In a few weeks’ time, the oyamel fir forests high in Mexico’s mountains will start to twitch. Millions of orange and black wings will lift from the trees, as overwintering monarch butterflies leave their winter shelter to start their last journey. The flight ahead is short compared with the one they made late last year, but their fat reserves from autumn feasting are down to just a few milligrams. The priorities now are to mate and to find milkweed on which to lay their eggs, providing food for the next generation.
These butterflies were themselves born under a milkweed leaf perhaps 2500 miles away, as summer neared its end last year. How did they find the energy for such an arduous journey, and how did they all find their way from across the northeastern USA and Canada to just a dozen overwintering sites in Mexico?
The flapping of wings would not take the monarch butterfly very far. The small body can scarcely carry enough fuel for a day of flapping, let alone two months of logging 50 or more miles a day. So the butterflies flap only until they find themselves in rising air and then they simply soar, conserving their energy by travelling wherever the air takes them. It’s not a perfect solution, and requires some adjustments because the wind does not always take them exactly where they want to go - and though the butterflies don't know where they are going, they know the direction. Southwest. They have even been known to keep their bodies angled towards Mexico as the wind carries them the wrong way.
The butterflies set a course from several sources of information. The position of the sun is their starting point, but useless if they don’t also know the time, so the monarchs combine the sun’s position with the circadian clock of their own body. The resulting solar compass allows them to calculate southwest at any time of day. If the sun is blocked by clouds, they turn to a backup source, calculating their direction from slight changes in the Earth’s magnetic field as they head south. On very cold or rainy days, the monarchs won’t fly at all. Their flight muscles are paralysed below about 13 ℃, and while they can absorb energy through their wings, they can’t store it. That's why they will sit with their wings spread in the sun before taking off.
En route to their winter perch, the monarchs gorge on nectar to boost their fat supplies, taking on enough fuel to survive the winter. As they approach their target, the mountains themselves probably act as a funnel, herding the butterflies together into the forests.
This long journey south and winter layover only falls to the last generation of monarchs each summer, the ones born the furthest north. They alone put their reproductive development on hold to better their chances of survival. At some point over the winter, the cold will flip their "compass", sending them northeast in spring, the start of a journey that is not theirs to finish.
Their offspring will be just the first generation of short-lived monarchs that take part in a family relay to carry the monarchs back to the northeast USA and Canada. You can follow their progress, and report your own sightings of monarch caterpillars, peering under milkweed leaves to spot the eggs. Follow their progress north, and then watch as the last butterfly of the summer heads in the other direction. Southwest to Mexico.
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| Monarchs in Motion/Image:Tarnya Hall/CC BY-NC-ND 2.0 |
The flapping of wings would not take the monarch butterfly very far. The small body can scarcely carry enough fuel for a day of flapping, let alone two months of logging 50 or more miles a day. So the butterflies flap only until they find themselves in rising air and then they simply soar, conserving their energy by travelling wherever the air takes them. It’s not a perfect solution, and requires some adjustments because the wind does not always take them exactly where they want to go - and though the butterflies don't know where they are going, they know the direction. Southwest. They have even been known to keep their bodies angled towards Mexico as the wind carries them the wrong way.
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| Image: Harry Comisso |
En route to their winter perch, the monarchs gorge on nectar to boost their fat supplies, taking on enough fuel to survive the winter. As they approach their target, the mountains themselves probably act as a funnel, herding the butterflies together into the forests.
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| Overwintering monarchs/by Bfpage at English Wikipedia/CC BY 3.0 |
Their offspring will be just the first generation of short-lived monarchs that take part in a family relay to carry the monarchs back to the northeast USA and Canada. You can follow their progress, and report your own sightings of monarch caterpillars, peering under milkweed leaves to spot the eggs. Follow their progress north, and then watch as the last butterfly of the summer heads in the other direction. Southwest to Mexico.
Further Reading:
Four Wings and a Prayer: Caught in the mystery of the monarch butterfly by Sue Halpern
Monarch Life Cycle, Monarch Butterfly Fund - Conserving the Migration
Monarchs in Peril, Scientific American
Monarch Life Cycle, Monarch Butterfly Fund - Conserving the Migration
Monarchs in Peril, Scientific American
Fascinating. Beautiful drawing!
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