If you've seen a monarch butterfly fluttering by recently, consider yourself lucky. The monarchs we see at this time of year are truly special, part of a “super generation” that's different than the previous three generations — the ones that journeyed north from Mexico to reach their summer breeding grounds.

Mid-August is the start of the monarchs' 3,000-mile journey to the Sierra Madre mountains in Mexico, according to the U.S. Fish and Wildlife Service. Monarchs have a life cycle that includes four or sometimes five generations each year. The fourth-generation monarchs — the ones we are just starting to see now — live very different lives than their parents, grandparents and great-grandparents before them. 

These super monarchs live longer, travel farther and reproduce on a different schedule than their relatives.

They live longer than other generations

Most monarch butterflies live for about a month and lay eggs when they are only a few days old. The super monarchs, though, live about eight times as long, typically emerging from their pupae in August or September and traveling to Mexico for the winter before starting the return journey back north beginning in March, the Fish and Wildlife Service reports.

Unlike most other migrating animals, the super monarchs will never complete the return trip. The super monarchs begin the journey north but typically only travel as far as the southern United States before mating and laying the eggs that will become the next generation, the first in the new year’s cycle.

They travel thousands of miles

Monarch butterflies spend the winter in the Sierra Madre mountains in Mexico and the summer in parts of the northern United States and southern Canada. The super monarchs complete the entire journey from their breeding grounds in the north to Mexico – a distance of up to 3,000 miles, according to the Fish and Wildlife Service. 

The super monarchs can travel 50 miles or even 100 miles per day, but their wings don’t do all the work. They often ride on thermal air currents, sometimes as high as 1 mile above the ground.

They congregate en masse

Monarchs only fly during the day, stopping off each night to rest, the Fish and Wildlife Service reports. They typically choose pine, fir and cedar trees for roosting, with dozens or even hundreds of the butterflies gathering close together for the night. In many cases, the super monarchs choose the same trees year after year, even though they are only distant relatives of the monarchs from the previous migration. 

Once the monarchs reach the Sierra Madre mountains, they roost in oyamel fir trees. Tens of thousands of the butterflies can cluster together, which helps them stay warm. The weight of the butterflies clustered together on a tree can sometimes be enough to cause the branches to break.

They take their cues from the environment

The super generation of monarchs travels thousands of miles south across North America, and their timing is the same each year. So how do they know when to start their journey? 

When the days start to get shorter and temperatures begin to cool, it signals to the monarchs that it is time to start migrating south. Not coincidentally, this is also when food sources for the monarchs — milkweed and other plants with nectar — become less abundant.

In addition to signaling when the butterflies should begin to migrate, the shorter and cooler days also affect the monarchs’ reproductive cycle. It keeps them in an immature state, called diapause, which delays mating and egg laying, according to the Monarch Joint Venture. They will remain in diapause until after they begin traveling back north in the spring. 

They remain a mystery to researchers

Scientific researchers understand a lot about how the super generation of monarchs is different from other generations, but they are still not fully understood. One of the biggest mysteries is how the super monarchs find their way back to the same oyamel fir forests in the Sierra Madre mountains year after year even though they have never been there before.

Scientists are continuing to research how they are able to successfully navigate the trip each year, but they believe both the magnetic force of the Earth and the position of the sun play a role, the U.S. Fish and Wildlife Service reports. The combination of the sun’s angle and position over the Earth as well as the Earth’s magnetic pull create an internal compass that is thought to guide the journey.