Every year, some 50 billion birds take to the air for their seasonal migrations. They may go 500 kilometers in a day and a few even travel from pole to pole. But how do they know when, where, and how far to fly? Although some of the answer lies in their DNA, nobody knew which genes or how they worked. Now ornithologists have pinned down one of those genes, and strange as it may sound, the length of that gene influences the length of the flights.
"If we understand the genetics underlying migratory behavior, we can understand more about how and why migration evolves," says Chris Guglielmo, who studies bird migration at the University of Western Ontario in Canada. "We may also be better able to understand how quickly migration can disappear in response to climate change."
As the moment for migration approaches, birds bulk up, adding muscle and fat. They hop and flap restlessly at night, shifting their internal clocks in anticipation of nighttime flights. Breeding experiments have shown that these shifts have a genetic basis, as do the timing, amount, and intensity of flights.
Since the 1970s, ornithologists at the Max Planck Institute for Ornithology in Starnberg, Germany, have studied European blackcaps (Sylvia atricapilla), a common warbler in Europe, which typically head to the Mediterranean for the winter. Some blackcaps had established a new wintering area in the past few decades. The researchers wanted to know the genetic basis for the change.
To hunt down a migration gene, Jakob Mueller and Bart Kempenaers of Max Planck, along with Francisco Pulido, now at the Complutense University of Madrid in Spain, selected six genes to evaluate. They picked ones known to influence how active a bird is at night or how much it tends to hop from branch to branch as it explores its environment.
The researchers evaluated 14 populations of blackcaps ranging from western Russia, through Europe, south to Africa. These populations varied in their inclinations to migrate. Blackcaps in Cape Verde, for example, never leave home, whereas those in Russia travel more than 3500 kilometers. Others fly shorter distances seasonally. The Max Planck team had previously captured these birds and taken blood samples, so studying their DNA was a snap.
When Mueller, Kempenaers, and Pulido compared the genes with the behavior, they found a link in just one gene, called ADCYAP1, as they report online today in the Proceedings of the Royal Society B. Different versions of the gene have different numbers of extra copies of a bit of DNA—called a two-base repeat—stuck on the gene's far end. The researchers found that the length of the gene was correlated with how much the birds hopped and flapped around their cages at night.
Such nighttime restlessness shows how eager a bird is to migrate, says Mueller. The more fidgety birds had more copies of the two-base repeat than calmer birds. Looking at the populations as a whole, the researchers found that groups that stayed put tended to have shorter version of the gene, whereas long-distance migrants tended to have longer versions. "We found a continuous relationship between gene [version] length and behavior," says Mueller. This gene specifies a peptide in the brain. Among other functions, the peptide influences daily rhythms and affects energy use—increasing body temperature, metabolic rate, and fat usage. These sorts of changes occur as a bird gets ready to migrate, Mueller points out.
Staffan Bensch, an ornithologist at Lund University in Sweden, is not convinced that this gene, not another nearby, is what's causing the variation in migratory behavior. But Guglielmo says the finding is significant. "It is the first demonstration," he says, "of a specific gene that is important for the expression of migratory behavior in birds."
It won't be the only one, says Mueller. The particular version of ADCYAP1 determines only about 3% of the migratory behavior. Dozens or even hundreds of gene variants might also be involved. And genes don't tell the whole story; the environment also influences migration. The study of the genetics of migration, clearly, is just getting off the ground.