What were the first birds like?

The origin of birds can be traced back to the age of the dinosaurs when they evolved within a group of these reptiles.

Although scientists have uncovered a number of transitional fossils of bird-like animals, precisely what was the first bird is a complicated question to answer. Birds and non-avian dinosaurs are categories created by us, but in reality there’s no clear line distinguishing the two.

What is a bird?

Over millions of years, birds evolved signature features that collectively set them apart from other animals alive today.

These include very noticeable characteristics, such as feathers and a pair of wings. Aside from bats, birds are the only living vertebrates capable of powered flight.

Less visible features usually include a hollow skeleton, relatively large brains compared to their head size and warm-bloodedness. Living bird species also have no teeth, although some species – often aquatic ones – have ridges on their beaks. These may function similarly to simple teeth by helping the birds to hold on to slippery prey.

All of these features appeared at different points in the evolutionary history of birds, making it difficult for scientists to be specific about what the first birds were. For instance, birds seem to have evolved a completely toothless beak around 120 million years ago, which is relatively late in their evolutionary history. Other features like a hollow skeleton may have arisen around 100 million years earlier.

Professor Daniel Field, a palaeontologist who studies the evolutionary history of birds, says, “the combination of these features allows us to recognise birds as being different from any other living group of animals”.

“I like to apply the term ‘bird’ to what I think of as the ‘modern bird group’, that is, the most recent common ancestor of all living birds and all its descendants. To me that’s an unambiguous definition. Anything that goes beyond that, when we go much further back in the evolutionary history of birds, is where the lines get blurred. Exactly when something is sufficiently ‘bird-like’ to be called a bird is very subjective.”

What did the earliest bird-like animals look like?

Many scientists now believe that the most prominent feature of birds – feathers – evolved in non-avian dinosaurs long before the earliest birds.

The first animals that scientists describe as bird-like are feathered dinosaurs. They’re thought to have appeared around 165 to 150 million years ago, evolving within a group of dinosaurs called theropods. Theropods are characterised by having three forward-facing toes and walking on two legs. As well as the ancestors of birds, this group also includes famous carnivorous dinosaurs such as Tyrannosaurus rex and Velociraptor.

Archaeopteryx is still widely referred to as the first bird, although many scientists dispute this title. Although Archaeopteryx had elements that were very bird-like, such as feathers and a wishbone, it also had many characteristics that we don’t see in living birds. These include sharp, pointy teeth, a long, bony tail and long claws on its hands. There’s also debate about whether Archaeopteryx could actually fly or only glide. One thing’s for sure – Archaeopteryx, though definitely bird-like, falls outside the modern bird group.

“Although you can’t unambiguously call Archaeopteryx a bird, it is unambiguously a dinosaur,” says Daniel.

“As it had some features that we see in modern birds, but others that were much more dinosaur-like, I’m uneasy about calling Archaeopteryx a bird. But it’s getting close!”

Archaeopteryx is about 150 million years old, which makes it arguably one of the oldest truly bird-like dinosaurs discovered so far. But the fact it still holds the title of ‘first bird’ in many people’s minds may have more to do with when the species was discovered than anything else.

When the first Archaeopteryx fossil was unearthed in Germany in 1861, it was unlike anything known to science. At that time, bird-like creatures weren’t known to have existed so far back, so it was given the title “first bird”, and the name has stuck.

But there are now other contenders for the earliest known members of the bird lineage. One of these is Aurornis xii, whose fossils date to around 160 million years ago.

Aurornis, which means dawn bird, was about the size of a pheasant. It had downy feathers, but lacked larger feathers and so probably wasn’t able to fly. Like Archaeopteryx, it had claws and a long tail, but some of its bones are much more primitive looking.

Similarly, Anchiornis huxleyi lived around 160 million years ago and had feathers on its legs and arms, and curved claws that may have helped it to climb trees. Experts think its feathers were too short to enable true flight and instead it may have glided between trees.

Scientists have also been able to study chemicals in Anchiornis feathers to work out the colour of its plumage. From analysing the preserved remains of melanosomes, which store pigment, they determined that Anchiornis had black and white feathers on its limbs and a red crest on its head.

When did birds first appear?

Bird-like creatures have been around for more than 150 million years, since the Late Jurassic Period. But what about modern birds?

The earliest animal that’s undisputedly considered a bird would be the most recent common ancestor of all living birds.

We think this ancestor lived during the Late Cretaceous Period, around 100 to 85 million years ago. Although it may not have looked exactly like any species alive today, it would’ve had the hallmark features of a living bird, such as feathers, the ability to fly, hollow bones and a toothless beak.

The earliest uncontroversial modern bird fossil discovered so far is Asteriornis maastrichtensis – more popularly called the wonderchicken.

Fossils of the wonderchicken date to 66.7 million years ago, just 700,000 years before the mass extinction that killed off the non-avian dinosaurs.

The wonderchicken is thought to have been a small, ground-dwelling bird that could reproduce quickly and fly, which may have helped it to survive the extinction event.

“The wonderchicken seems to be the most unambiguous early representative of Neornithes – the group that includes all modern birds – that we’ve found so far,” says Daniel.

“As it had the full complement of bird-like features that we see today, we know that all of these features must have evolved by the end of the Cretaceous.”

How did birds become so successful?

Over the millions of years since they first arrived on the scene, birds have evolved a huge diversity of shapes and sizes. More than 11,000 species now occupy various ecological niches in habitats around the world, from the ornate, forest-dwelling birds-of-paradise to the hardy emperor penguins of Antarctica.

But what makes birds so successful, and how did the early ancestors of modern birds survive the mass extinction event at the end of the Cretaceous Period when the non-avian dinosaurs died out?

It was likely due to several different factors, but mainly that flying allows birds to be very mobile. They can travel huge distances in search of more favourable conditions and colonise new land masses, and eventually evolve into different species to suit these new environments.

Their ability to adapt to new environments and occupy different roles in ecosystems relatively quickly has played a major role in their success. Bird beaks show this clearly, as they’re very diverse, having become specialised for different diets and feeding behaviours.

But another, less scientific, factor has contributed to the survival and success of birds – luck.

“Birds came very close to going extinct when the asteroid struck more than 66 million years ago,” says Daniel. “Our evidence shows that most bird-like animals that were around at the time didn’t survive. The odds were probably stacked against them just as it was for the non-avian dinosaurs.”

In fact, the early ancestors of only a few modern bird groups survived the mass extinction. These were the Galloanserae, the Palaeognathae and the Neoaves. Galloanserae is the group from which chickens, ducks and other fowl descended. Palaeognathae or ‘old jaws’ is the group that emus, ostriches and other ratites belong to and Neoaves contains almost 95% of all living birds.

The survivors were smaller, ground-dwelling birds that could grow, reproduce and adapt faster to the new treeless habitats in the aftermath of the asteroid impact that ended the age of the dinosaurs.

“I think luck is a really important part of the story,” adds Daniel. “It sounds a bit unscientific, but I think it’s important to recognise.”

“The truth is, our planet probably came very close to losing birds – and by extension, dinosaurs entirely – when that asteroid struck. So, I’m definitely glad they’re still around for us to appreciate today!”