In late summer and early fall, a special generation of monarchs is born. These Monarchs live much longer, up to eight months. Triggered by the decreasing daylight and angle of the sun, these butterflies delay sexual maturity and begin flying toward their overwintering grounds, in some cases up to 2,000 miles away. The Monarchs feed on flower nectar during the journey, attempting to build up fat reserves that will enable them to survive the winter months. At night, they may cluster together in small groups, but as winter approaches, they move on to more permanent overwintering sites.

After arriving at their destination, the monarchs cluster in large masses for protection from the elements. Since their flight muscles do not function well unless the temperature is above 55 degrees, they rest quietly on the trees, resembling dead leaves, until sunlight warms them enough to fly. On warm days, the butterflies will leave the trees entirely, seeking out nectar sources with which to replenish their energy reserves, but always returning well before evening to once again cluster in the trees.

The overwintering monarchs do not mate until the increasing temperatures and daylight hours in February trigger the development of their sexual organs. They can then be seen performing spiral mating flights, after which the coupled pair will rest overnight. The male passes a nutrient rich sperm packet to the female during mating. With newly fertilized eggs, the female will travel far in search of milkweed on which to lay them. By March, most of the butterflies have departed on their spring migration.