Recognizing the limits of species is one of the great challenges of biology. Some species are remarkably variable; humans with our ranges of skin color provide a nice example. On the other hand, some separate species can scarcely be separated based on their morphology. The willow flycatcher and alder flycatcher are dead ringers for each other, yet the distinctive songs of the males ensure that only willow females will mate with willow males and alder females with alder males.

A branch of biology called systematics is devoted to identifying the limits of species and to understanding the relationship among the species. We still follow the basic classification scheme erected by Charles Linnaeus nearly 300 years ago. Closely related species are placed in the same genus. Closely related genera are placed in the same family and so on upward in the scheme to orders, classes and phyla.

Systematic classifications were initially based on the structure of organisms. Over the years, systematists have used other traits to help understand species limits and interrelationships. Behavior can be an informative trait. The ability to interbreed is used by some systematists as an indication of a “good” species. In the past few decades, comparisons of DNA have greatly clarified and refined our understanding of the relatedness of various groups of organisms.

Like any scientist, a systematist regards her understanding of the relationships of a particular group of organisms (like the shorebirds, for instance) as a working hypothesis. The hypothesis is provisionally regarded as true but needs continued testing. As our knowledge grows, hypotheses have to be rejected. Like any science, systematics is a dynamic field.

A nice example of our changing understanding is the yellow-rumped warbler. Western birds have yellow throats while eastern birds have white throats. These birds were initially treated as separate species. The black-fronted warbler in Mexico and Goldman’s warbler in Guatemala, similar to the Myrtle warbler and Audubon warbler, also were described as distinct species. However, all of these four species based on additional systematics work were combined into the yellow-rumped warbler in 1973. It’s possible that some or all of these forms will be split back into separate species in the future.

To keep track of these taxonomic changes for birds, the American Ornithologists Union erected its Check-list Committee. This committee is charged with producing its Check-list of North American birds, the official source on the taxonomic classification of birds of North and Middle America. The current version is the seventh, published in 1998. You can see it at http://www.aou.org/checklist/north/print.php.

The committee monitors publications on bird systematics and revises the check-list as needed. The committee usually publishes a supplement in the ornithological journal, the Auk, every July to detail any changes to the official document.

Most of the changes in the 2013 supplement pertain to birds of middle America, so I will not cover them here. The supplement does describe some taxonomic changes to some shorebirds: surfbird, buff-breasted sandpiper, spoon-billed sandpiper, broad-billed sandpiper. The buff-breasted is a regular passage migrant in Maine and the ruff is an occasional vagrant. Each of these species was formerly placed in its own unique genus. Recent systematics work has shown these species are not sufficiently different from other shorebirds to merit their own genus. Each of these five genera has now been eliminated and all five species are now placed in the genus Calidris, which includes a number of shorebirds including sanderling, semipalmated sandpiper, red knot and Dunlin. If you are a stickler for the scientific names of birds, make those changes in your field guide.

The other change may affect your life list if you have birded in the West. The sage sparrow has been split into the northern sagebrush sparrow and the southern Californian Bell’s sparrow.

Herb Wilson teaches ornithology and other biology courses at Colby College and perches in South China. He welcomes reader comments and questions at:

whwilson@colby.edu