Migration

The days are getting shorter and cooler. Leaves are starting to turn and fall. Winter is approaching, and many animals are have already begun their seasonal migration. While some hibernate to avoid unfavorable seasonal habitat conditions, others move temporarily to more suitable habitat.

Migrations occur across all different animal classes: birds do it, and so do mammals, fish, insects, reptiles, amphibians, and crustaceans. Even some people do it. Florida and Arizona get plenty of snowbirds from up north every winter. More traditionally, nomadic tribes still exist in Africa and Asia living as they have for centuries.

We’re an ecology blog, not an anthropology blog so let’s forget about nomads for a minute. Perhaps the most visible migrations here in America are those of birds and monarch butterflies. Birds migrate to and from pretty much everywhere. Some stay within North America when migrating while others fly from pole to pole. Others, of course, are somewhere in between.

We’ll use the Baltimore oriole as an example of avian migration. They arrive in the eastern US from Central and northern South America in April and May. This is their breeding ground. They begin nesting right away and eggs usually hatch in June. Southern migration begins as early as July and has peaked by September.

Baltimore oriole (NPS)

Monarch butterflies journey to Mexico in a multi-generational migration. Northern monarchs go south to the oyamel fir trees in the mountains of central Mexico. Millions of them cover the trees. After winter, they head north again, laying eggs on milkweed plants. Caterpillars hatch in a few days and eat the plants before forming a chrysalis. A new butterfly emerges and continues the northward journey, stopping at milkweed to lay eggs and start the hatching and migrating process over again. The northernmost monarchs are able to return to Mexico in a single journey.

Monarch butterfly

The pronghorn in Wyoming migrates vertically. It moves from summer grounds in Grand Teton National Park about 150 miles south to lower elevation in the sagebrush habitats of the Green River area. This is North America’s longest mammal migration.

Pronghorn

Not all migration is seasonal. Zooplankton in the ocean migrate vertically through the water column on a daily basis. At night the move toward the surface, and by sunrise they are headed back to the briny deep. While a journey of hundreds or thousands of feet might not seem like much to you, keep in mind that these tiny critters probably can’t see as far as the other end of a yardstick.

Zooplankton (Nature International Journal of Science)

Fall migration is happening now. Who is moving through your neck of the woods?

This week's information comes from:

Journey North (Baltimore oriole)

National Geographic (monarch butterfly)

Wildlife Conservation Society (pronghorn)

Nature International Journal of Science (zooplankton)