Aves—A Taxonomy in Flux
There are big changes underway in bird taxonomy. You've probably noticed that the checklist is always changing. Suddenly the ducks are have replaced the loons in front of the NGS guide. Not too long ago the shrikes and vireos moved from next to the warblers to next to the jays. Maybe you've heard that vultures are storks, or that they're not really storks after all, or that North American tanagers are now cardinals. Then there are the Baltimore and Bullock's Orioles, or are they just Northern, Slate-colored and Oregon Juncos, or maybe it's Dark-eyed today. Not only are species being split or lumped, but everything is being rearranged. A new view of bird taxonomy takes shape as we watch.
There have always been questions and disagreements about bird taxonomy, and it has always been changing (see Bruce, 2003). Since Aristotle or before, people have tried to find a natural way to order all bird species (the very words “genus” and “species” carry the Aristotelean tradition). Early attempts were based on obvious similarities, such as grouping all waterbirds together. But what constitutes a natural order?
Linnaeus simplified scientific nomenclature by naming each type bird with a genus and species, but was unable to tell us what order was natural. It is not until Darwin that we get a clearcut way to order the birds. They should be ordered according to descent. Birds with more recent common ancestors should be grouped together.
Thomas Huxley (1867) was the first to construct a comprehensive bird list on Darwinian lines. He focused on the characteristics of the bird's skull to determine common inherited features that he could base his taxonomy on. Numerous other ornithologists have used morphological traits to classify the birds, with Livezey and Zusi (2007) being the most recent effort of this type.
More recently, focus has shifted to molecular methods, the most important of which is to directly study the stuff of inheritance---DNA. The culmination of the first wave of DNA based taxonomy are the publications by Sibley, Ahlquist, and Monroe (Sibley, Ahlquist, and Monroe, 1988; Sibley and Ahlquist, 1990; Sibley and Monroe, 1990). Sibley, Ahlquist, and Monroe used DNA hybridization to try to classify birds. This, together with increasing emphasis on cladistic methods, has revitalized interest in bird taxonomy. In the 1990's, DNA hybridization was replaced by the much more precise DNA sequencing, allowing direct examination of the genes themselves. This has opened the possibility of constructing a “tree of life” showing the evolutionary relationships between all living organisms. It has brought the prospect of producing a completely accurate taxonomic tree of all living bird species. It is the driving force behind the checklist changes.
A Guide to the New Taxonomy
This set of web pages contains a guess at what the avian part of the tree of life might look like. It examines recent taxonomic changes, and possible changes to come. Some of the projected changes are pretty solid, others are guesses based on current research. Keep in mind that some of my guesses will turn out to be wrong, and some of the research they are based will be wrong or misleading. There will doubtless be taxonomic surprises before the entire avian tree is worked out, just as there have been a number of recent surprises.
The tree I've put together is an interpretation of the genetic relationships studied by many authors. The higher level taxonomy (the non-passerines) I use is primarily based on Hackett et al. (2008), which refines the results of Ericson et al. (2006a) and Fain and Houde (2004). This taxonomy is slowly creeping into the checklists. The latest (2008) of Christidis and Boles Australian checklist incorporates some of these ideas, as does the AOU's South American checklist.
The purpose of these webpages is twofold: To present the new ideas that Fain and Houde's Metaves hypothesis has brought to the higher taxonomy (mostly above the family level), and to examine the wholesale changes that been going on within the various families. The latter has particularly affected the passerine families, which at times seems like they've been run through a blender (see the Sylviidae page for an example).
For more of this discussion, see the next page.
This set of web pages includes tree diagrams, comparisons with other checklists, a family list, an annotated list of species, and several checklists in csv format, which can be imported into Excel or other spreadsheets (world, AOU area, ABA area, South America). The checklists will be updated less frequently than the other pages.
The distinguishing characteristic of Version 2.5 is that it includes genus-level phylogenetic trees for over 90% of all extant bird families. This is all that seems to be feasible at this time.
The higher-level phylogeny follows Hackett et al. (2008) since version 2.1. I've created a pdf that summarizes the changes. For those who are interested, I've left a link at the very bottom of the page to an archived copy of version 1.1 (2007).