Evolution of birds

The evolution of birds began in the Jurassic Period. The earliest birds came 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, birds and crocodiles are the only living members of a group called Archosauria. This group also included many ancient reptiles. After a big event called the Cretaceous–Paleogene extinction event, 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 came from a group of dinosaurs called Maniraptora. This group includes dromaeosaurs and oviraptorids. Scientists found many dinosaurs that looked a lot like birds. This made it tricky to know where birds start and reptiles end.

Important discoveries happened in China, especially in Liaoning Province. These finds showed that some small dinosaurs had feathers. Examples are Sinosauropteryx and Sinornithosaurus.

Some of these dinosaurs, like Cryptovolans and Microraptor, could fly. This suggests that flight might have started in these dinosaurs before true birds evolved. Most scientists think birds came from saurischian dinosaurs. A few scientists think birds came from different early archosaurs.

Mesozoic birds

See also: Avialae

The earliest known bird, Archaeopteryx, lived during the Jurassic period. It helped scientists learn about how evolution works. It had feathers and wings but was not a strong flier. It was more like a glider.

As birds changed over time, they lost parts they did not need. For example, their long bony tail became a small bone called a pygostyle. Some early birds, like Confuciusornis, had this short tail and clawed fingers, maybe for climbing. During the Cretaceous period, birds started to look more like birds today. They had stronger bodies for better flight and special feathers called an alula that helped them control their wings when flying slowly. Some lived near water and looked like modern shorebirds or ducks. Others were swimmers, like 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 disappeared because of a big event at the end of the Cretaceous period, but a few survived and 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 opportunities, while others propose that many groups existed before the extinction. The birds that survived were probably ground-dwelling, which helped them endure changes. Climate changes also influenced bird diversification.

Classification of modern species

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

Scientists are still learning how to group and name modern birds. A book from 1990 by Sibley and Ahlquist helped us understand more, but we still update what we know. Most birds today share common ancestors. Scientists call these groups "clades."

Before, scientists studied bird shapes and fossils to see how birds are related. Now, looking at DNA with computers gives better answers, but some questions about bird families are still unknown.

Current evolutionary trends in birds

See also: Bird conservation and Sexual selection in birds

Bird species are disappearing faster than new ones can appear. This means we are losing many different genes forever. One reason for this is human changes to the land, like cutting down forests. This brings 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, and their wings have grown longer. These changes may be happening because of climate change, showing how birds are slowly evolving to adapt.