2013 Monarch Butterfly Migration

Monarch butterflies feeding and preparing for a night roost in Dallas Texas on the evening of October 8, 2013

It’s a sure sign of autumn and a rare sight for Dallas, a massing of over a hundred migrating Monarch butterflies in a single tree. Something one would usually travel a thousand miles away and into the mountains of Mexico to catch a glimpse of up close. These butterflies are actively feeding on nectar and preparing a colonial overnight roost in a tree.

Monarch Butterflies feeding on the flowers of a Roosevelt Willow Baccharis neglecta

In all the world, no butterflies migrate like the Monarch butterflies of North America. They travel much farther than all other tropical butterflies, up to four thousand miles. They are the only butterfly species to make such a long, two way migration every year. Amazingly, they fly in masses to the same winter roosts, often to the exact same trees. Their migration is more the type we expect from birds or whales. However, unlike birds and whales, individuals only make the round-trip once. It is their children’s grandchildren that return south the following fall. The Monarchs are the only butterfly that migrates both north and south as the birds do regularly, but no individual makes the entire round trip, because the migration period spans the life of three to four generations of the butterfly.

The Monarch butterfly (scientific name: Danaus plexippus) is perhaps the best known of all North American butterflies. It is easily recognizable by its bright orange-red wings, with black veins and white spots along the edges. The Monarch butterfly is famous for its southward migration from Canada to Mexico and the northward return back through the Great Plains to Canada in summer. Every fall, millions of these butterflies fly west to their wintering grounds in California and Mexico, covering the trees there with their bright shimmering wings.

 As fall approaches non-reproductive monarchs are born. These are the butterflies that will migrate south. They will not reproduce until the following spring. These late summer monarchs will travel hundreds and even thousands of miles to their winter grounds in Mexico and California.  They store fat in their abdomens that will help them make the long trip south and will help them survive the winter. During their five months in Mexico from November to May, monarchs remain mostly inactive. They will remain perfectly still hour-after-hour and day-after-day. They live off of the stored fat they gained during their fall migration.

The plant they are feeding from in the photos is known as Roosevelt Willow or Roosevelt Weed Baccharis neglecta . It’s a tall shrub with many willow-like branches covered with very dark green, linear leaves. After warm rains in late summer it produces a profusion of creamy white flower clusters which are followed by silvery plumed seeds that cover the plant with a white cloud. It grows from North Carolina to Arizona, and throughout Texas. Roosevelt Willow/Weed is one of the first plants to invade abandoned fields, roadsides and disturbed habitats. It is extremely drought tolerant, accepting wet or dry sites, and can grow in soils high in salt. The historical references of its common names purportedly come from the fact that after the great Dust Bowl, it was planted as a fast and easy way to revegetate the severely damaged soil.

Monarch Migration South Through Texas

The Monarch migration usually starts around October each year, but can start earlier if the weather turns cold sooner. They travel between 1,500 and 3,800 miles or more from Canada to central Mexican forests where the climate is warm. If the monarch lives in the Eastern states, usually east of the Rocky Mountains, it will migrate to Mexico and hibernate in Oyamel fir trees. If the monarch butterfly lives west of the Rocky Mountains, it will hibernate in and around Pacific Grove, California in eucalyptus trees.

Monarch butterflies use the very same trees each and every year when they migrate, which seems odd because they aren’t the same butterflies that were there last year.  How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research. Some believe the flight pattern is inherited. Other researches indicate the butterflies navigate using a combination of the position of the sun in the sky and the earth’s magnetic field for orientation.

The Monarch butterflies migrating through Texas all seem to focus and funnel into a 50 mile gap between Del Rio and Eagle Pass along the US-Mexico Border. Here they have a clear route through mountain passes to the Mexican Interior and highlands.

When they first arrive at their winter locations in November monarchs gather into clusters in the trees. These butterflies congregate into colonies, clustering onto pine and evergreen trees. In many cases, they are so thick that the trees turn orange in color and branches sag from the weight. It’s a remarkable sight that attracts scores of tourists. 

By December and January, when the weather is at its coldest, the monarchs will be tightly packed into dense clusters of hundreds or even thousands of butterflies. By mid-February these clusters of butterflies begin to break up and the monarchs will begin to gather nectar. In the spring they will reproduce and their offspring will make the return trip to the north.

 For many years, people puzzled where the millions of Monarchs that spend the summers in Canada disappear to in winter. Then in 1937, a Canadian zoologist named F. A. Urquhart started tracking the trails of the butterflies by tagging the wings of thousands of individual Monarchs. Nearly 40 years later, and with the help of thousands of volunteers across the country, Urquhart located the first known wintering refuge on a mountaintop in Michoacán, Mexico, more than 4,000 miles from the starting point of their migration. The area is now a World Heritage Site known as the Monarch Butterfly Biosphere Reserve. There are dozen such sites in Mexico and they are protected as ecological preserves by the Mexican government.

Night Roosting

 Monarchs only travel during the day and need to find a roost at night. Monarchs gather close together during the cool autumn evenings. Roost sites are important to the monarch migration. Many of these locations are used year after year. Often pine, fir and cedar trees are chosen for roosting. These trees have thick canopies that moderate the temperature and humidity at the roost site. In the mornings, monarchs bask in the sunlight to warm themselves.

The Monarchs seen here are consuming nectar from a blooming shrub. It is believed that the Monarchs might be following what biologists call a “nectar corridor” for food.

Nectar corridors are a series of habitat patches containing plants that flower at the appropriate times during the spring and fall migrations. These patches provide stopping-off points for the migrating butterflies to refuel and continue their journey. Having these islands of nectar sources is particularly important within large areas of urban and agricultural development. The discontinuous patches of nectar sources are “corridors” that monarchs will follow, like stepping-stones across a stream to complete their migration.

Monarchs and Milkweed

Many butterflies have a single plant required as a food source for their larval form called a host plant. Milkweed is the host plant for the monarch butterfly. Without milkweed, the larva would not be able to develop into a butterfly.

The larvae and the butterflies retain poisonous glycosides from their larval host plant, the milkweed, so they become distasteful to potential predators. These milkweed butterflies (Monarch, Queen, Soldier) eat only milkweeds as larvae. This highly effective defense strategy shields them against almost all predators that soon learn to avoid these species after attempting to eat them.

Milkweed contains a a variety of chemical compounds that make monarch caterpillars poisonous to potential predators. Milkweeds contain a cardiac poison that is poisonous to most vertebrates but does not damage the monarch caterpillar. Some milkweed species have higher levels of these toxins than others.

North Texans can attract Monarchs to their backyards by planting milkweed as a host for Monarch eggs and larvae. Easy to grow here in Dallas.

Some other species of Lepidoptera (butterflies and moths) travel long distances, but they generally go in one direction only, often following food. This one-way movement is properly called emigration. In tropical lands, butterflies do migrate back and forth as the seasons change. As it stands other butterfly flies further, attracts more attention or more curious onlookers than that of the Monarch.

Dispersal of monarchs into a nearby tree after sunset

As the sun sets in the cooling autumn air, the Monarchs head towards a nearby large tree to roost for the night. This is mostly for protection from predators like bats who might not see the bright orange and black coloration, the tell tale of the bad-tasting and poisonous Monarch. From the trees beyond the night crew of animals start up their evening calls. Ready to hunt under a rising crescent moon.

Male Great Horned Owl in a Texas Red Oak, Dallas, Texas, October 2013