We’ve known for more than a decade that the re-establishment of a migratory flock of Whooping Cranes in the eastern states would not be possible without the pilots of Operation Migration. They’re the ones who guide the birds south using ultralight aircraft.
What we didn’t know, until now, is that the birds teach each other the subsequent migration routes — north in spring and south in fall — and that it’s experience, not necessarily genetic relatedness or gender, that makes a good teacher.
According to a study embargoed until this afternoon and published this week in the journal Science, older cranes help younger birds learn the migration routes. Only one experienced bird is sufficient to keep a north- or southbound group on track. And the cranes get better at migrating with age.
The study is featured on the cover of the August 30 issue of Science, shown here.
Researchers analyzed data from all the ultralight-trained birds’ spring and fall migrations from 2002 to 2009 and found that migratory groups that included a seven-year-old adult deviated 38 percent less from a straight-line path than groups composed only of younger cranes.
One-year-old birds that did not follow older birds veered 60 miles (97 kilometers) from a straight flight path, on average. When the one-year-old cranes traveled with older birds, the average deviation was less than 40 miles (64 kilometers).
Individual Whoopers’ ability to stick to the route increased steadily each year up to about age 5 and remained roughly constant from then on.
Many migration studies examine short-lived species like songbirds, or compare young birds to older birds, says biologist Thomas Mueller of the University of Maryland, an expert on animal migration and the study’s lead author. “Here we could look over the course of the individual animals’ lifetimes, and show that learning takes place over many years.”
The intensive Whooper reintroduction project makes the new study unique, he adds. Each bird is hatched in captivity, so biologists know its family history and how much it is related to other birds in the flock. All of the birds in the population receive the same training with ultralights in their first year, and they are tracked via satellite transmitters, radio telemetry, and on-the-ground observers. The result is a record of the movements of individual birds over several years, all with known parentage and the same upbringing.
“This is a globally unique data set in which we can control for genetics and test for the effect of experience,” says William Fagan, a biology professor at the University of Maryland and a co-author of the paper. “And it gives us an indication of just how important this kind of socially learned behavior is.”
The eight years of data showed that neither genetic relatedness nor gender had any effect on the cranes’ tendency to stay on the shortest migratory route. They were surprised to find that the migrating groups’ size also made no difference.
“Many biologists,” says Fagan, “would have expected to find a strong effect of group size, with input from more birds’ brains leading to improved navigation, but we didn’t see that effect.”
The researchers hypothesize that older birds are better at recognizing landmarks and coping with bad weather. Whoopers tended to stray farther from their straight course during fall migration, possibly due to stronger autumn winds, Mueller notes.
He also suggests the study offers an unexpected insight into the cranes’ relative lack of breeding success. Based on the team’s findings, “we need to take into consideration that these birds may also reproduce more successfully as they age,” he says.
“These birds’ behaviors have evolved over millennia,” adds co-author Sarah Converse, a research ecologist with the U.S. Geological Survey. “Managers here are trying to restore a culture, that is, the knowledge that these birds accumulate over time. We need to give these birds the time and the opportunity to get the breeding right. We might need to be a little bit patient.” — Matt Mendenhall, Managing Editor