Western Monarch Butterflies (Danaus plexippus) in wintering cluster, coastal California. Many are opening their wings to gather warmth from a shaft of sunlight coming thru the trees. Monarch butterflies cannot fly if their body temperature is less than 86 degrees. We generally assume that monarchs can fly if it is above 60 degrees F, and above 50 degrees if it is sunny. The sun allows them to warm their flight muscles enough to fly.