Before the first Europeans landed their ships on the eastern shores of North America, there were more than ten thousand whooping cranes (Grus americana) flying the skies above the American midwest and central Canada. Nearly three centuries of habitat loss and overhunting meant that by 1870, the population of cranes plummeted to just 1500 individuals. In 1941, there were just 21 remaining in the wild and two in captivity. Still, it wasn't until 1967 that the species was declared endangered by the
IUCN U.S. Fish and Wildlife Service. Then, in the fall of 2001, a nonprofit called Operation Migration became involved.
The work to restore the whooping crane to a viable wild population began when the folks at Operation Migration imprinted a handful of infant cranes that hatched in captivity to costumed humans dressed in crane suits. When they were old enough, they were trained to fly behind an ultralight aircraft. It was the very same process that Bill Lishman used to train a group of Canada geese to migrate, which was dramatized in the 1996 movie Fly Away Home starring Jeff Daniels and a very young Anna Paquin.
Several months after leaving the breeding area in Wisconsin's Necedah National Wildlife Refuge, the metal-and-feather caravan landed at the Chassahowitzka National Wildlife Refuge on Florida’s Gulf Coast. The cranes remained there through the winter, and, relying on innate navigation abilities, began their annual flight northward on their own the following spring.
Thanks to the careful work of biologists and conservationists in the intervening years, there is data available on the genetic relationships between each individual whooping crane that now comprise the "eastern migratory population," now numbering around 100 individuals. Combined with telemetry data from small bands placed around the birds' legs, this meant that Thomas Mueller and colleagues from the University of Maryland, Smithsonian Conservation Biology Institute, the US Geological Survey, and the US Department of Fish and Wildlife have been able to finally address a question that for so long has eluded ornithologists: How much of a bird’s migration route - in many species, including this one, spanning thousands of miles - is innate, and how much requires learning?
Mueller and colleagues gathered up 8 years of migration data on reintroduced whooping cranes from the eastern migratory population. They found that flocks that included older, more experienced birds were more likely to stay on course to their summer breeding grounds, while flocks that were comprised mainly of younger birds were more likely to take longer, more meandering, less direct flight paths.
Specifically, the researchers found that the age of the oldest bird in a group improved migratory performance by 5.5% per year of age. This translated to a decrease in just over four kilometers of flight distance per year of age of the oldest bird in a group. In other words, adding a bird one year older than the oldest bird in a flock would make that group's performance 5.5% more accurate and four kilometers shorter.
What this meant in practice was that groups comprised only of one-year-old birds flew nearly thirty extra kilometers compared to groups that contained at least one eight-year-old whooping crane. Seven years of migratory practice therefore resulted in a 38% improvement in migratory performance.
Taking genetic data into account, the researchers discovered that closely related birds did not perform more similarly when it came to migration than unrelated birds, so the variation in performance among cranes was due entirely to experience. But practice alone isn't enough, argue the researchers. It's social learning that counts. "[M]igrations by naïve-to-migration, captive-reared juveniles flying in the absence of experienced individuals would be unlikely to lead to a successful journey," they write, "suggesting that cultural transmission of information" from older, more experienced birds to younger ones is critical.
But what is it, exactly, that this sort of learning provides that might aid the cranes in executing more accurate flight plans?
For one thing, the birds might gain improved spatial memory of visual landmarks. Other research on wild-born whooping cranes showed that they're aided by keeping track of landscape features, both small-scale, such as particular mountains, rivers, or lakes, and larger scale changes in topography.
More experienced birds might also have better knowledge of weather patterns. More than 75% of birds veered off-course by flying too far east. This pattern is consistent with the westerly winds which dominate the American midwest during the time that the cranes fly south.
The importance of these findings to conservation efforts are two-fold. First, and more obvious, strong limits need to remain in place on the hunting of whooping cranes. The migration success of this species is enormously dependent on older, experienced individuals, and those cranes must survive long enough to transmit their acquired knowledge to younger cranes. Luckily, according to the researchers the average age of the whooping crane within the eastern migratory population is increasing, suggesting that their migration accuracy will continue improving over time.
Second, prior research suggests a possible link between migration competence and breeding success. Thus, the researchers argue, "additional [migration] experience may also improve successful reproduction in the wild," providing an additional boost to the species' survival, and moving the species towards reduced reliance on humans in crane costumes and ultralight aircraft.
Mueller, et al. (2013). Social Learning of Migratory Performance. Science 341, 999-1002. Link.
Science podcast, including an interview with the researchers.
For more on spatial navigation and migration patterns:
Header image copyright Operation Migration USA Inc/Joe Duff. Whooping cranes led by ultra-light aircraft copyright Operation Migration USA Inc/Heather Ray. Postage stamp via Wikimedia Commons/Public Domain.