Thrush with light-level geolocator tracking device

How Do Birds Know Which Way to Fly South?

Just how exactly do birds know which way to fly south for winter? Tracking and genetic analysis provide us with some fascinating answers.


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.

monarch butterfly

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

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.

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Karyn Ho is a science animator and engineer who thrives at the interface between science, engineering, medicine, and art. She earned her MScBMC (biomedical communications) and PhD (chemical engineering and biomedical engineering) at the University of Toronto. Karyn is passionate about using cutting edge discoveries to create dynamic stories as a way of supporting innovation, collaboration, education, and informed decision making. By translating knowledge into narratives, her vision is to captivate people, spark their curiosity, and motivate them to share what they learned.