Monarch Butterflies and Predators at the Winter Sanctuary in Mexico

Monarch Butterflies

Monarch Migration News: February 25, 2016
By Elizabeth Howard

A butterfly colony looks like a bountiful source of food to hungry predators. How do monarchs defend themselves?

Beneath the colony Estela found the ground littered with butterflies that have not survived the winter, victims of predation, starvation, or other natural causes.

News from Mexico

Population Announcement
The monarch population size is expected be announced at a press conference on Friday, Feb. 26th at 12:30 CST.

This week monarch butterfly experts from Canada, Mexico and the US are meeting in Mexico City to coordinate conservation efforts. The three countries’ Environment Ministers will be in attendance on Friday so we expect the numbers to be released then.

Monarch Mating Begins!
Estela's weekly trip to the sanctuaries revealed an exciting change.

"By surprise, I came across the first three couples of monarchs mating in the season I have seen! One week ago, I was eagerly looking to find at least one, but all of them seemed to be ignoring my anxiety to see mating begin!"

Below the Clusters
When the monarchs arrive in November, not a single butterfly wing is on the ground. By March, the forest floor is peppered with dead butterflies. Estela walked through the sanctuary looking closely at the ground for signs of predation and other mortality. She took a sampling to give an idea how the sanctuary looks.

"In some places on the ground, below the colony itself, there could be an average of 40-50 monarchs dead per square meter all around. I looked up at the splendor of monarchs swirling overhead and their feat of survival struck me with a renewed admiration."

Population

Monarch Butterflies Begin Mating at Overwintering Sanctuaries in Mexico

Mating Begins

Monarch Butterfly Missing Abdomen, Eaten by a Predator

Below the Clusters

Monarchs and Their Predators in Mexico

Milkweed contains a chemical that is poisonous to many animals — but not to monarchs. Monarchs eat milkweed when they are caterpillars. The body of the adult monarch contains the poison it ate as a caterpillar. Most of the poison is stored in the cuticle (skin) of the butterfly’s abdomen.

Protected by Poison Milkweed contains a chemical that is poisonous to many animals — but not to monarchs. Monarchs eat milkweed when they are caterpillars. The body of the adult monarch contains the poison it ate as a caterpillar. Most of the poison is stored in the cuticle (skin) of the butterfly’s abdomen.

Dr. Lincoln Brower and his research teams worked for many years to identify the monarch’s enemies in the winter sanctuaries. The scientists discovered three main predators. Each predator has its own method of eating a butterfly, so leaves clues that reveal its identity.

Identifying Enemies Dr. Lincoln Brower and his research teams worked for many years to identify the monarch’s enemies in the winter sanctuaries. The scientists discovered three main predators. Each predator has its own method of eating a butterfly, so leaves clues that reveal its identity.

Mice feed at night. They eat butterflies that they find on the ground. The mice will eat living, dying, and recently dead butterflies. There are 4 species of mice in the sanctuary area, but only one species is known to feed heavily on monarchs. The Black-eared mouse (Peromyscus melanotis) can eat monarchs without being hurt by the poison. One mouse can eat about 37 monarchs a night. Mice leave a pile of wings on the ground.

Mice Eat Monarchs Mice feed at night. They eat butterflies that they find on the ground. The mice will eat living, dying, and recently dead butterflies. There are 4 species of mice in the sanctuary area, but only one species is known to feed heavily on monarchs. The Black-eared mouse (Peromyscus melanotis) can eat monarchs without being hurt by the poison. One mouse can eat about 37 monarchs a night. Mice leave a pile of wings on the ground.

Black-backed Orioles prey on monarchs. The Orioles avoid the poison by not eating the cuticle where most of the toxins are stored. An oriole uses its sharp beak to slice the cuticle open, and then eats the fat inside. The birds feed on monarchs in the morning and evening, when the butterflies are too cold to fly.

Orioles Eat Monarchs Black-backed Orioles prey on monarchs. The Orioles avoid the poison by not eating the cuticle where most of the toxins are stored. An oriole uses its sharp beak to slice the cuticle open, and then eats the fat inside. The birds feed on monarchs in the morning and evening, when the butterflies are too cold to fly.

It's easy to tell when a Black-headed Grosbeak ate a butterfly. The monarch's entire abdomen is missing. Grosbeaks do eat the cuticle. However, they selectively eat male monarchs who have 30% less toxins than females have. There are 37 bird species in the region that eat insects, but only the Oriole and Grosbeak prey on the monarchs.

Grosbeaks Eat Monarchs It's easy to tell when a Black-headed Grosbeak ate a butterfly. The monarch's entire abdomen is missing. Grosbeaks do eat the cuticle. However, they selectively eat male monarchs who have 30% less toxins than females have. There are 37 bird species in the region that eat insects, but only the Oriole and Grosbeak prey on the monarchs.

Millions of monarchs are eaten in Mexico. The ground under a butterfly colony is peppered with wings by the end of the winter. More than 15% of the entire overwintering population is killed by predators in a typical year, according to one estimate.

The Impact of Predators Millions of monarchs are eaten in Mexico. The ground under a butterfly colony is peppered with wings by the end of the winter. More than 15% of the entire overwintering population is killed by predators in a typical year, according to one estimate.

Beyond protection from poison, monarchs have other ways to defend themselves. "Forming a colony is clearly an anti-predator strategy," says Dr. Brower. "The probability of any one butterfly being killed is lower if the individuals aggregate, or group together."

Safety in Numbers Beyond protection from poison, monarchs have other ways to defend themselves. "Forming a colony is clearly an anti-predator strategy," says Dr. Brower. "The probability of any one butterfly being killed is lower if the individuals aggregate, or group together."

Monarchs are vulnerable when they’re on the ground and cold. They can’t crawl until their muscles reach 41°F or fly until they reach 55°F. Monarchs will struggle to fly to the trees — or crawl to the nearest twig — for safety.

Climbing and Flying to Safety Monarchs are vulnerable when they’re on the ground and cold. They can’t crawl until their muscles reach 41°F or fly until they reach 55°F. Monarchs will struggle to fly to the trees — or crawl to the nearest twig — for safety.

When at rest, monarchs are hidden in the forest by their stillness, their silence, and their coloration. The bright orange wings are closed. The wings’ undersides are dull and match the bark and lichens perfectly.

Quiet and Camouflaged When at rest, monarchs are hidden in the forest by their stillness, their silence, and their coloration. The bright orange wings are closed. The wings’ undersides are dull and match the bark and lichens perfectly.

Millions of motionless monarchs appear vulnerable in the colony, but the butterflies can defend themselves powerfully and effectively.

Silent Self-Defense Millions of motionless monarchs appear vulnerable in the colony, but the butterflies can defend themselves powerfully and effectively.

Who are the monarch's predators in Mexico?
How do the butterflies defend themselves?

Journal Page and Weekly Journal

Teaching Suggestions

Monarchs and Moisture at Overwintering Sanctuaries in Mexico

Report Your Sightings


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Next Update March 3, 2016