Oodles of Monarchs

Last September, hundreds of monarch butterflies congregated in our backyard trees (like those at right). We'd never seen anything like it before. Could the unseasonably warm weather have caused this gathering?

—Anita Bulgrin, Brooklyn Park, Minnesota

George: Monarch butterflies east of the Rocky Mountains migrate to and from Mexico in three to five generations. This means the butterflies that leave Mexico for northern destinations are not the same ones that arrive. Instead, they reproduce along the way north, and their offspring finish the journey. The monarchs that are in Minnesota in late summer and early autumn are probably the ones that will make the one-way flight to Mexico. In preparation for the trip, this generation loses interest in reproduction but becomes very sociable, forming large flocks to await favorable wind conditions for their flight south.