I have presented an analysis of the species density of the breeding birds of continental Canada, the United States (excluding Hawaii) and Mexico following the method of Simpson (1964) in his analysis of North American mammals; seabirds were not analysed. A map of species density was constructed using a 153 mile square-quadrant grid system super-imposed upon the range data of each species. Many of the same trends that Simpson found for mammals—latitudinal gradient, topographic effect, east-west gradient, peninsula effect—were also displayed in the avian species density pattern. There appeared to be a greater decrease in species density in southeastern United States in birds, and it is suggested that this may reflect historical factors in the form of repeated glaciation. A family density map and a species/family ratio map were also constructed, and shown to reflect trends similar to the species density map. Species density maps were constructed for the seven largest families of birds; these were the Anatidae, Accipitridae, Scolopacidae, Trochilidae, Tyrannidae, Parulidae, and Fringillidae. The characteristic pattern of each family is discussed in relation to historical factors and present ecological conditions.