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Migration 🪶

by Dr. Oakleigh Thorne, II

Two adult Cliff Swallows. One by their mud nest under a bridge and one by their mud roosting platform.

Migration is an interest- ing word. The dictionary says that it is “movement from one region or place of habitat to another.” We often talk about people or other animals, such as mammals or birds that have moved from one place to an- other, as “having migrated.” I am particularly interested in bird migration, since I am an ornithologist (a person who studies birds). Some kinds of birds migrate great distances, while others may not migrate at all.

One of my favorite birds is the Cliff Swallow. They nest under sandstone cliffs, but they also like to build their nests under road culverts (where a steam goes under a street or highway) or some- times under the eaves of barns or houses. In our Bird Banding class at Thorne Nature Experience, we usually band several hundred Cliff Swallows every summer. They are colonial nesters. This means that they build their mud nests right next to each other, like a “bird apartment house.” There may be as many as 100 nests un- der one road culvert. Since each nest represents a pair of birds, this means that 200 birds are flying in and out of that culvert! We drop a mist nest across the culvert opening and often catch 20- 30 birds at a time for banding.

Cliff Swallows eat flying insects, so they need to be where there is “perpetual summer.” That means that the ones nesting in the Boulder area must fly south, all the way to southern Brazil and Argentina in South America, during our winter. It’s summer down there then!

Cliff Swallows (and other kinds of swallows) migrate during the day so they can feed on flying insects as the go. They may only migrate 30 to 60 miles in a day, so it takes them a long time to get to South America.

As a bird bander, it is very exciting for me and my students to re-catch a Cliff Swallow that we have banded the previous year. That means it has flown to South America and back and survived! We caught one Cliff Swallow nine years after banding it. That means that it had made nine round trips to South America and was still alive!

Other kinds of birds, such as warblers, vireos, thrushes, flycatchers, and wrens, migrate at night (especially on a clear, moonlit night). They may fly over 100 miles in one night, then land and feed during the day, covering only a few miles. Nighttime is a good time for their migration because in this way they can avoid predators, such as hawks (that migrate during the day).

Some birds simply migrate up or down in altitude. Juncos are a good example. In the summer they nest in the subalpine evergreen forest, just below timberline at an altitude of 9,000 to 10,000 feet. In the winter they come down to the Boulder area, which is only about 5,300 feet in altitude.

House Finches are local birds that do not migrate. They are here all year round. House Sparrows (sometimes called English Sparrows) are a different kind of bird that also does not migrate. These were introduced to America by an Englishman who released a few in Central Park, New York City. They multiplied rapidly and from there they gradually spread across the whole country. His reason for bringing them over here, he said, was “all birds mentioned by Shakespeare should be brought to America.” This also included the European Starling, which also spread across the country.

I think that bird migration is fascinating. I hope that you will enjoy learning about it, too. There is a lot to learn about how and why birds migrate!

Dr. Thorne is founder and honorary president of Thorne Ecological Institute in Boulder. They have helped “connect kids to nature” for more than 55 years. For more information about Thorne Natural Science School classes for children, check or e-mail or call (303) 499-3647.

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