Monarch Overwintering


Do monarchs need to eat during the winter?

Monarchs that migrate to Mexico or the coast of California do not rely on nectar to be available during the overwintering months. They consume as much as possible during their migrations, and actually gain weight along the way. This allows them to store enough lipids, or fat reserves, to survive the winter without eating. They can do this because the cooler temperatures at the overwintering sites allow them to stay relatively inactive during the winter, which helps them to save energy. They do need water during the winter, however, and visit streams or other water sources near the overwintering colonies. 

What kinds of trees do monarchs overwinter on in California?

If you’ve ever been to the coast of California, you’ll have noticed that most of the trees there are eucalyptus trees, and this is the main tree that the monarchs use. They also use pine trees where these are growing near the coast, and just about any other kind of tree that is growing in the right locations.

When do monarchs arrive in Mexico and when do they leave?

Typically, monarchs arrive in Mexico around the same time as the Day of the Dead in early November. They spend the winter in the Mexican overwintering sites and then the colonies start to break up and migrate back towards the U.S. starting in March. 

How many overwintering sites are there in Mexico and California?

This numer varies from one year to the next, usually depending on the size of the monarch population and the quality of the habitat. 

In Mexico, monarchs return to the Monarch Butterfly Biosphere Reserve, a large area of land where the overwintering sites are located. Within this reserve there are typically between 5 and 14 different sties known to be used by monarchs. Not every site is occupied by monarchs every year, but monarchs are able to find the same specific sites each year, even though they have never been there. 

In California, there are many more overwintering sites than are in Mexico. Hundreds of sites along the California coast have historically been the home to overwintering monarchs, but these sites are not as consistent as those in Mexico, nor are they as large. Some of the larger sites, such as Pismo Beach Grove, or Pacific Grove typically have monarchs each year. Some sites in California are considered transitional sites, as they do not have monarchs present for the entire overwintering period. There is more movement between sites in California, whereas this does not occur in Mexico that we know of. In 2013-14, 162 sites were visited and monitored during the overwintering season. 

How many monarchs are in one overwintering colony in Mexico?

Each of the site that monarchs occupy in the Monarch Butterfly Biosphere Reserve are different in size. Scientists measure the size of each colony or overwintering site by measuring the area of the trees/land that is occupied by monarchs. With these measurements, they use an estimate of between 10-50 million monarchs per hectare of land occupied. 

How do they count the butterflies at overwintering sites?

In California, volunteers help to count the butterflies at each location by participating in the Western Monarch Thanksgiving Count. Groups of volunteers go out with a leader and estimate the number of monarchs in each cluster by counting individual butterflies. This is possible because there are many fewer butterflies at the California sites (as opposed to the Mexico sites). 

In Mexico, it would be impossible to count each and every butterfly at a particular overwintering site. Researchers actually measure the area occupied by monarchs and then using this measurement, estimate the number of individuals based on an estimate of between 10 and 50 million monarchs per hectare. 

What are the characteristics of an overwintering site in Mexico?

Monarchs in Mexico overwinter in high elevation oyamel fir forests in the Transvolcanic mountain range of central Mexico. Typically the locations that they find favorable are about 2 miles above sea level, or about 3,000 meters. These sites are typically found on moderately steep south-soutwest facing slopes and are usually in close proximity to a stream or source of water. The temperature at these locations is fairly cool, typically just a bit above freezing. It can get warmer during nice days, but nights remain fairly cool. 

What do monarchs do at the overwintering sites?

Monarchs need to consume enough milkweed as caterpillars and nectar as adults before reaching the overwintering sites so that they do not need to eat to survive while they are overwintering. There is not enough nectar available at the overwintering sites to sustain that many monarchs for the entire winter, but there is typically some nectaring behavior during the winter months. 

Monarchs do need water during the winter, so on warm days they find streams or water from dew or fog/clouds on the mountain. They need moisture so that they don’t become dehydrated. 

They remain fairly inactive clustering in trees to conserve energy/lipid reserves, but do fly some on warm days and warm their wings in the sun. Towards the end of the season, monarchs may also begin mating in Mexico as they prepare for the journey northward.  

Predation by birds also happens at overwintering sites, but monarchs cannot do much to avoid this. 

Are there predators in Mexico?

Yes, there are two bird species that can eat monarchs in overwintering sites in Mexico. They are the black-headed grosbeak and the black-backed oriole. There are also about 5 species of mice that prey on the monarchs on the ground.