The whole of continental America, from the Arctic regions to almost the extreme of South America, and from ocean to ocean, is inhabited, so far as known, by but this one species of _Tinnunculus_. But in different portions of this vast extent of territory the species experiences modifications under the influence of certain climatic and other local conditions, which are here characterized as geographical races; these, let me say, present their distinctive characteristics with great uniformity and constancy, although the differences from the typical or restricted _sparverius_ are not very great. The _F. sparverius_ as restricted, or what is more properly termed var. _sparverius_, inhabits the whole of North and Middle America (both coasts included, except those of the Gulf of Mexico and Caribbean Sea), south to the Isthmus of Panama. Throughout this whole region it is everywhere nearly the same bird. This variety appears to represent the species in its greatest purity, being a sort of central form from which the others radiate. The most typical examples of the var. _sparverius_ are the specimens in the large series from the elevated regions or plateau of Mexico and Guatemala. In these the rufous of the crown is most extended (in none is it at all restricted), and the ashy portions are of the finest or bluest and lightest tint. All specimens, of quite a large series, from the peninsula of Lower California, are considerably smaller than any others, the smallest (1,693 ♂ ad. San José; J. Xantus) measuring, wing, 6.50; tail, 4.20, and tarsus, 1.30; the dwarfed size of these, however, is their only distinguishing feature. Two specimens (50,199, ♂, Cape Florida, and 10,345, ♀, Indian Key) from Florida differ from others in the unusual development of the bill, which toward the end is more suddenly curved, and the point considerably lengthened; these specimens have, also, only a tinge of rufous on the crown, thus showing a proximity to the var. _isabellinus_. The large bill, however, is no more than would be looked for in specimens from that region. Along the Gulf border of the United States, and the Caribbean and North Atlantic coasts of South America (probably the whole Atlantic coast of tropical and subtropical America), the true _sparverius_ is changed into what Swainson has called “_Falco isabellinus_,” which differs from the former only in having the cinereous of the crown and wings considerably darker (as well as less bluish), approaching plumbeous; the rufous of the crown is totally absent, or only present in faint touches; the lower parts are of a deeper ochraceous, and the black spots on the breast and sides sparse.