Did you see a Monarch Roost? 

Monarchs only migrate by day. They rest in overnight roosts during fall migration. Migrating monarchs cluster together in trees at night, forming what is called a roost. A roost may have a handful of butterflies or more than you can count. Sometimes a large roost forms in several adjacent trees. If you see a roost, please report the number of monarchs in the roost along with habitat conditions that may indicate what attracted them to the area (e.g., are there nectar sources nearby, are they protected from the wind, etc). Because roosts are dynamic and often reform over multiple nights, please submit a new report for each day you observe the roost. 

Fall roost reports should reflect the number of monarchs observed within a roost for a single night. In comments, include date roost formed (if known), how you estimated the number sighted, available nectar source(s), species of tree(s) in roost, and whether the monarchs are shielded from the wind in their location. 

Note: If roost remains more than one night, keep reporting daily as long as you see this roost. 

Resource: https://journeynorth.org/monarchs/resources/slideshow/slideshow-why-overnight-roosts