Although we occasionally see Monarchs in the spring, these are generally few and far between. Like the occasional western shift of southerly migration routes due to storms, a strong storm at the right time can shift some north-bound spring migrants our way. In North America, there are two geographically distinct Monarch populations, separated by the Rocky Mountains. In the eastern population, Monarch butterflies and caterpillars are common summer sights on and around milkweed plants from the Great Plains to the East Coast and from Mexico into Canada. After they spend several generations in the Midwest and Northeast, the shorter days and cooler temperatures of late summer begin to trigger the Monarchs' migratory behavior.
Migratory butterflies are much more active than non- migratory ones.
They will regularly take flight in large numbers, with groups often flying hundreds of feet up into the sky. This activity increases as southerly moving storm fronts approach areas where Monarchs are present. When both migratory activity and weather patterns come together, thousands, and sometimes hundreds of thousands, of Monarchs will fly up into the fast moving frontal winds pushed ahead of these weather systems. Once airborne, the Monarchs ride these winds, much like a surfer rides a wave, as they undertake their journey to the mountain sites in central Mexico, where they will spend the winter. These mountain locations provide the environmental conditions that allow the millions of Monarchs that have traveled from as far away as Eastern Canada to survive the winter.