Bird migration: Why does it happen?

By Sally Zimmerman, MNA Intern

The Kirtland's warbler migrates from Michigan to the Bahamas. Photo by Cindy Mead.

The Kirtland’s warbler migrates from Michigan to the Bahamas. Photo by Cindy Mead.

Many people know that birds migrate south when it starts to get cold and that they can be seen flying in a V-shape as they leave, but not much more than that. The large-scale movement of birds as the weather changes is imperative to their survival and is stamped into their genetic makeup.

The migration of birds happens because there is a difference between their breeding home and nonbreeding home. Their breeding home is where they spend the summer months building nests and laying their eggs. The nonbreeding home is where they spend the winter months. The two main resources birds look for when they migrate are food and nesting locations. They typically move from areas of decreasing resources to areas of increasing resources.

In the northern hemisphere, birds migrate north in the spring. They thrive on insect populations and budding plants. There are also plentiful nesting locations for the birds to build their homes. When it starts to get cold, the insect populations decrease and the plants are no longer budding, so the birds migrate south.

Birds can cover a span of thousands of miles in their migration. They often travel the same route with little deviation from year to year. Researchers believe birds use a variety of techniques to navigate, including navigation by the stars, changes in the earth’s magnetic field and smell. Birds have important stopover locations in their migration pathways that provide food supplies.

Raptors such as hawks, eagles, vultures and falcons use landmasses such as the Keweenaw Peninsula as a stopover point in their migration. This shortens their flying time over Lake Superior and the updrafts carry the birds so they exert minimal energy.

The Kirtland’s warbler is another bird species that uses Michigan as one of its homes. The bird arrives in Michigan anywhere from late April to early May and constructs its nest between mid-May and early June. Egg-laying occurs from late May to mid-July. Incubation takes about 15 days with fledging occurring nine days afterward. The bird migrates from northern Michigan where it nests exclusively in young jack-pine forests to the Bahamas in the winter, where they nest in pine woods, broad-leafed scrub and Australian pipe. The Kirtland’s warbler makes the journey to the Bahamas sometime between August and October without any stops, following a narrow band that crosses Ohio, West Virginia, Virginia, North Carolina and South Carolina.

Recently, there have been increasing migration hazards for birds as more communication towers and tall buildings are being constructed. The lights of tall buildings attract birds and are accountable for millions of collision deaths each year. Food supplies, weather and exposure to predators can also affect the journey. Organizations like MNA have been working to protect these important flyway areas for migratory birds. You can read about MNA’s efforts to discourage the building of a cell phone tower on Brockway Mountain in the Fall 2013 issue of Michigan Nature magazine

Scientists use banding and satellite tracking to study migration, with the hope of locating important stopover and winter spots. The goal is to make steps toward protecting and saving these important locations.

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