Why Roost?
Roosting behavior may be temperature-driven. When overnight temperatures are warm, monarchs may not roost or aggregate as tightly. Roosting is a known anti-predator strategy at the overwintering sites in Mexico. Cool temperatures paralyze monarchs, making them vulnerable to predators. A roost provides safety in numbers. Perhaps monarchs shift to roosting behavior only when cold overnight temperatures make them vulnerable.

Patty Moss    Wainfleet, Ontario    September, 2012