Imagine you could fly. Now imagine giving that up. WHY would you ever do that?! Ostriches, emus, kiwis, penguins… one thing these birds all have in common is that they can’t fly. A non-flying bird feels like an oxymoron, but the maybe most surprising thing about these creatures is that they all once flew.

Ostrich

They all evolved not to fly. At one point, scientists thought that maybe some of the birds living today evolved from flightless ancestors. More specifically they thought this about ratites – a diverse group of large, flightless avians which includes ostriches, emus, and moas. This has since been proven wrong. Thanks to DNA studies, we now know that all of the today’s flightless birds came from ancestors that flew, but they all independently evolved to lose their ability to fly.

And they did this all around the world. Penguins in Antarctica, steamer ducks in South America, emus in Australia, kiwis in New Zealand, and ostriches in Africa. It’s astounding to think that given such different environments all these varied bird populations would go through the same type of evolution- or convergent evolution to be exact. But why would the evolution of flight loss happen over and over again? Well, two possible explanations largely influenced by Darwinian theory, are that changes in the environment either decreased pressure for flying or increased pressure to develop another trait. This means that nature slowly selected for certain traits over others, and along the way, took away some birds’ ability to fly.

Emu

To illustrate how increased environmental pressures for traits other than flying work in practice, let’s dive into the biomechanical explanation. In a study published in the Proceedings of the National Academy of Sciences, researchers studied different types of flying and wing-propelled diving seabirds. By analyzing energy costs and morphology, they found that as a bird’s functional ability to be a good diver increases, its ability to fly decreases – eventually to a point where flying ceases to be possible. A larger body and short wings may help seabirds dive and swim better and thereby help them get more food for example, but this body type makes taking off more difficult. In other words, penguins may have lost their ability to fly because evolving to be better swimmers provided a greater advantage than flying – and the two abilities required a trade-off. You can’t be both a great flier and swimmer.

Kiwi bird

So, the environment puts pressure on birds to evolve certain abilities at the expense of others, but a lack environmental pressures can also lead to change. If the environment doesn’t strongly require a bird to fly, the bird may lose this ability over time. For example, in a study published in the journal Evolution, researchers argue that steamer ducks eventually lost their ability to fly in part because their South American coastal environment was suitable to live in year round. The birds did not have a need to migrate, and thus had little selective pressure to fly.

Another example of diminished selective pressures is when the dinosaurs went extinct. During this period, birds were left without predators… so without the need to fly away from hungry t-rex’s, the birds evolved other qualities – the ostrich, for example, became huge and better suited for running rather than flying. This lack of predation in the environment helps explain why so many flightless birds are found on islands that were isolated for extended periods of time. Why use up energy to fly when you can just strut around?

Penguins

Now, these changes happened slowly over time through natural selection, but I’d be remiss if I didn’t also mention that you and I and all humans are also making birds flightless – and very quickly – through artificial selection. Our turkeys and chickens would look very different if we weren’t breeding them to be large and meaty for our consumption – So big that they can no longer fly like their ancestors once did.

Steamer ducks

Still, while we certainly have a hand in their flightlessness, today’s flightless birds evolved to be this way. Which, in some cases might be good. Could you imagine how terrifying a flying ostrich would be? On a kind of related note, evolutionary change doesn’t just affect living creatures – it also affects the technology we use every day.

What trait would you be thrilled to evolve or evolve out of? Let us know in the comments!