How whooping crane youngsters learn from their elders
Young whooping cranes on their first migration learn important lessons from older, more experienced birds – such as how to deal with crosswinds that could knock them off track, researchers find.
When it comes to annual migrations, young whooping cranes need old dogs to teach them new tricks – with refresher courses to keep the rookies on track.
That is the implication of a new study exploring the migration patterns of these majestic but endangered birds.
Such studies are important, researchers say, not only in shedding light on how large groups of animals migrate over long distances generally. Successful migration trips also help determine a species' breeding success, since the migrations take place between summer breeding grounds and winter locations where the animals can find food.
Researchers have known for a long time that whooping cranes – long-lived, social birds – probably can't migrate successfully without some kind of early guidance, says Thomas Mueller, an ecologist at the University of Maryland in College Park who led the team conducting the study.
It's also clear that some of the birds' key migration abilities are hard-wired into them – timing, general direction, even the knowledge that they are supposed to migrate.
But which is more important, nature or nurture?
For migrating whooping cranes, the results suggest that the alarm signaling departure time for the long flight between Florida and Wisconsin is built into the birds. But their ability to know where to go once the alarm goes off appears to be the result of training.
And it turns out that it's not just a one-off course in navigation. The most striking result, Dr. Mueller says, showed that learning continued well after cranes' first migrations. The migration groups that strayed the least from a straight-line track were the groups that included the oldest birds.
Over the eight years the study covered, the modeling that the team conducted suggested that the accuracy of a group's flight path should increase by about 5.5 percent for every additional year in the oldest bird's age. For groups that included a 7-year-old bird, that would imply a 38 percent improvement over a group composed entirely of first-timers.
The team's field data suggest that at least for a crane's first five years, it indeed gets better each year at staying on course.
"There's continuous learning over many years of migration" as information is passed from the older birds to the younger birds, Mueller says.
And unlike many other bird species, the instructors aren't parents, they are complete strangers, suggesting a strong cultural component to the activity of learning, says Sarah Converse, an ecologist at the US Geological Survey's Patuxent Wildlife Research Center in Laurel, Md.
Conservation of migrating whooping cranes remains an ambitious work in progress. The birds are native to North America, but by the 1940s, the number of known individuals had plummeted to 14, Dr. Converse explains, in addition to a couple of populations of nonmigrating whooping cranes.
Breeding programs have rebuilt a population that migrates between northern Canada and the Texas Gulf Coast to more than 250 birds. An effort to reestablish an eastern population run by the Whooping Crane Eastern Partnership has been under way for 12 years. The population started at zero and now hosts more than 100 birds.
But these birds all have been born in captivity at the Patuxent Wildlife Research Refuge and a handful of other breeding centers. At about 6 months old, they are shipped to the Necedah National Wildlife Refuge, about 80 miles northwest of Madison, Wis., to join with other whooping cranes summering there.
There, the youngsters are trained to view an ultralight airplane as one of them, following it as it taxis around the summering grounds. When it comes time to head south, the pilot leads a migration group full of rookies. More recently, the effort has used older birds to guide groups as well.
A one-way trip to the wintering grounds at the Chassahowitzka National Wildlife Refuge, about 20 miles south of Chrystal River, Fla., is 1,140 miles.
Mueller's team used exhaustive amounts of data the Whooping Crane Eastern Partnership has gathered on each bird it has raised – including travel routes and genetic information – for its analysis of annual migrations from 2002 to 2009.
The team looked at the ages of each individual crane on each flight over the seven-year period, the age of the oldest member in a group of migrating birds, the size of each migrating group, and the genetic relationships among the birds to tease out the relative importance of innate navigation skills and those acquired by experience.
Then they looked for patterns based on how far from the straight-line path migrating groups strayed.
On average, groups of one-year-olds that included one or more older birds strayed from the straight-line course by an average of about 40 miles as they made their way south. Groups without the older birds strayed by an average of 60 miles, with about 25 percent of these groups wandering off course by nearly 100 miles.
The patterns held regardless of gender or genetic relationships among the birds.
Having an elder or two among the group also appeared to add a bit of weather wisdom. The researchers found that groups of the youngest birds that had an experienced migrator with them tended to hew closer to the straight-line course than did groups of young birds without older members. This suggested that the elders were passing along some street smarts in coping with crosswinds.
Researchers hope the cultural care-of-strangers aspect to training young whooping cranes about migration spills over into the birds' mating efforts. So far, the eastern population isn't as successful in producing offspring as conservationists had hoped.
Scientists are beginning to hunt for the reasons for that, says Converse, after returning from a whooping-crane conservation meeting in Wisconsin where sex and the single whooping crane was a significant concern.