Kings & queens of long trips
Cool nights, clear blue skies, shorter days, leaves changing colour and kids returning to school: all this screams, “Time for migration!”
It’s not just birds that take off, either. Reptiles and amphibians will soon seek out the mud at the bottom of local ponds to go dormant for the winter. Mammals like skunks and groundhogs burrow down deep in the ground to hibernate. Fish metabolism slows with the cooler temperature of the water. Everything slows down for winter, even the trees!
However, other animals leave and travel to warmer places to survive our Ontario winters. Most local birds must migrate to find food. Amazingly, one notable insect travels thousands of kilometers to spend the winter in a remote forest in Mexico. Imagine that! It’s a unique story.
Let’s start in the spring, when the sun shines longer every day, and the temperatures are warmer. Adult monarch butterflies have been roosting in their millions in the oyamel fir forests, high in the mountains of Mexico, all winter long. The increase in daylight triggers a hormonal change that leads to the maturation of their sexual organs. They mate, and then the males die, but the females fly north, laying eggs as they go on milkweed plants (Asclepias syriaca), eventually dying, themselves.
The eggs, only as big as a pinhead, hatch in a few days and the tiny black, yellow and white striped caterpillars begin a new generation. They are voracious eaters, eventually able to eat a single leaf in about five minutes, and they gain about 2,700 times their original weight! They eat constantly for about two weeks, shedding their skin as they grow from under 1 cm to over 5 cm. However, the next phase is nothing short of miraculous.
The caterpillar attaches itself to a branch or leaf with strands of silk, then sheds its skin and forms a hard shell. This vase-shaped pupa looks like a shiny jade-green vase with beautiful gold dots. Gradually, over the next nine to 14 days, it turns see-through, and the fully formed orange, black and white butterfly emerges. It pumps fluid from its abdomen into veins in its wings to make them unfurl. It takes about an hour for them to dry, then the butterfly can take flight. There’s no time for training – away they go!
Their wings are about 10 cm across, and each butterfly weighs about 0.5 grams. That means that three butterflies together weigh less than a dime (1.75g).
Adult butterflies feed on nectar from flowers, including milkweed, clover and goldenrod. They continue to spread north for two or three more generations, each taking about one month to complete their life cycle.
The final generation is different from the others. These butterflies hatch at the end of the summer and begin the migration south. They are not sexually mature yet, and store fat in their abdomens. Like birds they use updrafts of warm air to help them preserve energy on their 4,000 km journey from the Great Lakes to the central Mexican mountains.
They fly at speeds ranging from 18 to 40 kilometers an hour, and have been observed over one kilometer above the ground.
It takes about two months to reach their wintering grounds. One tagged monarch was recaptured over 400 km from where it had been released the previous day! What a record! And they do this all by instinct, since they’ve never been here before.
The cool temperatures in the high mountains slow their metabolism, so they don’t use up all their fat reserves. They cluster together by the millions and are protected by the trees from the cool damp conditions. They can survive short periods of freezing temperatures, but a big snowfall or prolonged cold spell could prove disastrous.
The Mexican authorities have protected over 160 square kilometers of forest in the Sierra Madres in the Monarch Butterfly Biosphere Reserve. However, monarchs face a perilous journey north every year through the U.S. Midwest past fields sprayed with pesticides, and from reduced habitat due to agricultural practices for both reproduction as well as feeding. It’s estimated that the population has dropped by 90 per cent since 1995, resulting in the loss of nearly one billion butterflies.
We can help these regal insects by growing milkweed plants to provide breeding habitat and nectar-rich flowers to provide a food source. Conservation groups in the U.S. are encouraging the planting of “Monarch Waystations.” They are also lobbying transportation departments and utilities to let milkweed grow along roadsides and power lines. These efforts may result in migration corridors to preserve these amazing long-distance travellers!
Nancy Melcher is The Nature Nut. Send details of your sightings or questions about the natural world to: email@example.com