Birder's Guide

OCT 2014

Birder's Guide is the American Birding Association's newest publication. Each issue focuses on a key subject, providing tips from experienced birders on a wide variety of topics like Travel, Listing & Taxonomy, Gear, and Conservation & Community.

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23 October 2014 | Birder's Guide to Listing and Taxonomy ue to rigorously debate not only the most appropriate concepts to use for their feld, but also how to apply them. Nowadays, there is strong appreciation for the use of phylogenies in making decisions about what constitutes a species, and this has driven the current trend towards more splitting than lumping. Recently, near-ly every successfully passed AOU proposal for species splitting has incorporated a gene-based phylogeny. When a proposal that includes this information fails, it is usually due to limited sampling of individ- uals, limited sampling of genes, or both. In 2013, proposals advocating splitting Sandwich Tern and Nutting's Flycatcher ran afoul of the above issues. Belief in the importance of monophyly of species also has become more important, and was a ma- and Unitt 2002; Remsen 2010; Zink 2004; Burbrink 2001). Statistics are frequently used in the rec- ognition of subspecies (Amadan 1949), which has led to "the 75% rule". It states that a population is a subspecies when at least 75% of the population can easily be identifed as belonging to that subspe- cies. Other authors have argued for even stricter guidelines, calling for 90 or even 100% accuracy (Patten and Unitt 2002; Remsen 2010). Unfortunately, most sub- species were described before these rules came into place, in the 1800s and early 1900s, and most checklists—even works such as the Handbook of Birds of the World and Clements—largely just parrot the sub- species classifcation from works well over a half a century old (Remsen 2010). Even proponents of subspecies are skeptical at how many are valid; Remsen (2010) roughly estimated that around three- fourths of all described North American bird subspecies are invalid. Because of the dubious nature of many currently described subspecies, some re- searchers have called for wholesale aboli- tion of the concept. Subspecies have tradi- tionally been based on plumage and ana- tomical characteristics, but many of these features may vary continuously within a population and are inappropriate for use in defning discrete units. An example of this can be seen in birds like Song Sparrows, which show extensive variation in dark- ness of their plumage, but these changes are subtle and populations grade into one another. (This is referred to as clinal variation.) A subspecies defned on the basis of tail length might designate a com- pletely different entity than one based on bill length; the two features needn't show the same pattern of geographic variation. Finally, (usually old) subspecies classifca- tions often strongly disagree with (usually new) phylogenies based on genetic data, suggesting that subspecies may not prop- erly defne evolutionary units. Species Concepts: Today and the Future With all the taxonomic concepts described above (and many more!), scientists contin- also have some sort of unique trait or set of traits that separates them from other popu- lations/species; this is referred to as diag- nosability. Unlike in the BSC, the ability to interbreed is not the primary criterion used in delineating species, although reproduc- tive isolation often leads to diagnosability and monophyly over time. The PSC has been endorsed by many researchers, as it can easily accommodate issues of allopatry and hybridization that are troublesome within the BSC. But the PSC has been heavily criticized; diagnos- ability can be subjective, and it's uncertain how different a group of organisms must be to be considered a separate species (Lee 2003). In addition, phylogenetic diagrams may vary by which genes (and how many) are used. This can sometimes manifest in misleading differences in how species should be organized (Moore 1995), as well as in misdiagnosis of species and/or in dif- ferent delineations being recognized by different studies. Finally, some researchers have worried that application of the PSC may lead to an overproliferation of species, making it diffcult to concentrate conserva- tion efforts (Isaac et al. 2004). What about Subspecies? The species is probably the most familiar "unit" of biodiversity, but there are units below it. The most well known is the subspecies. Subspecies constitute units of geographic variation below the rank of the species. This geographic variation can take the form of differences in plumage coloration, body size, and size and length of features of the anatomy, such as bill depth or wing length. In recent decades, many birders have paid increasing atten- tion to subspecifc variation because they enjoy the identifcation challenges; some birders are holding out for the possibility that a subspecies may later be elevated to species rank. Species concepts have re- ceived extensive coverage and debate in the literature, but the subspecies concept has been largely neglected. What debate has occurred has focused on how exactly to defne them as geographic variants, and whether they should be used at all (Wilson and Brown 1953; Amadon 1949; Patten Taxonomy can have very serious implications. In the case of the "Dusky" Seaside Sparrow, being downgraded to "just" a subspecies may have been a deathblow for the now- extinct taxon. The "Cape Sable" Seaside Sparrow, shown here, is found only in a small portion of southwestern Florida, and its status as "just" a subspecies may similarly hinder its future conservation. Photo © Brian E. Small

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