Songbirds Use More Energy at Rest Than Aloft
Courtesy of Melissa Bowlin/Princeton University |
If you ever walked to a bus stop in winter and felt toasty until you stood awhile waiting for the bus to come, you can sympathize with the migrating thrush. Weighing just an ounce each, the Swainson’s thrush and the hermit thrush migrate each spring from Central and South America to Canada. During the journey to colder climes, the tiny songbirds fly all night and log an average of 165 miles before stopping to rest for a day or two. How do they keep it up? In June a study of two annual migrations revealed that the flying part is relatively easy—the thrushes use half as much energy as they use to keep warm during stopovers.
A team led by Martin Wikelski of Princeton University intercepted the birds at a stopover habitat in Illinois. Each thrush was weighed, equipped with a radio transmitter, and given an injection of water containing two stable isotopes. Released, the birds were tracked for 24 hours, caught, and weighed again. Then a blood sample was taken. The degree to which the isotopes had been diluted served to quantify the creatures’ metabolic activity.
Each thrush maintained about the same body weight and fat content, but the birds that had rested that day expended nearly as much energy as those that had continued their migration. Over the entire migration, the team projected, the thrushes expended only 29 percent of their energy while airborne. “It turns out that keeping warm is a very expensive activity for these birds, and flying is a relatively cheap activity,” says Ronald Larkin, a wildlife ecologist at the Illinois Natural History Survey and a member of the research team.
—Michael W. Robbins
Monarch Butterflies Bounce Back From Winter Disaster
Monarch butterflies recovered this year from a freakish winter storm that wiped out roughly 500 million of them in Mexico in January 2002. The black-and-gold lepidopterans had just completed their annual migration from Canada and the eastern United States when the storm hit, pelting them with rain and snow and carpeting the ground with their frozen carcasses. Fortunately, last year’s migration was unusually large to begin with—the population varies each year depending on many environmental factors—and enough survived to breed what scientists hope will be an average-size migration of 3 billion in 2003.
That was only one of the monarchs’ many problems. Lincoln Brower, a biology professor at Sweet Briar College in Virginia, says outlaw logging in the monarchs’ wintering grounds cut out nearly half their habitat since scientists had found it in the mid-1970s. At the same time their migration is getting tougher. Widespread herbicide use in Texas and the Midwest is eliminating milkweed, a staple food during their journey each spring and fall. Extinction is not imminent, but Brower worries about another severe storm hitting Mexico in a year when the population has become smaller for other reasons. “It may just be a matter of time before we lose this migratory phenomenon,” he says.
—Michael W. Robbins
