The original version of the IOC World List (Gill and Wright 2006) 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.
DNA sequences clarify (or erase) some of the connections between the deepest historical branches of avian evolution (Hackett et al 2008). Accordingly, we can revisit the compositions of the classical Orders of birds and the relationships among them. Some groups (clades) of bird species are not as closely related to each other as previously thought. DNA-based resolutions of the phylogenetic relationships among world birds have increased dramatically in the past few years, and will continue to multiply. Many of the latest results are available on the web as part of the Tree of Life (TOL) project. Other excellent resources include John Boyd’s website Taxonomy in Flux .
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 splitting 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 increasing instability of working classifications for a decade or so. Continually improving sets of DNA sequences and analytical approaches also may override previous conclusions, so cautious revisions of higher category taxonomy and new sequences of taxa seem prudent.
Our policy, therefore, is to change higher level classification and sequences of taxa conservatively. Stability is important, and being creatures of habit we all find a particular taxon or species more easily in a familiar sequence, even if dated.