Two types of flight responsible for birds’ migration

Gossamer clouds drifted across the sky, never dimming the full moon as they passed in front of it. The night air was cool and quiet.

It was a perfect night for birds and for bird-watching.

I almost held my breath as I trained my binoculars upward and waited. Suddenly a small form flitted across my field of vision, silhouetted against the sphere of light cast by the moon. Soon another and another appeared, and just as quickly disappeared into the milky darkness. Migration was under way.

Birds migrate by both day and night. Flight characteristics and the behavior of night air are among the factors that determine which it will be.

Basically, birds engage in variations of two types of flight: powered and soaring, or gliding, flight. A bird that uses powered flight must flap almost continuously to maintain altitude. Birds that soar, or glide, use powered flight to attain a certain altitude; they then set their wings to let the currents carry them. In this way, they are able to travel greater distances before they have to flap again.

Powered flight requires much more energy, and creates more body heat, than does gliding flight. It makes sense, then, for powered fliers to migrate at night, when the air is much calmer and cooler. Songbirds, shorebirds and owls are among the nocturnal migrants.

In contrast, birds that use soaring flight need the turbulent air that exists during the day. Like a sailboat that is dead in the water without wind, a bird in the air without the right kinds of currents sinks quickly, unless it resorts to flapping.

This is where large birds, such as eagles and hawks, are at a disadvantage. Because of their greater body mass, they must expend more energy for powered flight than do smaller birds, such as warblers and others. They are the avian counterparts of SUVs-not very fuel-efficient, especially when compared with a species such as the blackpoll warbler. This tiny bird, which weighs about half an ounce, may migrate over 5,000 miles between its breeding and nonbreeding ranges – using only powered flight.

I thought of these stalwart migrants as I scanned the night sky, wondering if any of the birds I was seeing were blackpolls. More experienced birders, I knew, would be able to identify the birds by their call notes as they passed overhead. I just stood in wonder at this awesome pageant, and, as one author described it, the call notes twinkling down out of the darkened sky were indeed “the voices of stars.”

Chris Corio, a volunteer at Fields Pond Nature Center, can be reached at fieldspond@juno.com.