CLASSIFICATION

Birds are egg-laying, feathered vertebrates in the class Aves (Latin for "birds"). They are divided into orders (recognized by the suffix "-iformes"), large groupings that include many different birds that appear to be close evolutionary relatives. For example, the order Pelecaniformes includes pelicans, tropicbirds, cormorants, darters, frigatebirds,

The distinctive ridgwayi subspecies of Northern Bobwhite, Masked Bobwhite, is an endangered bird of northern Mexico. A few hundred also persist in southern Arizona at Buenos Aires National Wildlife Refuge.

boobies, and gannets, all of which have completely webbed feet and specialize in eating aquatic prey. Orders are divided into families (suffix "-idae"); within families there are often subfamilies ("-inae"). Families are further divided into genera (plural of genus) and species. A species is traditionally defined as a population of like organisms capable of breeding together and producing fertile offspring. Every species is assigned a scientific name, which consists of a genus name (a taxonomic rank that may include one or dozens of different species) and a species name. For example, Northern Bobwhite is a bird (class Aves) in the order Galliformes and the family Odontophoridae; it is placed in the genus Colinus (derived from a Central American word for "partridge") and given the species name virginianus (from Virginia); the scientific name Colinus virginianus identifies this species unambiguously to any ornithologist in the world.

Subspecies are geographically distinct populations, or "races," of a species that show different characteristics; they are assigned a three-part scientific name, called a trinomial. The trinomial of the distinctive northern Mexican subspecies of Northern Bobwhite known as "Masked Bobwhite" is Colinus virginianus ridgwayi. The subspecies found in much of the Southeast is Colinus vriginianus virginianus and is called the "nominate" subspecies because it was the first to scientifically named and described. (Nominate subspecies bear the same subspecies name—in this case, virginianus—as the species name.) When referring to multiple subspecies, this guide often uses the shorter word "taxa" (plural of taxon).

	Canada Goose Branta canadensis
	Cackling Goose Branta hutchinsii

Ongoing changes: The classification of genera and species is often revised to reflect new information and changing opinions. Many populations of birds (especially gulls, waterfowl, and hummingbirds) regularly hybridize—that is, they interbreed with other species—and some produce hybrid young capable of breeding. Some species hybridize so readily with other species that in certain areas hybrids may actually be more common than either parent species.

Modern ornithologists see instances of hybridization not as a troublesome fly-in-the-ointment but as fascinating evidence of the ongoing process of speciation, in which various populations evolve and interact and over time may give rise to new species. Every year, somewhere in the world, ornithologists elevate a subspecies of bird to the level of full species. The four smallest subspecies of Canada Goose were recently split into a full and separate species under the English name Cackling Goose and the scientific name Branta hutchinsii. Species are also regularly combined (or lumped), becoming a single species; the former species are then often designated as subspecies. For example, in 2005 White Wagtail and Black-backed Wagtail were combined as one species, White Wagtail.