The Monarch of the Air

Monarch butterflies are sort-of the Passenger Pigeons of the modern world. They migrate into California and Mexico at two relatively small sites, where the entire population spends the Winter. If anything were to occur to either of those two sites, huge populations of Monarch butterflies would be wiped out. The Passenger Pigeons were killed when they nested for the night. At the moment, Monarchs are not in danger of extinction, but there is always that possibility.