Why Do Birds Migrate?
Last time we asked HOW birds manage the biological miracle of migration. Now that we’ve wrapped our heads around that, the next question is… why? Why do birds make this incredible trip south every fall and, maybe more importantly, if everything is so much better for them down there… why do they come back every year?
EPISODE NOTES
This episode was an example of a genuine blind spot for me when I started working on it. Migration was an obvious topic to cover, but as soon as I started poking around at it I realized both that a) I had always taken it for granted and b) it kind of made no sense. The questions that this episode answers were very much the ones that immediately struck me as soon as I thought about it, and I was so happy that a lot of very smart people had done this work so I could actually get some clarity on them.
I spent a long time trying to figure out how to visualize the shift of birds on an evolutionary time-scale so it made sense. When you’re dealing with both season-to-season temperature changes and broader, planet-wide climatic shifts, it’s hard to hold both in your head at once when thinking about movement that happens twice a year. Hope I didn’t leave anyone more confused than when we started.
TRANSCRIPT
Last time, we looked at how birds manage the biological miracle of migration. And we were left with two questions.
First, how did migration evolve? Evolution happens by increments, not thousands of kilometres at once.
Second, why, if birds head south for better weather and more food… do they come back, like clockwork?
At the heart of the first question is the fact that long-distance migration would have to have evolved gradually, over many many generations. And that’s a clue.
So what would prompt that gradual shift? Well, once you look at the phenomenon on an evolutionary time-scale, all of a sudden the earth gets a lot more dynamic. Its climate cycles between warm and cool periods, and livable territory for birds expands and contracts with it. It lets them inch a little further north - or be pushed a little further south - each year, adding up to a long-distance migration.
And that’s at the heart of two theories on the origin of bird migration that have been duking it out for a century. Known as the ‘northern home’ and ‘southern home’ hypotheses, they’re basically mirror images.
‘Northern home’ says that as the climate cooled, birds living at higher latitudes were forced to go a little further south every winter to survive.
‘Southern home’ says that as the climate warmed, tropical birds were able to expand their range a little further north every summer.
The problem is both theories are equally plausible on their face. So how do you prove one vs the other? In 2014, researchers finally find a way.
They undertake a huge study, gathering the evolutionary lineages of 800 species of migratory songbirds. Then they actually reconstruct their ancestral ranges through an evolutionary tree to see how, and in what direction, they shift over time.
That study seems to prove that the large majority of species follow the northern home hypothesis. They start off in higher latitudes but increasing seasonality pushes them slowly further south in the winter.
Though interestingly, it’s likely some bird species followed the southern home hypothesis, just not nearly as many. So the answer to the century long debate between the two is… sort of, both!
That answers one of our two big questions: how did birds incrementally evolve long-distance migration?
But there’s still the other one. If all these birds migrate south to better conditions, why not just stay there?
The best answer we’ve got is: philopatry. That’s the tendency for animals to return to where they were born and evidently it’s strong enough to drive birds back thousands of kilometres.
So why would that be baked into bird behaviour? Well, evolutionarily, the most important thing you can do is reproduce. And the place you were born represents a rare certainty in a chaotic world. It’s the place you were successfully hatched, and maybe where you’ve had broods of your own. It’s your best bet. And those odds are worth the trip home, no matter how long.
Mind you, philopatry isn’t just a bird thing - many animals exhibit the behaviour, which just confirms how advantageous it must be.
And it answers our second question. It’s worth more to return to the certainty of your successful nesting site than to gamble on living south year-round.
So since we’ve gotten to the bottom of how migration evolved and why birds do it, that just leaves the slight inconvenience of the birds who… don’t.
Almost 80% of Canada’s birds migrate. That leaves 20% who stay here all winter. So, next time: what’s up with them?