Not all proposals sent to the AOU Checklist Committee are passed. However, looking at the specifics of each proposal reminds us that distinctive subspecies could become new species in the future, (recent splits of Winter Wren and Whip-poor-will, e.g.).
A proposal has been made to split Mountain Chickadee (Poecile gambeli) into two species. In 1956, Behle divided seven Mountain Chickadee subspecies into three groups, the Rocky Mountain “gambeli” group; the Great Basin “inyoensis” group, and the Pacific “abbreviatus” group. In 2007, Spellman et al. used mitochondrial DNA sequences to investigate the phylogeography of Mountain Chickadee populations. Based on the results of their research they recommend splitting Mountain Chickadee into two species. They suggest the common name, Gambel’s Chickadee (Poecile gambeli), for birds found in the Great Basin and Rocky Mountains and Bailey’s Chickadee (Poecile baileyae) for birds found in coastal California, Sierra Nevada, and the Cascades (the name honoring Florence Merriam Bailey, a pioneer ornithologist).
In 1973, the AOU lumped Myrtle and Audubon’s warblers into Yellow-rumped Warbler (Dendroica coronata) based on studies in the contact zone between the two groups; in particular examining the width of the zone of contact. Four subspecies of Yellow-rumped Warbler are currently recognized, D. c. coronata, D. c. auduboni, D. c. nigrifrons, and D. c. goldmani. Mitochondrial genetic variation has been studied across the populations of all four taxa. All four groups are visually distinct and there are fixed differences between at least one genetic marker in each group. The recent proposal suggests it may be time to a) split all four taxa into four species, b) split only D. coronata from the rest, or c) split D. coronata and D. goldmani, and leave nigrifrons as a subspecies. D. goldmani, sometimes called Goldman’s Warbler, is found in the high mountains of southern Chiapas, Mexico, south into western Guatemala. D. nigrifrons, part of the Audubon group, resides in northwest Mexico, reaching the Arizona/Mexican border. D. nigrifrons individuals have black mottling ventrally extending onto the belly (Howell and Webb).
To learn about other recent proposals check www.aou.org/committees/nacc/proposals/2010-A.pdf