How Do Birds Migrate?
Happy Canada Day… week! And we’re back! With the start of a new series. We can sometimes take bird migration for granted, but it’s a pretty remarkable feat, and one we’ll be exploring over the next four weeks. To start us off, a simple question: how exactly do they do it?
EPISODE NOTES
I took nearly a year off from this project because of a sudden, unexpected glut of work I had to prioritize so this is the grand return. It doesn’t quite feel like that though, since I had this whole series at least written and recorded in July 2023. It was stuck in a state of half-doneness until late June of 2024.
No one can seem to agree on that ‘40% of all birds migrate’ number - it was the most consistent one, but there’s a bracket around it. It might be counting different thresholds of distance as a ‘true’ migration, but I was never able to get real clarity on that.
Likewise, arctic terns and their marathon migrations - no one disputes that they hold the world record, but the actual distance gets muddied by what route they take, whether you’re counting there and back, etc. 70,000 km (there-and-back) is both a well-cited number and one of the most conservative I found, so it felt like a safe bet.
I mention Wheaters in the episode as an example of a crazy migration - from Canada’s eastern arctic across the Atlantic and down to western Africa. Another population of Wheaters makes an even more extreme trip, across the Bering Strait, Russia and Asia to land in eastern Africa - but they start in Alaska and I wanted to stick with Canadian birds. In defence of our own Wheaters, their shorter route also means they have to fly for 3-4 days straight as they cross the Atlantic.
TRANSCRIPT
Zugunruhe. That’s a German word we don’t quite have an analogue for. It describes the behaviour of captive birds when, in the spring and fall, they start to repeatedly flutter to one side of their cage. Even in captivity, they’re restlessly trying to migrate.
Migration’s a fact of life for a lot of birds - about 40% of all species. And in regions with more seasonality like here in Canada, that percentage is much higher. Of our 450ish species, around 350, or 78%, migrate.
At its most basic, migration means consistent seasonal shifts in location. In Canada, most birds head south to escape harsh winter conditions and return in the spring to breed. But migration can have a lot of variation, even within species.
Short-range migrants might just move up and down a mountainside. Medium-range migrants travel a few hundred kilometers. Long-range migrants can cross continents.
Wheaters, for example, breed in the eastern Canadian arctic but travel 7500km and cross the Atlantic Ocean to overwinter in Western Africa.
Canada’s also home to the world champions of migration: arctic terns, who can travel 70,000 km in a year.
Before we knew about migration, we came up with all kinds of wild theories to explain the seasonal absence of certain birds. That migratory species had transformed into other birds, that they buried themselves in mud for the winter, or that they went to the moon. Those all seemed more plausible than a fragile little bird flying thousands of kilometers.
But they’ve been doing it for hundreds of thousands of years. In a 1991 experiment, scientists cross-bred blackcaps from two populations with very different migration lengths and directions. The offspring attempted a migration that was in between in both. Migration is written in their genes. And can result in dramatic transformation.
As the temperature starts to drop, food becomes scarce and days shorten, photoreceptors in the birds’ brains trigger big hormonal changes.
They molt to grow stronger feathers for the long flight. They eat voraciously. Some can double their body weight building up the fat reserves they’ll need. To make room, they can shrink some of their internal organs while increasing the size of their heart to promote blood flow.
That’s all before they take off. Once on these marathon flights they have even more specialized adaptations. Some shift to sleeping in 9-second increments, while others employ unihemispheric slow wave sleep: allowing half their brain to sleep at a time while the other half stays alert, letting them stay airborne almost indefinitely if they need to.
And they’ve perfected the routes as well. Some will take different paths north and south, to take advantage of different seasonal weather patterns and food sources.
So birds have evolved into migration machines. But that still leaves a couple questions.
First: how did they evolve this way? Evolution is an incremental process. No bird’s gonna suddenly decide to go on a ten-thousand-kilometer jaunt in one generation.
Second: there’s still the question of why? Not so much why they go - escaping harsh winters makes sense. But if everything’s better down south… why come back?
So next time, we’ll aim to answer those questions. Because it turns out they have lead to a century-long clash over the origin of bird migration.