Definition bird

Definition
Aves and a sister group, the order Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[29] 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. This was done by excluding most groups known only from fossils, and assigning them, instead, to the broader group Avialae,[30] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

Gauthier and de Queiroz[31] identified four different definitions for the same biological name “Aves”, which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. He assigned other names to the other groups.

 
 
 
Crocodiles

 
 
Birds

 
 
 
Turtles

 
 
 
Lizards (including snakes)

 
The birds’ phylogenetic relationships to major living reptile groups.
Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
Aves can mean those feathered dinosaurs that fly (alternately Avialae)
Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a “crown group”, in this sense synonymous with Neornithes)