Monarch Butterfly Field Notes from Mexico by Dr. Bill Calvert

Field Notes from Mexico by Dr. Bill Calvert

February 25, 2004

Photo: Jim Edson

Dr. Calvert Calling Journey North from Mexico

(Audio File mp3)

NASA Visible Earth

"Yesterday a clean, cool, high altitude wind blew onto the Sierra Chincua, reminding our New Englanders of a crisp autumn day. These westerly winds prevail at this time of year, and sometimes bring Pacific moisture to the southwest slopes of the Transvolcanic Belt. Yesterday the cool, dry air could only desiccate the mountain forest and the butterflies.

"The Chincua colony covered about 1 acre. It was centered on a small stream in a deep arroyo that drains north. Many outlying butterfly groups were scattered along the slope above the main aggregation. The high winds that we had experienced in the ridges were calmed by the protective ridge and deep forest of the canyon.

"This forest is impressive. Trees likely exceed 35 meters in height. The ingredients include a mixture of oyamel, pine and a few oaks and cypresses. There are patches of natural regeneration of oyamels and pines. Only near the ridge is there evidence of clandestine cutting. The impression is one of a natural, unmolested forest, very different from the slopes of El Rosario which appear hacked and abused. Still no evidence of the massive mortality seen at Rosario. The impression is of a happy, healthy colony.

"But all might not be so well in butterfly land. There is little evidence of floral development. The deep forest and the north slopes have virtually no understory. Many late-season bloomers around the fields of Rosario and La Salud have been zapped by the cold. More about this in subsequent weeks.”

Journey North's Elizabeth Howard asked about the dry season.

"The most intense part of the dry season is beginning now--late February, March and April--until the rains begin. The forest is literally drying out. And the butterflies no longer can take water from dew patches or wet areas in the forest because they’re drying up. They have to go down to streams. And fortunately, on this north slope of Sierra Chincua there is a stream that they can use. And as I said in the report (above), the colony is right now centered in the bottom of that drainage, right around that stream.

"The place at Rosario has no stream close to the colony. I’m not sure about down the canyon more, I haven’t had a chance, they won’t let you go down there. There has to be some water somewhere down there. And they’re going to be flying down towards that water and the colony will eventually move down in that direction.

"When it’s dry they risk drying out a lot more quickly. The wind and the lack of moisture in the ambient dries them quickly."

Journey North's Elizabeth Howard asked, Do you have any sense of how many dew drops they need to drink in a day?

"None. But that would be easy to measure in cage experiments. Somebody could do that. You know, just figure out how much water—or how much nectar—they’re taking in a day. They can get water from nectar, as well as sugar.

"I’m beginning got worried that there’s not enough nectar this year. I hope that was clear from the report. It’s not clear that there’s going to be as much nectar available for the butterflies as there has been in the past. And we don’t really know the consequences of that. It seems like they won’t be able to replenish their fat reserves.

Maps and images of monarchs at their winter refuge in Mexico.

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