The monarchs again put on a sterling performance last week! During our last day in the area, we witnessed a massive flow of tens of thousands of butterflies flying out of the colony down across a pasture. All of the butterflies within 3 m of the ground were flying in the same direction giving the impression of a massive sheet of orange and black colored creatures streaming slowly downward. At the middle of the pasture there was a seep of water. Thousands of monarchs were at the seep drinking water from the water soaked mud and from open pools, but the majority were flying on pass the seep. Above 3 m fewer butterflies were flying in the opposite direction back towards the colony.
Descending the Mountain
This colony was the Rosario "bud colony." It was the lower part of the Rosario colony that had budded off from the main colony, which occupied a site in the same drainage, but at a higher elevation. Each day more butterflies left the upper (original) colony and joined the bud colony. At this point in time (March 11), it was hard to tell whether the bud colony or the main colony had more butterflies. The bud colony was stopped from descending to even lower elevations by fields and pastures that came all the way up to 3000 m elevation. Earlier in the year the butterfly colony was located above the Llano del Conejo at 3240 m elevation. Monarch colonies always descend the mountain during the course of the monarch's 5 month tenure. This descent accelerates during late February and March when the combination of intense sunshine and lack of clouds and moisture in the air warms up the ambient considerably. The descent is almost always associated with a particular arroyo or drainage.
At Rosario they usually follow the drainage called Arroyo Los Conejos. However, this year they used another drainage about 1.5 km to the northwest of Los Conejos. This small drainage is not named on the local topographic map, but is called (inappropriately) the Rio Grande by the locals. When we were there (well into the dry season), there was only a little water flowing in it. In contrast, at Chincua, the monarchs first formed one of their colonies at 3100 m and descended to 2800 m by mid-March. At Chincua they had plenty of dense forest with a good supply of water to shield them from the intense tropical sun and associated warmth. The lack of forest at the field edge in El Rosario did not stop them entirely however. During the day butterflies poured out of the bud colony and over the ridge at the little community, La Salud, towards Angangueo.
These butterflies are undoubtedly part of the return migration to the United States and Canada. Each day tens of thousands pass through the town of Angangueo. They are all going in the same direction - northward. Back towards Rosario, many thousands are taking nectar from flowering plants, especially eupatorium and scenecios along the road to Angangueo. Many of these do not return to the bud colony mentioned above. Instead they bud again, forming smaller aggregations in remnant pieces of woods along the Angangueo-Rosario road. These small remnants of woodland may be very important to them in offering nighttime shelter from cold and predators.
We witnessed many fires burning in forests all over the states of Mexico and Michoacan. Fires were so frequent and dense that a permanent haze was evident in the sky. None of these were 'serious' fires such as the crown fires that we hear about in our northern forests. All were ground fires burning along the forest floors. They created a lot of smoke and locally, a lot of heat. One such fire was burning near the Chincua colony located in the Arroyo Honda about 5 km northwest of Angangueo. Although the smoke from this fire was clearly visible, it apparently has not affected the Chincua butterfly colony.