The annual migration of monarch butterflies in North America is not only a unique phenomenon but also a sight to behold. Monarchs make the 5,500 km migration from Canada and the US in their millions, clustering tightly in the fir and pine forests in the Western area of Mexico City. Since it is virtually impossible to count them by individuals, they are counted by the total area that they cover.
While most insects can survive harsh weather, monarchs cannot survive the harsh conditions of an extended cold winter. This is why every fall, the monarch butterflies in North America head south to roosting sites where they spend the winter. Amazingly, they fly in their numbers to the same roosting areas and in many cases occupy exactly the same trees.
The migration of monarch butterflies is special because it is the type of migration that one would expect from whales or birds and not insects. However, unlike whales and birds, the monarchs that make the trip to the roosting areas don’t survive to make the return trip. In fact, it is their great grandchildren that head back south the next fall.
How do monarchs know it’s time to leave?
The life cycle of a monarch butterfly is a complicated one. This is because the monarchs that emerge at different periods in a year don’t do the same things. For instance, while a monarch that emerges in both the spring and summer periods becomes reproductive in a matter of days, a monarch that emerges in the fall emerges with its reproductive organs in a state of reproductive diapause or suspended development. This means that they will live until the next spring before they can lay eggs or mate.
The monarchs that emerge in the summer and early fall will have different physical and behavioral characteristics from those that will emerge in the summer. The changes are triggered by the cooler air, shorter days as well as milkweed senescence of late summer.
The end of the diapause period varies considerably across individual monarchs in an overwintering colony. An overwintering population comprises of monarchs that are from a wide range of geographical areas that have different environmental conditions. In most cases, diapause in males is shorter than in males.
How do monarchs know the location of overwintering sites?
Researchers are still trying to figure out how monarchs manage to find their overwintering sites. It is highly likely that they mostly rely on their instincts. Other likely directional aids include the sun’s position and the earth’s magnetic pull.
How do monarchs stay warm?
In order to stay warm, monarchs will cluster together in colonies. It is normal for tens of thousands of monarch butterflies to cluster on one tree. Although a single monarch weighs less than a gram, the weight of a cluster of tens of thousands can cause a tree branch to break.
Monarch numbers are shrinking
This year, there was a drop of 27 percent in the population of monarchs wintering in Mexico compared to last year. Experts have cited shrinking forests as a highly probable cause. Many scientists have raised alarm about the effect of illegal logging on the future of monarchs in the coming years. Storms and cold weather have also been cited as factors.
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