When monarchs roost

It’s autumn, and monarchs are gradually heading south. During their migration they make stops, sometimes in large numbers. Let’s take a look at the astonishing phenomenon of monarch roosting.

Why do monarchs roost?

Unlike certain migratory birds such as geese, monarchs migrate by themselves. Since these butterflies fly only during the day, they make a stopover every evening and set off again the next morning, weather permitting. From the air, monarchs identify interesting sites, and since they’re all looking for the same thing, a number of them will end up in the same spots. At that point they can be counted by the dozen, or even by the hundred!

What makes a good migratory staging site?

First of all, monarchs look for an abundance of nectar. So they’ll be observed in places that are rich in nectar-producing plants. In eastern Canada, they are attracted to plants like the Canadian goldenrod and New England aster.

To spend the night, monarchs prefer being sheltered; they settle on the leaves of trees, under the wind. At our latitudes, they’re often observed on conifers and maples, whose foliage is especially dense. A preferred migratory stopover site therefore consists of nectar-producing plants in bloom and trees in sufficient quantity to block the wind.

Where can we see monarch roosts?

Certain spots are known for playing host to monarchs in large numbers every year. That’s the case with Point Pelee National Park in southern Ontario, which acts as a bottleneck for butterflies migrating along Lake Erie. They assemble there while waiting for a favorable wind to help them cross that large expanse of water.

However, the location of migratory stopovers also varies from year to year. The phenomenon isn’t rare, but it’s not well documented. In fact, if monarchs touch down at the end of the day and take off again the next morning, the odds are that roosts will go unnoticed! Nevertheless, every year hundreds of roosts are reported in Canada and in the United States to citizen-science programs like Mission Monarch and Journey North. If you happen to witness the phenomenon, tell us about it by sharing your observations on the Mission Monarch website!

Using the “Flash Data” form, note the basic information about the sighting (place, time, and date) and estimate the number of butterflies observed. Then, in the comments section, briefly describe the roosting site, indicating whether it is near a body of water and/or flowering plants. You can also indicate on which type of tree, plant, or structure you observed monarchs roosting. The information collected will help better understand this phenomenon.