Journey North Monarch Butterfly Spring 2003

Field Notes from Mexico
by Dr. Bill Calvert

March 12, 2003

News from the butterfly zone. It was cooler but dry in the Sierra Chincua. The butterfly colonies have again moved down, but not so far. At Chincua they’re on the other side of the Baranca Hondo, 100 meters above the stream. The colony is much reduced, to maybe a quarter of a hectare.

A surprising find was the number of trunk clusters, a method of roosting that’s rare this time of year. The long “tongues” (the bottom of the V’s), of the trunk clusters were facing south, just the opposite of what they were when the colony was on the north side of the Baranca. Possibly they extend lower on the upslope side to maximize exposure to moisture. Late-season trunk clustering itself may be a response to low moisture. It still hasn’t rained this year, although big cumulous clouds were visible on the horizon yesterday.

I don’t remember in past years in late season seeing any trunk clusters. So is it some sort of microclimate condition? Seems like long shot, but would bear investigating why there are so many trunk clusters. Roosting on trunks would minimize desiccation due to dry winds whacking through clusters. Here’s an analogy: The clusters hanging from branches are like clothes hanging on a line. The wind dries out the clusters, and on a windy day, they dry much faster. So a trunk location, instead of a branch, might avoid this drying.

Mating has increase many-fold. Take downs occurred throughout the colony. There’s still heavy packing of clusters in some places, as well as the smaller, highly dispersed clustering characteristic of late season.

Strangely, last week when we walked in the forest was just filled with butterflies. The butterflies were everywhere, and they were drinking and nectaring. This week, not so. And I’m thinking that it has a lot to do with the coolness. That’s my first thought. It’s just cooler this week, and they’re not searching for nectar and water so much.

There’s a reasonably large number of butterflies in the town of Angangueo now, so it’s obvious that some of them are going north. They’re flying around the town, and a portion of these seem to be heading north. So it’s clear that some of them are beginning to head back. Once they’ve come down the mountains this far, they don’t go back. Of course I have no way of knowing this, but one piece of evidence is there are transient clusters forming in the valleys on the way down from La Salud, and way, way down to Angangueo. In the evenings you can see clusters forming in these little barancas. So that clearly means they’re not going all the way back up to the colonies. They’re transient clusters; here one day gone the next. And they break up totally during the day. There’s nothing left mid-day.

Other Observations:

Cerro Pelon: No colony is descending the depauperate forest on the west face of the Cerro Pelon.
Piedra Herrada: Streaming down to water is still evident at Piedra Herrada, to the delight of our school teacher group from New Jersey, Tennessee and Georgia.