Evolution of birds

The evolution of birds began in the Jurassic Period, with the earliest birds coming from a group of theropod dinosaurs called Paraves. Birds are now known as a special group of animals called Aves. For many years, scientists thought a small dinosaur named Archaeopteryx lithographica was the first bird. This dinosaur lived during the Late Jurassic period.

Today, we know that birds and crocodiles are the only living members of a group called Archosauria, which also includes many ancient reptiles. After a big event called the Cretaceous–Paleogene extinction event 66 million years ago, only four main groups of birds survived. These groups eventually led to birds we see today, like ostriches, ducks, chickens, and many other types of birds.

Scientists study how birds evolved by looking at their family tree. Birds are defined as all the descendants of a common ancestor shared by modern birds, like the house sparrow, and either Archaeopteryx or other ancient birds. This helps us understand how birds are related to dinosaurs and other creatures that lived long ago.

Origins

Main article: Origin of birds

See also: Feathered dinosaur, Paraves, Avemetatarsalia, and Dromaeosauridae § Alternative theories and flightlessness

Birds evolved from a group of theropod dinosaurs called Maniraptora, which includes dromaeosaurs and oviraptorids. Over time, scientists have found many dinosaurs that looked very similar to birds, making it harder to tell where birds begin and reptiles end. Important discoveries in China, especially in Liaoning Province, showed that some small theropod dinosaurs had feathers, like Sinosauropteryx and Sinornithosaurus.

Some of these dinosaurs, such as Cryptovolans and Microraptor, could even fly, which suggests that flight might have first appeared in these dinosaur groups before true birds evolved. Most scientists believe birds came from saurischian dinosaurs, though a few argue birds evolved from different early archosaurs.

Mesozoic birds

See also: Avialae

The earliest known bird, Archaeopteryx, lived during the Jurassic period and was a key discovery that helped scientists understand how evolution works. Though it wasn’t the direct ancestor of today’s birds, it showed us what the first birds might have looked like and how flight could have developed. It had feathers and wings but was not a very strong flier, more of a glider.

As birds continued to evolve, they began to lose parts of their bodies that weren’t needed, like the long bony tail, which became a small bone called a pygostyle. Some early birds, like Confuciusornis, had this short tail and still kept clawed fingers, perhaps for climbing. During the Cretaceous period, birds began to look more like modern birds, with stronger bodies for better flight and special feathers called an alula that helped them control their wings at low speeds. Some of these birds lived near water and looked like modern shorebirds or ducks, while others were swimmers, similar to today’s grebes and loons. Many of these early birds had teeth and sharp claws, but later birds lost their teeth. Most of these ancient birds went extinct due to a huge event at the end of the Cretaceous, but a few groups survived and eventually evolved into the birds we see today.

Radiation of modern birds

See also: Mesozoic–Cenozoic radiation

Modern birds likely appeared near the end of the Early Cretaceous or the start of the Late Cretaceous. They split into two main groups: the paleognaths and neognaths. Paleognaths include the tinamous and the large flightless ratites, such as ostriches, rheas, cassowaries, kiwis, and emus.

Scientists debate when different bird groups diverged. Most agree that modern birds originated in the Cretaceous period, but they disagree on whether many bird groups appeared before or after the massive Cretaceous–Paleogene extinction event. Some studies suggest birds diversified rapidly after this event due to new ecological opportunities, while others propose that many groups existed before the extinction. The birds that survived were probably ground-dwelling, which helped them endure the destruction of forests. Climate changes also influenced bird diversification, with new species appearing more often during cooler periods.

Classification of modern species

See also: Sibley–Ahlquist taxonomy of birds and dinosaur classification

The way we group and name modern birds is still being studied and discussed by scientists. A very important book from 1990 by Sibley and Ahlquist helped shape our understanding, though it continues to be updated. Most groups of birds today seem to share common ancestors, forming what scientists call "clades."

Traditionally, scientists looked at bird shapes and fossils to guess how different birds are related. But now, studying DNA with computers gives clearer answers, even though some questions about bird families remain unsolved.

Current evolutionary trends in birds

See also: Bird conservation and Sexual selection in birds

Bird species are disappearing at a faster rate than new ones can appear, which means we are losing many different genes forever. One reason for this is that human changes to the land, like cutting down forests, bring different bird species together that used to live far apart. This can cause them to mix and lose their unique traits.

Scientists have noticed changes in birds' bodies over time. A study looked at birds that hit buildings in Chicago from 1978 to 2016. They found that birds' legs have become shorter by about 2.4%, and their wings have grown longer by 1.3%. These changes may be happening because of climate change, showing how birds are slowly evolving to adapt.