Select a thumbnail representing one of the forty currently described Bird Orders, from Struthioniformes (bottom left) to Passeriformes (top right). Each order is a clade, i.e., it groups a set of species derived from one common ancestor. Orders should be monophyletic, so that they comprise all the descendants of their common ancestor and these descendants only.

In turn, the 40 bird orders are derived, not from each other, but from a set of ancestral precursors referred to as Bird Superorders. The seven superorders are shown. Again, the latter are monophyletic clades, as defined above.

This figure is based on studies by Hackett et al (2008) and Jarvis et al (2014). The former used 19 aligned nuclear DNA sequences from 15 different chromosomes, whereas the latter performed analyses on the whole-genome. This new strategy ensured better resolution for the early branches of the tree.

These recent works replaced former nomenclatures which counted less than 30 orders.