It's Not Just Your Grandparents—Older Eagles Leave Early Too
Kids these days have it so easy. Scientists say young golden eagles display a laissez-faire attitude toward migration, leaving later and waiting for good weather before setting out, while their older relatives perform the avian equivalent of walking to school uphill both ways in the snow. The researchers published their findings in The Auk: Ornithological Advances.
With beautiful russet feathers, a wingspan up to 7.5 feet, and a sensational diving speed, the golden eagle (Aquila chrysaetos) is an impressive sight at any age. They're chiller than their notoriously daring Scandinavian cousins, which carry off bear cubs [PDF] and attack reindeer and wolves.
A. chrysaetos populations in eastern North America spend their summers mating and raising chicks in Quebec, Ontario, and Labrador, then head south to the Appalachian Mountains for the winter.
Ornithologists wondered how the birds’ lengthy trips were affected by environmental conditions like sunlight and wind speed. To find out, they captured 90 eagles, outfitted each one with a light backpack containing a GPS tracker, recorded the bird’s approximate age, then let them go. The birds and their backpacks flew back and forth for years, transmitting information all the while.
Combining the GPS data with local meteorological records revealed some interesting patterns. All the birds flew both farther and faster when tailwinds were in their favor. But when the weather went bad, a split emerged along age lines. Older birds hit the air anyway to make sure they arrived at their nesting sites in time to breed. But younger birds apparently felt no need to battle the elements and waited until clearer days arrived.
Paper co-author Todd Katzner of the U.S. Geological Survey says the two groups’ divergent travel styles are a product of their different needs. Old birds are slower and have fewer breeding seasons remaining; they don’t have time to waste hanging around like the whippersnappers do, so they get going right away. “Younger eagles just need to survive the summer,” Katzner said in a press statement, “so they can be choosy about when they travel north and only migrate when conditions are really ideal for fast soaring flight.”