As October comes to an end, temperatures continue to decline from what has been record-breaking warmth throughout much of the United States.
Residents of the northern regions of the U.S. and the southern parts of Canada may have noticed that there have been a larger number of monarch butterflies fluttering through the skies for this time of year.
This is no coincidence.
Warm weather and strong headwinds have delayed the annual monarch migration.
Millions of monarchs travel about 3,000 miles to overwintering sites in southwestern Mexico, as temperatures drop and the winter season begins.
“This has been the slowest migration that I have seen, and I have been doing this since 1992,” Founder and Director of Monarch Watch Chip Taylor said.
Climate and temperature play a role in their migratory patterns and monarch development.
Monarchs prefer to travel when temperatures are in the 60s and 70s F, but temperatures have been higher than average for this time of year, Taylor said.
Unlike many other insects in temperate climates, monarchs cannot survive a long cold winter, according to the University of Minnesota Monarch Lab.
“Monarchs as migratory organisms need habitat not just in one place, but suitable climate in all different areas of their range,” University of Minnesota Monarch Lab Dr. Kelly Nail said in a U.S. Fish and Wildlife Service (USFWS) web seminar.
Monarchs need a “goldilocks” climate, not too hot and not too cold, according to Nail.
Therefore, these monarchs depart to a few mountains in Mexico, where they huddle together on branches of Oyamel fir trees for the winter season.
These trees create a micro-climate for the butterflies, which protects them from extreme temperatures and weather events, according to Nail.
The monarchs cluster vertically by the millions, centered at 12 meters in height, which protects them from freezing. The dense forests act as a wind break and a blanket of thermal insulation, according to Nail.
At the end of the winter months, the monarchs prepare for their journey back north.
It usually takes the monarchs two to three generations to complete the northern migration.
The summer monarchs live only about a month as adults and begin laying eggs when they are only a few days old, according to the Monarch Lab.
Whereas, the monarchs that spend the winter in Mexico are the final, or migratory, generation of the annual life cycle. This migratory generation of monarchs can live up to nine months.
Decreasing day length and temperatures, along with aging milkweed and nectar sources trigger a change in monarchs; this change signifies the beginning of the migratory generation, according to the Monarch Joint Venture.
Most butterflies in the final generation begin their lives in the north and then migrate thousands of miles to mountaintops in Mexico, which neither their parents or grandparents have ever seen.
This generation of monarchs will begin to head north in March after spending several months in Mexico. They will begin to lay eggs in northern Mexico and southern U.S., starting the cycle again.
Mystery largely surrounds why these creatures migrate, and how they know where to migrate to.
Millions of monarchs travel from regions all over eastern and central North America to accumulate in a very small area in the mountains of central Mexico. However, there is no parent to lead the route to Mexico, as implied by the annual life cycle of monarchs.
Therefore, it is fairly certain that monarchs rely on their instincts to find overwintering sites.
Sun compass and magnetic compass are considered to be the two most likely cues that monarchs use, according to the Monarch Lab.
There is also speculation that the migration is the result of monarchs following the bloom of their primary food source, milkweed.
Monarch development relies largely on the availability of milkweed, which acts as the primary food supply for the earlier stage of the monarchs' life span.
Monarchs are at a disadvantage when traveling late, like this season. There will be less flowering at this time of year; therefore, less fuel for the monarchs on the migration, according to Taylor.
“Monarchs gain their masses and fuel as they migrate. If they make it to Mexico, they won’t be as prepared,” Taylor said.
The monarchs had a good reproductive year, but the late migration could impact the population.
The current avocado craze also places this year's monarch population at risk. The increasing demand for the fruit has led to greater incentive to expand avocado orchards into the Monarch's overwintering sites in Mexico, according to The New York Times.
Between 1974 and 2011, about 110,000 acres of forest across Michoacán’s central highlands were turned into avocado orchards, according to a study by the National Autonomous University of Mexico.
The avocado boom further complicates issues surrounding deforestation and monarch protection.