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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
Before the migration begins, please help us document where monarchs have been sighted this winter. Next week's update will include a map, so please report your winter observations immediately.

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.

Why do you think the scientific name of the oyamel fir is Abies religiosa?

Because their lives will soon end, both males and females must attempt to pass their genes to the next generation. Their behavior suggests different strategies for males and females, and a form of deception is occurring during these early matings: The females will probably not use the sperm from early matings to fertilize their eggs. Instead they may use the spermatophore* for nutrition. One theory holds that during these early matings, females are collecting nutrients to make egg-yolk. The spermatophores delivered by males to females contain nitrogen--an important in ingredient in egg yolk--which cannot be obtained from nectar or from the female's fat reserves. So, despite the male's effort to reproduce, these early matings probably do not produce offspring for the male. Instead they serve to increase the female's fitness.

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:
Predator Clue
Black-headed Grosbeak The monarch's abdomen is entirely missing.
Black-backed Oriole The monarch's abdomen is present, but it appears to be "unzipped" (slit open) and the thorax is gorged.
Black-eared Mouse Caches of butterflies are found on the ground, and only the wings remain.

Challenge Question # 8
Who Ate These Butterflies?

Inspect these butterflies carefully and see if you can determine who the predators were:
(Click on photos to enlarge.)




(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:

  • Orioles and grosbeaks visit daily in the morning and afternoon, in flocks of as many as 5-60 birds.
  • Mice feed at night on live, dying and recently dead butterflies which they find on the forest floor, another reason monarchs are at risk when on the ground.
  • More butterflies are eaten on cold days than on warm days.
  • In smaller colonies, predation levels by birds alone can be as high as 44%. This is known as the "edge effect"; predators are able to penetrate more easily into smaller colonies because they have more surface area.

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.

Challenge Question # 9
"Can you think of a reason why there were no female monarch wings found?" (Clue: How do monarchs defend themselves? Remember: Butterflies can't bite, and it's often too cold to fly away.)

Until next week,
Bill Calvert

*When mating, males deliver sperm to females in a package called a spermatophore.

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?

Ms. Thurber's 4th grade students in Ferrisburgh, VT:

"A hectare is 2. 47 acres. That would about the same as 2.4 football fields. It would be a little bit less than that. First, we looked up the word hectare in the dictionary. Using a Metric System table, we saw that a hectare was 2.47 acres. Then we found out how big an acre was. An acre is 43, 560 sq. ft. We knew that a football field was 100 yards by 50 yards, which means it is 5000 sq. yards. We changed that to sqare feet and a football field is 45,000 square feet. Then, we found out how many sq. ft. in a hectare. There are 107,593.2 sq. ft. That is almost the same as 2.4 football fields. In third grade, we had measured an acre of our playground. Our playground is closer to a hectare than an acre. If you count the surrounding playing fields, our playground is bigger than a hectare." (

How to Respond to Today's Monarch Challenge Questions

IMPORTANT: Please answer ONLY ONE question in each e-mail message!

1. Address an e-mail message to:
2. In the Subject Line of your message write: Challenge Question # 5 (or Challenge Question # 6)
3. In the body of EACH message, answer ONE of the questions above.

The Next Monarch Migration Update Will Be Posted on March 2, 1999.

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