Wild Georgia: For the hummingbird, fat is beautiful

Most of these tiny birds, however, are pass-throughs: They will hang around a few days to rest and refuel before continuing on to summer nesting grounds as far north as Canada. Later this month, “our” ruby throats -- those that nest here -- will start coming in.

This spring, I have a new perspective on ruby-throated hummingbird migrations because of a scientific study that I recently read. The author, James Hargrove, an associate professor of foods and nutrition at the University of Georgia, says his research on the way hummingbirds store and use energy offers important lessons in treating and preventing obesity, diabetes, heart attack and stroke.

The basic message, Hargrove says, is that fat “should not be regarded with fear and loathing.” Fat, he says, “is part of an ancient, genetically inherited energy regulatory system in most, if not all, animal species.” His study was published in the December 2005 issue of Nutrition Journal.

Hummingbirds, Hargrove says, are superb examples of creatures that use fat for energy and survival. He says untold numbers of ruby-throated hummingbirds this spring will depart from Mexico’s Yucatan Peninsula and fly 500-600 miles nonstop (about 20 hours) over the Gulf of Mexico to reach the shores of Florida and other Gulf Coast states. From there, they will fly hundreds of more miles to reach our backyards.

An adult ruby-throated hummingbird, Georgia's smallest bird, usually weighs 2.8 to 4.5 grams, Hargrove says. A U.S. penny, by comparison, weighs 2.5 grams. Before migration, the tiny birds go into feeding frenzies to double their percent of body fat to about 40 percent. Amazingly, the creatures can get across the gulf on the extra bit of fat that weighs a little more than a gram. Hargrove says it’s the best use of a gram of fat anywhere on Earth.

When not migrating, nectar-feeding hummingbirds get their energy almost entirely from blood sugar and stored carbohydrates, or glycogen. If the birds relied solely on carbohydrates to make their trek over the gulf, they would have to boost their weight by 12 grams -- making them too heavy to become airborne, Hargrove says.

Instead, he says, the ruby throats have the unusual ability to switch quickly from carbohydrates to fats just before departing on migration. Come breeding season, he says, the birds will become lean again “and maintain an extremely accurate energy balance.”

In the sky: The moon is full this weekend. Watch the moon next week “shrink” from full to last quarter by Friday, said astronomer David Dundee at the Tellus Science Museum. Mercury is low in the east just before dawn. Venus, shining brightly, is in the west just after dark and sets about three hours later. Mars rises out of the east a few hours after sunset. Jupiter is very low in the west at dusk. Saturn rises out of the east a few hours after sunset and will appear near the moon Saturday night.

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