When Mom and Dad Have Different Migratory Routes, Kids Fly Right Down the Middle

By Elizabeth Preston | July 29, 2014 11:02 am

swainson's thrush

It sounds like the setup to a bad joke told by zoologists: What do you get when you cross a bird that always flies to the west with one that always flies east? But the punch line is weirder than you’d guess. Birds’ migratory routes are partly coded into their DNA. A baby that inherits genes for two different routes doesn’t commit to either path. Instead it bounces between them and may take a wild zigzag straight through the middle—even if that route is perilous.

Laboratory tests in the past have hinted that this might be true. One study of captive warblers, for example, crossed a type that orients itself to the southwest when it’s ready to migrate with a type that orients southeast. The offspring pointed themselves directly south.

Kira Delmore and Darren Irwin, researchers at the University of British Columbia, wanted to put this idea to a tougher test. So they captured wild birds, attached geolocator tags to their backs, and sent the birds out into the world again. When the travelers returned, the researchers could see exactly where they’d chosen to fly.

The birds Delmore and Irwin studied are Swainson’s thrushes. Two subspecies, the olive-backed and russet-backed thrushes, meet up in western Canada each summer. When the weather gets colder, they migrate to Mexico and Central and South America. But they take different routes to get there. Russet-backed thrushes hug the west coast tightly. The olive-backed birds go eastward, swinging wide over the center of the United States before hanging a right and flying south over the Gulf of Mexico.

Where the two subspecies’ ranges overlap in Canada, they have a chance to (ahem) mingle. Delmore and Irwin trapped birds in this overlap zone and fitted them with tracking devices before releasing them. In the next year, after the birds had migrated south and north again, the scientists recaptured as many tagged birds as they could find. They ended up with recorded routes for 21 thrushes. A genetic test told them which of these birds belonged to each subspecies, and which were hybrids that had resulted from the two types mingling.

The tracking devices revealed that the olive-backed birds had migrated through the middle of North America, as expected, while the russet-backed subspecies had traveled along the west coast. But the hybrids clearly flew to the beat of their own drums. They took “tremendously variable” routes, many combining legs from both parents’ trips. Some birds took one parent’s migratory route south and the other parent’s back north. A few of the birds even flew right down the middle between the two paths.

Hybrid birds seem to follow a mixed-up set of genetic directions, with pieces inherited from both parents. But Delmore says it’s still a mystery how, exactly, a bird’s DNA tells it where to go.

“It’s believed that for their first migration, birds have genetic instructions on which direction they need to head and how long they need to fly,” she says. This kind of travel is called “vector navigation.” Circadian clock genes may play a role, along with light receptors in the birds’ eyes. Beyond that, no one knows. “It’s really an up-and-coming field,” Delmore says.

The in-between route taken by hybrids is an incredible trick of genetics, but it may also be a problem. The middle ground between the two subspecies’ routes includes much of the dry and mountainous western United States. Some birds, at least, survived this trip and made it back to the researchers. But it was probably no picnic.

Delmore thinks this might be one of the mechanisms keeping these two types of thrush separate. For animal species or subspecies to stay distinct from one other, there need to be barriers—physical or not—that stop them from interbreeding. In the case of the thrushes, one of these barriers might be the harsh terrain that hybrids fly across. Their jumbled internal maps might make these birds less likely to survive than purebreds.

The researchers are now doing modeling experiments to find out whether a hybrid route really is worse. So far, Delmore says, it looks like that’s true. If this had been a joke after all, the chicken would have been better off staying on its own side of the road.

thrush with tracker

Images: top, Darren Irwin; bottom, Kira Delmore.

Delmore, K., & Irwin, D. (2014). Hybrid songbirds employ intermediate routes in a migratory divide Ecology Letters DOI: 10.1111/ele.12326

CATEGORIZED UNDER: birds, evolution, genes, navigation, top posts
  • Rietha Crafford

    I agree, follow nature and don’t interbreed ……. even in humans people should do the same…..no in breeding with different cultures. You can’t go wrong living according to nature…..a “hybrid” is most likely to be weaker, more aggressive, not being able to produce off spring as in donkey and horse etc, etc….it is just a no, no

    • Jean Freed

      Sorry, but hybrids are often not only smarter than either parent but more intelligent as well. Consider your example of the mule as an indication of this. Your thinly veiled racism is not an addition to this forum.

      • Rietha Crafford

        Got nothing to do with racism…..typical you americans, if you have nothing else to say that is the truth, you call the other party a racist…..fat of the matter is that hybrids are not so smart as this hybrid songbird is clearly indicating……………also a mule might not be any smarter than a horse, but perhaps a donkey……fact is they can’t reproduce and so is the situation of a goat and a sheep or a zebra and a donkey…..they are also more susceptible
        to disease and not such a long life span either………….Nature is not intended to interbreed, neither are humans, no matter whether you are Asian, Black, European or Chinese

    • amphiox

      No such thing as a hybrid human.

      All humans are a single species (and a single subspecies, for that matter).

      Human breeding between different cultural groups has in fact been part of the pattern “according to nature” since the beginning of human existence. The evidence is everywhere in the human genome.

    • ECarpenter

      All humans are the same species, so there’s no inter-species breeding possible among us. So all breeding between people is “living according to nature”.

      You are using the word “interbreed” in a completely different way than this article’s use. Your ideas are not backed up by any scientific evidence at all.

      • Rietha Crafford

        don’t agree with you…………..you want to tell me that breeding a mountain zebra and a low veld zebra does not make any difference. It surely does believe you me. You get “funny looking zebra’s coming from that, not the true bred zebra and is standing out in the pack, almost the same as an albino animal, vulnerable to be eaten rather than it’s mate.(not sure if you would know the difference between the different zebras not being from Africa)………….in humans you get the same…..just does not gel as it should…..it is only accepted in life because we are/BELIEVE we are above nature
        “Human breeding between different cultural groups has in fact been part of the pattern “according to nature” since the beginning of human existence”……………only because we have FORGOTTEN TO LIVE ACCORDING TO NATURE

        • http://inkfish.fieldofscience.com/p/about.html Elizabeth Preston

          (This commenter has now been blocked. Thanks for everyone else’s calm and scientifically based responses.)

  • Cyndi

    I’m almost afraid to post given the inanity posted so far but…

    Seems to me this sort of “hybrid” programming is a really good evolutionary adaptation. We see what happens with migratory birds who depend on a particular fresh water source which no longer exists; they can’t feed and they die. Although in our modern world those sources are disappearing quickly, they still would change over time before human activity was a factor. The species will only survive if there are new routes established that contain the fresh water and other resources the birds need en route (as well as the beginning and end points). Even if only a few of the new “hybrid” routes are fruitful, it gives the bird species alternative lines of growth. Of course this only works with healthy populations of birds who do not need to change their routes very often. I would really love to see what happens with 3rd and 4th generation “hybrid” birds.

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Inkfish

Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also Editor of the children's science magazine Muse, where she frequently writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.

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