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 (and palaeontologists in particular) still adhere to the tradition of using the term Aves in a broader sense to include 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. Today some 10,500 bird species are inhabiting the Earth, with almost six hundred species having already gone extinct in historic times because of human activities, primarily due to deforestation and hunting (Sayol et al., 2020; Cooke et al., 2023; Matthews et al., 2024).

Reconstructions of phylogenetic relationships among representatives of the avian stem group are almost exclusively based on fossilised bones. Nuclear and mitochondrial DNA, routinely used to reconstruct relationships among extant taxa, are not available. Inferring relationships among members of the stem group by means of comparative osteology is inherently problematic (e.g. Černy & Simonoff, 2023; cf. Schmitt & Edward’s, 2022; Ericson & Qu, 2024). Not surprisingly, the skeletal evolution towards crown birds seems to exhibit a conflicting pattern of independent (parallel) acquisitions and/or reductions of phenotypic traits.

Nevertheless, numerous potential skeletal apomorphies accompanying the successive transformation from early protobirds into more and more bird-like creatures are given by Gauthier (1986), Makovicky & Zanno (2011), Smith et al. (2015), Cau (2018), Pittman & Xu (2020), and Rauhut & Foth (2020). 

The following graphic depicts just one plausible reconstruction of avian evolutionary history (without stating apomorphies): 

Reconstruction of the origin of birds. Evolution from the earliest protobirds towards modern birds progresses along the ancestral avian lineage (red line). For many clades the supposed age is given in million years (Ma, Mega annum), mostly following Allen et al. (2021) and Wang et al. (2021). Note that the popular Dinosauria clade comprises almost all stem-group taxa as well as modern birds. This means that dinosaurs didn't become extinct (a popular misconception), but are represented in the present-day fauna by modern birds. In fact, dinosaurs probably never were as diverse as they are today being represented by ~11.000 bird species.

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