"Wisdom begins with putting the right name on a thing"
(Old Chinese Proverb)

Classification

The  IOC World List mostly followed  the classification and sequence of families in the third edition of the Howard and Moore Complete Checklist of the  Birds of the World (Dickinson 2003). That classification reflected the  phylogenies based on DNA data available at that time, summarized in the chapter  on “Avian Higher-Level Phylogenetics” by Cracraft,  Barker, and Cibois.

We  now have the ability to to distinguish taxa of similar rank that are reciprocally  monophyletic. This lets us redefine some groups of species that in hindsight  were paraphyletic. Changes in taxonomy follow, especially realigning of orders,  families, and genera to define monophyletic taxa that can be reconnected to  their closest relatives in the classification. Genera also change with corrections of nomenclature and the rules of priority. As a result, we can expect ongoing instability of working classifications for a decade or so.  Continually improving sets of DNA sequences and analytical approaches  may override previous conclusions, so cautious revisions of higher category taxonomy and new sequences of taxa are prudent.

Excellent  recent resources include Joel Cracraft’s (2013) Avian higher level relationships and classification in Howard & Moore 4, Jon Fjeldsa’s (2013) Avian Classification in Flux in HBW Special Volume and  John Boyd’s website Taxonomy in Flux. For comprehensive views of the relationships among Orders, see Rick Prum et al’s (2015) A comprehensive phylogeny of birds (Aves) using targeted next generation DNA sequencing, and Erich Jarvis et al’s (2014) Whole genome analyses resolve early branches in the tree of life of modern birds.