey North keeps and posts data collected by citizen scientists on where roosts are being observed. 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.