Ask Sam: Which Animals Migrate Backwards?
Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”
Brendan from Tuscon asks: “A couple of years ago in the fall I saw what looked like a migratory flock of birds, based on their V-shape, flying north and it got me thinking: are there any animals that chase colder weather rather than warmer weather like most species?”
There are actually two interesting phenomena in this question, and I’d like to start with the one this caller actually saw: why the heck would you spot birds flying north in the fall? This is called reverse migration: sometimes birds fly the exact opposite direction from the one we would expect them to fly.
There are a couple of explanations and they range from totally reasonable to slightly inexplicable.
For instance, just before large water crossings “lots of juvenile birds, or birds that were very lean and didn’t have enough fat reserves, when they got to that point they would turn back and go back to probably refuel and try again another time,” explains Cecilia Nilsson, a post-doc at Cornell University, who has used tiny-gps tracking backpacks on song-birds to study the moments when birds decide to turn around.
Totally reasonable, right? Migration is hard and sometimes they realize they need to go back to fatten up.
But also, sometimes birds just totally go the wrong direction. In the birding community are sometimes called vagrants. This is when some bird that is supposed to spend the winter in—say—southeast Asia, but winds up in Europe. It can happen for a lot of reasons: a bird migrating over-water might hitchhike on a passing ship and wind up way off-course; they might get blown off-course by a storm; they might actually be flying to a smaller breeding ground that we didn’t know about; but also they can just get confused.
A lot of birds navigate using the ability to sense the earth’s magnetic field, and a bunch of studies have shown that sometimes birds just get flipped and go kinda the wrong direction for no good reason.
But What Species Chase the Cold on Purpose?
First stop in answering this is to point out that animals aren’t chasing warm weather. “They’re looking for the places where there’s the most amount of resources at that particular time,” says Nilsson.
The classic migratory model is that animals are shuttling back and forth between breeding grounds and feeding grounds. So the feeding grounds have got to have food, they don’t have to be warmer, it’s just often the case that they are warmer.
However, in one notable example, the exact opposite is true: out in the ocean, arctic and antarctic waters are incredibly nutrient rich. This means that many whale species (and most famously the humpback whale) are actually seeking out cold waters when they search for their feeding grounds. However, they are still headed to the higher latitudes over the summer, so still not quite what I think this caller had in mind.
The best examples I could find? One might argue that polar bears seek out the cold, since they head out on sea-ice all winter to hunt. And let’s not forget emperor penguins, whose rookeries depend on stable sea-ice, so they lay their eggs in Antarctica in the middle of the winter, and as Morgan Freeman explained to us that leads to a pretty substantial "march" to go find open water to feed. Both of these are species whose migrations don't follow the north-south oscillation you might expect.
So the short answer to your question, Brendan from Tuscon, is that the vast majority of migratory creatures do look like they’re chasing warm weather, but a couple of outliers show us that it’s not really about the weather; it’s about chasing tasty noms.
Sam Evans-Brown, is host of NHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to firstname.lastname@example.org, OR call the hotline, 1-844-GO-OTTER.