It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.The earliest known avialan fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago.

However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system.

Gauthier defined Aves to include only the crown group of the set of modern birds.

Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling.

Many of these early "stem-birds", such as Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.

Birds are descendants of extinct dinosaurs with feathers, making them the only surviving dinosaurs according to cladistics.

DNA-based evidence finds that birds diversified dramatically around the time of the Cretaceous–Palaeogene extinction event 66 million years ago, which killed off the pterosaurs and all the non-avian dinosaur lineages.

Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them.

Recreational birdwatching is an important part of the ecotourism industry.

Songbirds, parrots, and other species are popular as pets.

Guano (bird excrement) is harvested for use as a fertiliser. About 120–130 species have become extinct due to human activity since the 17th century, and hundreds more before then.

This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae, identified four different definitions for the same biological name "Aves", which is a problem.