Prehistoric birds are various taxa of birds that became extinct before recorded history, or more precisely, before they could be studied alive by ornithologists. They are known from subfossil remains and sometimes folk memory, as in the case of Haast's eagle from New Zealand.
Birds (Aves) are generally believed to have evolved from feathered dinosaurs, and there is no real dividing line between birds and dinosaurs except of course that the former survived the Cretaceous–Paleogene extinction event and the latter did not. For the purposes of this article, a "bird" is considered to be any member of the clade Neornithes, that is the bird lineage as exists today. The other lineages of the Aves also became extinct at the end of the Cretaceous.
Taxon extinctions taking place before the Late Quaternary happened in the absence of significant human interference. Rather, reasons for extinction are stochastic abiotic events such as bolid impacts, climate changes, mass volcanic eruptions etc. Alternatively, species may have gone extinct due to evolutionary displacement by successor or competitor taxa – it is notable for example that in the early Neogene, seabird biodiversity was much higher than today; this is probably due to competition by the radiation of marine mammals after that time. The relationships of these ancient birds are often hard to determine, as many are known only from very fragmentary remains and due to the complete fossilization precludes analysis of information from DNA, RNA or protein sequencing.