Three thousand years ago, Aristotle observed that cranes travel every winter from the Eurasian steppes to the headwaters of the Nile. It is the first surviving observation of bird migration, and ever since naturalists have been investigating the complex, fascinating puzzle of how birds choose and navigate their annual routes. A new study from scientists from the University of British Columbia reveals a specific genetic basis for the route that two groups of thrushes take during their yearly wanderings, and the findings have strong implications for how birds have evolved into different species.
Science has a pretty good idea of how birds navigate during migration. It’s a complicated combination of sensing the earth’s magnetic fields, the position of the sun and length of day, and astonishingly accurate mental maps consisting of visual and even olfactory clues. Individual birds tend to get better at migration the more times that they make the trip, but even a recently fledged animal still has the instinct and capability to make the journey perfectly the very first time.
So how is it that a bird is born with the knowledge of continent-spanning journey? Scientists have long suspected a genetic component. To identify that component, zoologist Kira Delmore and her colleagues studied two separate groups of Swainson's Thrush; the two groups have separate migration paths and winter roosting sites. Occasionally birds from these two groups will interbreed and the hybrids, instead of using the migration path of one parent or the other, use an intermediary path that splits the difference between the routes their parents would take. By comparing the genetics of specific birds (both hybrid and non-hybrid) to the paths they took across the continent, the scientists managed to isolate a cluster of approximately 60 genes that mostly control neural development, circadian rhythm, and metabolism. This cluster seems to act as a genetic “map” and accounts for the difference in routes.
This discovery would be interesting enough on its own, but Delmore and her group noticed something else: The middle route that the hybrid birds were taking was a lot less friendly than the two original routes. It crossed deserts, mountains, and other hostile terrain, and the hybrids were less likely to survive the trip. This explains why, even though they occasionally interbreed, the two groups of thrushes have managed to stay genetically distinct from each other—the hybrids are more likely to die en route while their non-hybrid cousins have a much easier trans-continental journey. The non-hybrid birds, being more likely to survive, are also more likely to breed and thus their genes persist in the population while the hybrid genes don’t, a process that can eventually lead to speciation.
Currently the two adjacent populations of Swainson’s Thrush are still the same species, but as each group continues to breed in what is effectively reproductive isolation from each other, tiny mutations will accumulate in each population until they’re different enough so that they become sister species. By monitoring these two specific populations over time, we can gain significant insight into how migratory birds in general developed such amazing diversity in the first place.