Friday, September 7, 2007

Summer's End

Nature Watch
September 7, 2007

By Susan Benson
Director of Education
Cable Natural History Museum

It may still feel like summer, but the birds know better. Migration is peaking for many species, especially broad-wing hawks, and there’s no better place to witness this than up in Duluth at the Hawk Ridge Bird Observatory.

Broad-wings can be seen passing through, with the right winds, in enormous numbers. Like most raptors, they are reluctant to cross large bodies of water. When they migrate south and encounter Lake Superior, the birds naturally veer southwest along the lakeshore.

Broad-wings migrate at high altitudes and seldom stop to hunt during the days of their travels. Because of their dependence on cold-blooded terrestrial prey species, they migrate all the way to Central and even South America. In order to conserve energy on their long journey, they float upward on vertical air currents as high as they can go, and then shoot forward. These air currents, called thermals or updrafts, often are found above rock outcrops, buildings, or parking lots—surfaces that heat the air above them.

When one broad-wing discovers a thermal or updraft, others quickly join it, all swirling upward in a “kettle.” Their migration is one of the most exciting spectacles of the natural world. The Hawk Ridge Bird Observatory’s popular Hawk Weekend is scheduled for September 21-23 this year. In addition to broad-wing hawks, you may see various other raptors, the rarest of which include the peregrine falcon and gyrfalcon.

Use your sense of sound to pick up another sign of summer’s end. You’re probably familiar with that persistent hum in the trees on warm summer afternoons. That’s made by an insect called a cicada, using a pair of drumskin-like organs on the base of its abdomen. These vibrate at a high speed, making the buzzing noise, when the male cicada calls for a mate. However, that mating call drops off in mid-September, so the sound of fall—at least where the cicada is concerned—is actually the sound of silence.

The colorful Monarch butterflies we’ve seen all summer are beginning their migration south now. While most butterflies don’t stray too far from the area where they hatched, the Monarch is an exceptional exception. Every fall, in what is one of the greatest long-distance feats in the natural world, millions of Monarch butterflies embark on a trek that will take them hundreds, and in some cases, thousands of miles to wintering grounds in central Mexico.

North American monarchs are actually members of a group of tropical butterflies called Danaids. Like other tropical butterflies, Monarchs cannot withstand the freezing temperatures we get in this part of the world. But somehow, this creature has developed the capacity to avoid the cold winters by migrating south. Unlike many migrants, Monarchs make the trip without benefit of experience; they only live long enough to make the trip once.

A fall treat in the flower world is the New England aster, which you might find in roadside ditches or meadows, and is a glory to the eye in the fall landscape when most other flowers are gone. The New England aster comes in various shades of purple and can reach a height of four feet.

Become a phenologist! Mark your calendar each day you see a Monarch butterfly, then note the last entry this season. Seek out the last of the season’s wildflowers on your next walk or hike. Find a natural way to bid farewell to another fine summer.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at

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