Dating back to the year 1768 when the tenth edition of Systema Naturae of Carl Linnaeus was published, birds are referred to by zoologists as CLASS Aves. However, there is a dissent among scientists concerning the proper definition of the term. As first suggested by Jacques Gauthier (1986), I prefer considering Aves as being represented by the last (i.e. the most recent) common ancestor of all extant birds and all its descendants. According to this definition the term Aves is synonymous to modern or crown birds. In contrast, numerous avian researchers still adhere to the tradition of using the term Aves in a broader sense to include the famous Archaeopteryx and other bird-like fossils. In this case, modern birds are referred to as Neornithes.

Fossils not included in modern birds, but still closer to this group than to any other group of living organisms, are treated as members of the avian stem group. These fossils either pertain to the ancestral lineage, in which case they represent direct ancestors of crown birds, or to extinct side branches. Stem-group and crown birds together constitute the pan-Aves, or total birds. The informal name birds should be restricted to crown-group taxa, while the term protobirds might be used for representatives of the stem group.

Pan-Aves originated ~250 million years ago, when the last (most recent) common archosaurian ancestor split into two species, one giving rise to the crocodilian lineage, and the other to the bird lineage.

Reconstructions of phylogenetic relationships among representatives of the avian stem group are based on fossilised bones. Nuclear and mitochondrial DNA, routinely used to reconstruct relationships among extant taxa, are not available (except in rare cases of ancient DNA). Therefore, inferring relationships among members of the stem group is inherently problematic (e.g. Černy & Simonoff, 2023) and the figure below depicts just one plausible hypothesis. Despite this limitation, a relatively coherent picture of avian stem-group evolution developed over the last decades:

The successive transformation into more and more bird-like creatures probably started with the enlargement of the rear limbs. Initially, ancestral birds possibly ran on their hind legs only for short periods, while moving on all four legs most of the time. Bipedalism may have evolved to enhance the accelerator phase when trying to escape from predators. In the course of evolution, protobirds became lighter and smaller, showing increased levels of activity and higher body temperature. Recent birds have mean levels of resting body temperatures above 38°C. 

Along with increased body temperature, protobirds developed feathers for insulation and possibly display. Later the forelimbs became significantly longer than the hindlimb, and the long bony tail was lost. 

Today some 10,500 bird species are inhabiting the Earth, but almost six hundred species have already gone extinct in historic times because of human activities, primarily due to deforestation and hunting (Sayol et al., 2020; Cooke et al., 2023).

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