Roberts VII and Sibley and Ahlquist's sequence
The other potentially confusing aspect of the new list is its overall sequence. For years we have listed the families and orders of birds in the Wetmore sequence, starting with ostriches and ending with canaries and buntings. However, in the last few decades the increasing use of genetic information to infer evolutionary relationships has resulted in a new sequence proposed by Charles Sibley and Jon Ahlquist. The advantage of using genetic data rather than the morphological characters used to infer bird relationships in the past is that genetic data are less subject to the confounding effects that living in a common environment can have on morphology. For example, grouping pelicans, cormorants, gannets, frigates and tropicbirds in a single Order Pelecaniformes on the basis of having all four toes webbed is misleading. This similar foot structure is a convergent character resulting from their common aquatic lifestyles; these water birds are not all closely related and thus the Pelecaniformes should be dissolved.
The Sibley and Ahlquist sequence still starts with the ratites (ostriches and their allies), and ends with the passerines, but the sequence of orders, families, and, in some cases, the taxa within them, have been reshuffled. One of the main strengths of the new sequence is that it makes sense of the bewildering variety of passerines, where attempts to infer relationships based on bird structure are confounded by convergent evolution. There hasn’t been a final decision to use the Sibley and Ahlquist sequence for the new Roberts, but we present the draft list here in the revised sequence to familiarise people with the concept. While Sibley and Ahlquist's arrangement is not absolutely 'right', it is much more accurate than the Wetmore sequence.
Revisions to the passerine systematic sequence
The passerines comprise more than half of all bird species, and their relatively uniform structure has posed significant problems for taxonomists and systematists for many years. Many different sequences have been proposed, but it is only with the advent of molecular techniques that character sets free from convergence have allowed seemingly sensible interpretations of passerine relationships. Sibley and Ahlquist's rearrangement of the passerines based on DNA-DNA hybridisation was probably their finest achievement, and has won greater acceptance than many of their other proposals regarding relationships among the orders of birds. However, there has long been concern about the ability of DNA-DNA hybridisation to reconstruct accurate phylogenies. Two recent studies using conserved nuclear genes independently suggest that although Sibley and Ahlquist were on the right track, some of the details regarding the phylogeny of passerines were incorrect. Specifically, it now appears that the Passerida are not a separate radiation from the Corvida, but merely an extremely speciose lineage within the corvid radiation. This is exciting for southern hemisphere birders, because it implies that all passerines evolved in the south, around the time of the break up of Gondwanaland, some 80-85 million years ago. The new studies also rearrange several passerine families, for example placing larks as sister to the Cisticolidae. Although it is early days, the concordance in overall structure revealed by the two studies argues that the structure inferred by Keith Barker and his colleagues likely is closer to the true evolutionary history of the passerines than that proposed by Sibley and Ahlquist. Many more taxa still need to be added to the data set before we have a robust passerine tree, but we enthusiastically adopt the newly-proposed sequence as a step in the right direction.