The epic nature of animal migration is astounding to imagine: individuals crossing huge distances instinctively. Every major branch of the animal kingdom contains animals that migrate, including fish and crustaceans, amphibians and reptiles, insects and mammals.
Curiously, little is known about how exactly animals know where to go, and how to get there.
One perplexing example is the monarch butterfly, whose lifespan is shorter than the time it takes to complete the migration. No one butterfly ever makes the complete journey. It isn’t possible for young butterflies to simply follow their elders who have taken the trip and already know the way.
A migrating monarch butterfly will never complete the full journey
To expand our knowledge on this mystery, researchers Kira Delmore and Darren Irwin at the University of British Columbia tracked the migration routes of two closely related groups of songbirds called Swainson’s thrushes. These groups of birds meet and interbreed in the coastal mountains northeast of Vancouver each year before heading south for the winter, but each takes a very different route to get there: one group hugs the coastline, while the other travels inland.
But what happens to the hybrid children of parents who would take different routes south?
To track the birds, the study outfitted the thrushes and their hybrids with light-level geolocators that recorded their migration paths. The study also collected DNA samples to look for patterns.
Thrushes were tracked with coin-sized light-level geolocator tracking devices
Credit: Kira Delmore, UBC
It turns out that the hybrid birds were likely to take a completely different route than their parents, splitting the difference and taking a path between. These routes took the hybrid birds through deserts and mountains, making their trips more dangerous. This may be a factor in keeping the two groups of thrushes separate: even though they can interbreed, hybrid offspring follow more hazardous migration paths.
The genetic analysis identified a small cluster of genes that are linked to the changes in migration patterns; other smaller studies have found genes in this cluster to be important to migration in other organisms. The cluster is involved in the birds’ circadian, nervous, and cell signalling systems.