Monarch Butterfly Habitat Needs in Mexico
Dr. Calvert Describes the Monarch Colony's Use of Space

Contributed by Dr. Bill Calvert

Popular photos of monarchs overwintering in Mexico suggest that the butterflies are condensed into discreet colonies with well-defined borders at any time of the year. The butterflies appear to be surrounded by a sea of available fir trees. So why do they need so much space?

The pictures that we see are misleading to the extent that they are purposely taken to show the tight concentrations which so dazzle us. But the butterflies are not so concentrated in the beginning of the season or at the end. Although they are concentrated at night and during periods when it is too cold for them to fly, at almost all times of the year they use entire watersheds. There they search out nectar and water and, later on, mates. Finally, even within the proper habitat, their choice of sites varies from year.

In late October and November, they are spread out over acres and acres of ridge lines. In the end they are distributed along miles of canyons and are flying the length of those canyons every day. Their needs for space change with the seasons:

After an initial period of arrival, and condensation along ridge lines lasting well into December, the colony drops down slope, usually within the shelter of an arroyo (streambed). They seem to time this movement so that they are on relatively steep slopes during the coldest part of the year. The steep slope allows cold air, which has accumulated near the ground, to drain off and be replaced by warmer air from above. These butterflies take clear advantage of microclimatic regimes occurring in the mountain forests.

Beginning about mid-February, the weather begins to warm up. Surviving the cold is no longer of great concern but desiccation (drying out) and starvation (running out of their lipid reserves) becomes important. Butterfly colonies creep slowly, or occasionally jump quickly, down slope in the direction of water and nectar. The colonies often split up into smaller groups that are spread out along the canyon bottom. Later in March, on warm days with the correct wind direction, a portion of the butterflies spill out of the mouths of the canyon to begin the long journey north.

Another concern are the spaces between colonies. This is still poorly understood, but there is evidence of an active exchange between butterfly colonies – at least between some of them. Disruption of the forest between these sites would likely create a barrier preventing that exchange, the consequences of which are unknown.

So during the course of the year, the butterflies have occupied miles of ridgelines, entire arroyos down from those ridgelines, and large portions of the canyons below. What’s more, because they don’t always choose the same sites, they may occupy a different ridge, arroyo, and canyon system the following year. This is why they need so much space.


Special thanks to Dr. Lincoln Brower for sharing his lifetime collection of photos from the sanctuary area, a few of which are shown below, as indicated.

      Butterfly's view of surrounding landscape.

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