Birds are considered reptiles under the phylogenetic classification system. This method of categorizing organisms was developed in the 1940s and is based on common ancestry.
In phylogenetics, all animals, including birds, mammals and dinosaurs, that have genetic connections to early reptiles are reptiles. Birds are traced back to the group of feathered dinosaurs that did not die off during the massive dinosaur extinction 65 million years ago. Over time, these dinosaurs evolved into birds. However, in an 18th century classification scheme, the Linnaean system, organisms are grouped together based on common traits. Since reptiles are cold-blooded and have scales, birds do not fall into this category following the Linnaean system.