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This is Episode Three of a four-part Fascination on really big birds. You can listen to Episode One here and Episode Two here.

Anusuya Chinsamy-Turan: So, yeah, so very unlike a bird that you typically think of sitting at a bird feeder—you know, definitely not that kind of bird.

Flora Lichtman: I’m Flora Lichtman, with Scientific American’s Science, Quickly. Today: Episode Three of our four-part Fascination on really big birds. We’re going on a gargantuan goose chase.

Chinsamy-Turan:  I love talking about these big birds because, as you see, most people, you know, think ostrich, and they think that’s big. But actually they were real giants around at one time.

Lichtman: This is paleontologist Anusuya Chinsamy-Turan of the University of Cape Town in South Africa. She’s talking about the dromornithids. That’s their scientific name. But in big bird circles, you’ll hear them called thunderbirds.

Chinsamy-Turan: And I think the name the thunderbirds is really quite wonderful because it kind of immediately—you know, you can understand these really large animals are coming.  

Lichtman: Thunderbirds lived in Australia, and they had a long run. The earliest members of the group date back 50 million years or so, and one species persisted to about 40,000 years ago. And there was one thunderbird that was more thunderous than the rest.  

Chinsamy-Turan: The largest are the thunderbirds called Dromornis stirtoni. And they are known from the late Miocene, about eight million years ago. 

[Music]

Lichtman: Here’s what to picture.

Chinsamy-Turan: It’s like an overgrown ostrich, you know. Think about an ostrich, just kind of bigger—big beaks; the bones would have been heavier, more bulkier, more robust ...

Lichtman (tape): But, like, a lot bulkier and more robust.

Trevor Worthy: Absolutely. They are some of the hugest birds on the planet—were on the planet.

Lichtman: This is paleontologist and thunderbird expert Trevor Worthy.

Worthy: They got up to a whopping great 1,300 pounds, which is quite a lot, ten times a human.

Lichtman (tape): A bird the size of a cow. 

Worthy: Yeah.

Lichtman: They were huge in … almost every way. 

Worthy: They’ve got no discernible wings. The bit that pokes out of the side of the body is four inches. It isn’t going to make a dent in the feathers.

Lichtman: They’re not flying. Their closest living relatives are ducks and geese. And that’s another thing you could picture: just a gigantic goose trundling across a lush landscape—Australia was greener eight million years ago—grazing on plants and fruits.  Trevor says the thunderbirds’ vegetarian diet might partly explain their size.

Worthy: You can get big; you can have a big gut. And then you can eat a lot of rubbish. So if you can ingest a lot of low-quality food, then that is one way of living in an environment.

Lichtman: Their big body probably helped them in other ways, too. Here’s Anusuya again.

Chinsamy-Turan: Their body size probably evolves in the context of what else is around at the time. In Australia, there were these thylacines. There are these marsupial lions, and I think a big body size would have enabled them to escape predation.

Lichtman (tape): They’re just too big to eat. 

Chinsamy-Turan: Yeah. Or otherwise it’s too much of trouble to deal with them. You know, you rather go after something that’s smaller, that’s not going to harm you. Because, I think, you know, we know ostriches certainly are not predators, but an ostrich can do pretty serious damage to, to any animal that attacks it just by kicking.

Lichtman (tape): And if they’re related to geese, I mean, we all know how terrifying geese can be. 

Chinsamy-Turan: Yes. You know, I had a friend earlier this year that was attacked by a goose, and you’re not going to believe this, but literally, she had to be hospitalized because this goose attacked out of nowhere. 

Lichtman (tape): Just imagine if that goose was, you know, 1,300 pounds.

Chinsamy-Turan: Exactly. Exactly.

Lichtman: But how did they grow so big? That’s something Anusuya wanted to understand. To drill down on that question, she took a very thin section of a thunderbird bone and looked at it under the microscope.

Chinsamy-Turan: And this is fascinating because it tells us about the way in which the bone was deposited. So it tells us about the rates of growth.

Lichtman: Bones have rings that correspond to growth. Like tree rings…

Chinsamy-Turan: Where you have a white band that is growth during the favorable season, and then you have a growth line that indicates growth stopped for a period of time, and then afterward, it resumes. So just like tree rings, you can count the number of rings, and you can work out the age of the animal, and you can work out how long it took for an animal to grow to an adult body size. 

Lichtman: This bone ring method is a feather in Anusuya’s cap—she developed it when she was just a fledgling Ph.D. student working on dinosaurs. And when she applied the method to this giant thunderbird, she found ...

Chinsamy-Turan: These birds lived life in the slow lane. We can find growth cycles in the bones that suggest that they took about 15 years to reach an adult body size. And this is really a long time, compared to modern birds that grow up very quickly.

Lichtman: And it took them years to have babies, too, the data suggest.

Anusuya and Trevor say that gives us some insight, into why these birds don’t exist today.

[Music]

If life is easy—if there is enough food and not a lot of pressure from predators—you can take your time to grow up and have babies. But this can be a riskier strategy when life is tough. And life did get tough for thunderbirds. After eight million years ago, Australia became more arid. And that’s when the giant D. stirtoni disappears. Humans arrived in Australia around 50,000 years ago, and soon after, you don’t find any species of thunderbirds at all.

Worthy: Having the strategy of life in the slow lane didn’t facilitate being able to deal with something new in the environment, which is taking out numbers of their young. 

Lichtman: This theory is backed up by another bird tale. Consider the emu, a different big bird that lived alongside the thunderbirds.

Worthy: An emu, in contrast, it will grow to full body size in 14 months and is breeding shortly after that.

Lichtman: And while the thunderbirds went extinct, the emu survived.

Chinsamy-Turan: It survives because its growth rate is so much faster, so it is able to reach an adult size very quickly, it reproduce very quickly, and it’s able to recoup its numbers very quickly. 

Lichtman: And that’s something to think about.

Because many of the birds at our feeders, flapping around our cities, pooping on our cars—they live life in the fast lane. They grow up quickly and have babies early. And in an uncertain world, filled with two-legged jerks who make life hard for other animals, this strategy has advantages.

Chinsamy-Turan: So I think that when you see a pigeon or a robin, one should appreciate that they are the end products of a long evolutionary history, that there were many other forms that came before them. And I think the giant birds, they also are part of this evolutionary history of all modern birds that we see today.  

Lichtman: On the next Science, Quickly: when a big bird becomes an axe murderer.

Federico Degrange: Terror birds do not follow the general rules of, of birds. They use the beak as an axe to kill prey.

Lichtman (tape): That’s terrifying. 

Degrange: Well, that’s why they are terror birds. 

Lichtman: You’ve just listened to Part Three in a four-part Science, Quickly Fascination on really big birds.

Science, Quickly is produced by Jeff DelViscio, Tulika Bose and Kelso Harper. Our theme music was composed by Dominic Smith.

Don’t forget to subscribe to Science, Quickly wherever you get your podcasts. Head over to ScientificAmerican.com for in-depth science news.

For Science, Quickly—I’m Flora Lichtman.

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