New Theory On How Birds Evolved Flight

Privacy | Contact | Affiliate Disclosure

Search Soapbox Videos

In this video, biologist Ken Dials presents a fascinating new hypothesis for how birds evolved flight.

I grew up around a lot of chickens. My grandmother raised chickens for eggs to sell. Although she didn't have anywhere near the number of chickens as large-scale chicken farms, at any one time, she had 300 to 400. Since they actually were able to roam around outside during the day, and act like chickens normally act, I've been able to observe chickens a lot. Primarily, this resulted in a very advanced chicken imitation. But, as well, I've seen chickens using their wings.

It is actually not true that chickens are completely flightless birds. They actually do use their wings for very short rapid flights. Smaller breeds like the bantam can fly for a more sustained time, and get further up off the ground. Observe a bantam chicken flying, below.

If you've never seen chickens given lots of room, then you may have never seen chickens use their wings and actually fly, although for a very brief duration. How much they do this depends on their breed and size, but some chickens will actually fly up into trees to roost! For others, they will use their wings to achieve a very good long jump. Their wings are just too small and weak for any kind of sustained flight, not to mention that we've bred them, and fed them, to be so very large and heavy. They also use their wings to help jump/fly up to a higher perch or to assist in climbing up on something. The reason I mention all this is that, we could look at chickens for a glimpse into how flight evolved.

Birds Evolved from Theropod Dinosaurs

You may have heard that birds evolved from dinosaurs. They actually evolved from theropod dinosaurs, specifically a small coelurosaurian theropod. Tyrannosaurus actually fell into this group, but also ornithomimosaurs and compsognathids. Then there were the maniraptorans. Yes, you know these from the Jurassic Park and World movies: the raptors. Birds are part of this group. That's right, birds are raptors. These dinosaurs first appeared during the Jurassic period. Just like the Raptors, they move about on two feet and have three clawed fingers. And that's only for starters.

Feathers Did Not Evolve to Enable Flight

Some of these theropods starting developing feathers on their bodies and arms. We can see the evidence of these feathers in fossils. Of course, these early feathered dinosaurs could not fly. It is a mistake, in fact, to think of feathers as having been evolved in order to allow flight. They were necessary for theropods to fly, but this is not the same as saying that flight was a prerequisite of feathers. And, remember, bats can fly, and they have no feathers. As an aside, the popular assumption that bats cannot fly as well as birds is a myth. They have a degree of control over their flight that birds can never match so feathers did not necessarily make birds superior flyers. And the pterosaurs were soaring through the sky long before birds appeared, although these flying dinosaurs more closely resemble bird than other reptiles.

Feathers Have Other Functions

Even today, feathers have other functions, some of them much the same as that of hair or fur. Feathers are, after all, a form of hair. They are made from keratin just like our hair. The earliest feathers were very simple tubular structures. It took five stages of evolution before feathers with asymmetrical vanes suited for flight emerged. Each of these innovations may have evolved to suit a different function. In general, however, the earliest feathers may have served primarily as insulators, to conserve heat, for camouflage, water repellency, defense, and perhaps to assist in courtship displays. Whatever survival advantage the first feathers provided, we may never know.

The Two Primary Flight Evolution Hypotheses

As for how birds actually evolved flight there are two primary opposing hypotheses: The arboreal, or tree-down hypothesis, and the cursorial, or ground-up hypothesis.

Arboreal or Trees-down Version

According to the trees-down version, flight began with birds gliding down from trees. This is seen as a precursor to actually flapping the wings, or powered flight. So, according to this view, you have to become a good glider before you can develop powered and sustained flight. Perhaps, at first, slight movements of the wings would have been used to correct the flight path, and this developed into more active and complex use of the wings to actually sustain and control flight for long periods.

Cursorial or Ground-up Version

The ground-up version is, as the name implies, the opposite viewpoint. According to this hypothesis, birds started by running along the ground, and, by using its wings, for whatever reason, such as to clear obstacles or to evade a predator, the bird became airborne for brief periods, much like the chickens I mentioned, above. So, here, flapping occurred first, and the birds actually learned how to overcome gravity and power their own flight before they started gliding.

Biologist Ken Dial's Fascinating New Observation

Biologist Ken Dial, who has been studying bird flight for many years, reveals, to me, quite an amazing discovery, in regards to how young birds who are learning to fly use their wings. He was inspired by a challenge to Darwin's Theory of Evolution. According to Darwin, a wing would have evolved from a forelimb in states. A fellow named George Jackson Mivart, after Darwin published On the Origin of Species, challenged this view, saying "What use is half a wing?" It's not a terrible question. Sure, we know some of the functions of early feathers, but, in truth, we have no idea why feathers would have developed some partial aerodynamic abilities or indeed, why primitive wings that were not yet able to take a bird aloft would have been any use at all. We can explain the use of many early and primitive features that were not yet fully useful. But even Darwin really couldn't answer the question and we still cannot be sure about "what use is half a wing." The quite fanciful depiction of Deinonychus antirrhopus, which, evidence suggests, had feathers, begs the question. What good, indeed, would those wings have been?

Model of Deinonychus antirrhopus feathered dinosaur by Stephen Czerkas at the Royal Ontario Museum

Model of Deinonychus antirrhopus by Stephen Czerkas at the Royal Ontario Museum
Image by Aaron Gustafson via FlickrImage Credit

Model of Deinonychus antirrhopus feathered dinosaur by Stephen Czerkas at the Royal Ontario Museum

Model of Deinonychus antirrhopus by Stephen Czerkas at the Royal Ontario Museum
Image by Aaron Gustafson via FlickrImage Credit

But Dial looked to bird chicks, who have small immature wings, to observe how they used their wings as they learned to fly.

At first, he was putting young birds in a room with a smooth floor and expecting them to go up a slick vertical wall, in order to reach their siblings. A rancher came in once and said, "What are these birds doing on the ground? They hate being on the ground. Give them a bail of hay. Give them something to get up on." Dial got some hay bales for the chicks and something interesting happened. According to Dial's son, who had been looking after the birds during his absence, the day's data was horrible because the birds 'cheated.' They had run straight up a vertical surface. So much for flying!

A New Theory on the Evolution of Flying?

But this should have been impossible. How could these chicks run up a steeply angled surface? Well, remember when I said that chickens would use their wings to assist in climbing up things? I always assumed they were using their wings to get just a little lift…to boost them up a little as they tried to scramble up some slope. In the video, when a little chick is given a log propped on hay bales at an angle too steep to just walk up, it does indeed use its wings. But it doesn't use them to push it upwards, as an assist to its legs. It uses them to pull itself toward the tree in order to stay in contact with the tree as is climbs up. This is a function for a wing that few people have ever imagined, and Dial has observed it in many birds as they scramble up trees or other surfaces. Wings not used to fly, but to, in essences, keep their feet planted.

This suggests a third possibility for the evolution of flight. That the little wings developed in the first winged theropods were used to run up to a higher elevation, such as into a tree. This would have been handy for escaping predators. Then, they would flap back down when it was safe. Powered flight would have developed from here.

You may be interested in learning about a couple of the early non-avian winged dinosaurs that probably exhibited flight, such as the microraptor, with its four-wing design, and the archaeopteryx, long viewed as a transitional between non-avian feathered dinosaurs and modern birds.

Posted on 07 Apr 2018 02:14

© 2018 by Soap Box Videos. All Rights Reserved. Please contact for permissions. Videos property of their creators.