There’s a longtime assumption about bird migration: the more years a bird follows the long path from its wintering grounds to its northern breeding territory, the more efficient it becomes. That’s what USGS research wildlife biologist Todd Katzner expected to find in the reams of data he’s collected from the small migratory population of golden eagles, Aquila chrysaetos, in the eastern United States. Over the last decade, Katzner and his collaborators have captured and attached trackers to about 90 eagles, mapping their movements down to the minute. “One of the things we know from other studies and projects is that there’s this typical relationship where as birds get older, their migratory performance improves,” says Katzner. “They learn how to fly more quickly, in a straighter line, and how to use updrafts. That’s established ecology, and works in general for all birds.”
But when undergraduate research fellow Adrian Rus crunched the numbers, the team found something very different. The juveniles flew faster and in straighter lines than the adults, which flew more slowly, taking more circuitous routes. It was essentially the opposite of every other study out there. “It was counterintuitive,” says Katzner, co-author of the study in the journal The Auk: Ornithological Advances. “It means all that experience might not be helping them out like we thought. They were moving slower than the younger inexperienced eagles.”
When the team looked closer at the numbers and correlated the eagles’ movements with the seasons and weather data, they began to understand the discrepancy. It turns out that juvenile eagles, which usually do not begin breeding for five years, and breeding adults face very different pressures. Adults need to make it to their breeding range as soon as possible in order to claim their nests and guard their territories.
This means heading north in February and March when weather conditions aren’t the best. Sometimes that…