Like all organisms, species of birds are grouped into hierarchical categories. In theory, these categories help illustrate evolutionary relationships. Although this basic classification system was developed in the 1700s by Linnaeus, a Swedish scientist who described many North American species, it is still being changed today as new discoveries are made and recent advances in DNA technology illuminate relationships among groups of birds. Starting from the highest level, the categories are: Because the system is hierarchical, the highest levels contain the largest numbers organisms, related by the most general characteristics. As you reach the lower levels, fewer kinds of organisms share the more specific characteristics that define the group. Of course, because scientists like to quibble, there are many divisions of the basic categories, for instance superorders, subfamilies, etc. For birds, the taxonomy would be: All birds are in the same kingdom, phylum, and class. At the next level, order, the birds begin to diverge. For instance, the loons are in the order Gaviiformes while the pelicans and cormorants are in the order Pelecaniformes. Next comes the family, with the pelicans in the family Pelecanidae and the cormorants in the family Phalacrocoracidae. At the genus level, the pelicans are in the genus Pelecanus. At the species level, there are the American White Pelican, Pelecanus erythrorhynchos, and the Brown Pelican, Pelecanus occidentalis. The scientific names are all in Latin so that scientists around the world can communicate in a common language. The species is considered the basic biological unit. It is defined as a potentially interbreeding group comprising various populations that are reproductively isolated from other such groups. However, even within species there are sometimes discernible subunits, which are called subspecies. An example would be the Northern Flicker. In the eastern United States, it is called the Yellow-shafted Flicker because of its bright yellow areas under the wings and tail. In western flickers, these areas are red rather than yellow, and the bird is appropriately called the Red-shafted Flicker. In the areas where the ranges of the two overlap, considerable interbreeding results in hybrids between the two. The Yellow-shafted Flicker is given the name Colaptes auratus auratus while the Red-shafted Flicker is Colaptes auratus cafer. They are considered to be the same species but are recognizably different. In the species lists, subspecies that are recognizably different will be mentioned. For example, the Red-shafted Flicker is called Colaptes a. cafer where the a. stands for auratus. The Yellow- shafted Flicker, the nominate form, is referred to as Colaptes auratus. Sometimes, two closely related species hybridize. Such is the case with the Golden-winged and Blue-winged Warblers. Since their offspring have characteristics of both parents and are easily identifiable in the field, they are mentioned in the lists as Vermivora chrysoptera x pinus, where the x indicates hybridization. The nomenclature used in the BBS Home Page follows The AOU Checklist of North American Birds(Seventh edition, 1998), with supplements published in The Auk in July of odd-numbered years.