Even in well-studied groups such as birds, there are specimens that experts cannot identify. For example, the Pterodroma petrel shown at right was closely examined and photographed after it was captured on a cruise ship outside Maui in 2003, but not conclusively identified. More generally, some closely-related bird species cannot be reliably distinguished morphologically, particularly juveniles and adults in non-breeding plumage, limiting the value of banding efforts in determining population size, mortality, and range, for example.
When morphologic identifications are routinely incomplete, it may be worthwhile to routinely analyze DNA. For birds this is usually simple as a single breast feather plucked from a live bird and stored dry at room temperature generally contains sufficient DNA for barcode analysis.
Of course, for reliable identifications, a comprehensive reference library of DNA sequences is essential. The need for a well-stocked library is highlighted by an article in April 2007 Ibis on skuas. Skuas are large, brown, gull-like predatory birds that nest in polar regions and migrate widely in open oceans, although their usual routes are not known. Young skuas do not reappear on breeding grounds until they are 3 years old, and any reports on non-breeding birds are of great interest. In an earlier paper (Ibis 146:95, 2004) using mitochondrial DNA analysis (698 bp of 12s and cytochrome b), the researchers concluded that 2 birds found in England in 2001 and 2002 constituted the first records of Brown Skuas (Stercorarius antarctica) in Europe. In this year’s follow-up paper, they dropped that conclusion, as sampling of a larger number of individuals showed that the 3 south polar skua species, S. antarctica, S. chilensis, and S. maccormicki are not distinguished by 12s/cytb mitochondrial DNA sequences.
All told, this is a lot of academic effort for an identification question that could have been answered quickly by a web inquiry if there were a comprehensive library of avian DNA barcodes. So far, researchers have contributed about 8,600 DNA barcode sequences from about 1,800 of the 10,000 known avian species (see www.barcodingbirds.org ), helping create an enduring reference work that will be of long-term use by a large community of scientists, regulatory personnel, and general public who are interested in birds.
In closing, one interesting finding in the 2007 “retraction” paper is the general absence of mitochondrial genetic variation among south polar skua species. Two recent studies of southern polar skuas (Polar Biol 2006 29:153; J Ornithol 2006 147 (suppl):238 ) showed regular hybridization with normal reproductive success between C. antarctica and C. maccormicki, and analysis of mitochondrial hypervariable regions indicated “strong gene flow and little genetic differentiation among southern hemisphere taxa”, which is what population biologists usually say when they study single species. Biologists are generally loathe to “lump” species, but these findings suggest it may be more accurate to consider the south polar skuas as one species.