Monarch Migration Update: March 2, 1999
Today's Report Includes:
First Monarchs May Begin to Leave Mexico Next Week!
Spring is rapidly advancing, and the typical flurry of monarch activity before their departure has begun. Dr. Calvert predicts the first butterflies may begin to depart as early as next week. Although it takes 3-4 weeks for all the butterflies to leave Mexico--and much longer to reach you--please sharpen your observation skills now. Review this checklist so you'll be ready to make good field observations this spring:
Reminder: Report Sightings of Over-wintering Monarchs Now
Field Notes from Mexico by Dr. Bill Calvert
February 28, 1999
The weather in Michoacan continues to be warm. As the butterflies respond to the increasing heat, the clusters are clearly changing character. While still enormous they are becoming thinner: Where thick clusters and curtains of hanging butterflies darkened the forest, there now appear green spots where the oyamel foliage shows through. Trunk clusters are not as numerous as they once were either.
Down-slope a different kind of cluster is forming and increasing in number. These new clusters are higher in the trees, smaller in size, and much more dispersed. Because of this dispersion, the area covered by the colonies is significantly greater. (Unfortunately, Eligio Garcia won't be able to take measurements until Thursday this week, but they will be sent in next week's update.)
Increased Mating Signals End of Wintering Season
Mating attempts and successes have increased dramatically. In the last week, 1-5 mating pairs could be seen at one time in the trails below the colonies. Leading up to the monarchs' departure, we'll see three times as many mating pairs.
Mating proceeds like this: A male patrolling the canopy grasps the wings of a female, using the claws on his feet as he encounters her. The pair parachutes down to the ground. If he manages to attach to her (about 10% of the time), she folds her wings and he then flies off with her, if he can, to a safe perch where mating is completed.
Millions of Monarchs Eaten by Predators
Before the monarchs arrived here in November, there was not a single butterfly wing on the ground. Now, the forest floor is peppered with dead butterflies. In a typical year, we estimate upwards of 12-15% of the entire over-wintering population dies due to predatory activities. (You can estimate how many monarchs this is, and how many die in a single day during the 5 month period.) Predation this year appears to be at normal levels, though the results of the Mexican government's study are not yet in.
There are 3 main monarch predators in the sanctuary, one mouse species and two bird species. Close inspection of a dead butterfly gives a clue as to its predator:
(To respond to this question, please follow the instructions below.)
The millions of monarchs clustered together are a rich food source and easy prey--but only for those predators who can successfully prey upon them. While conducting research over the years, these are some of the observations made:
Last week, we looked carefully at a few monarch wings left by a predator near a water hole. We noticed all the dead butterflies were males.
Monitoring Predation and Mortality
Biologist Benigno Salazar estimates the impact of predation and other causes of monarch mortality. He made transects
through the forest, counted dead butterflies and determined the causes. As we walked through the forest, Benigno
showed us the tip of the oyamel branch and pointed out that the scientific name of the oyamel fir is Abies
religiosa because the tip of the branch forms a "religious" Christian cross.
Discussion of Challenge Question #4
What the Heck is a Hectare?
From Ms. Thurber's 4th grade students in Ferrisburgh, VT:
How to Respond to Today's Monarch Challenge Questions
1. Address an e-mail message to: firstname.lastname@example.org
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