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. 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.
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 .