This Hawk constructs no nest, but makes use of hollow trees, the deserted hole of a Woodpecker, or even an old Crow’s nest. Its eggs are usually as many as five in number, and Mr. Audubon once even met with seven in a single nest. The ground of the eggs is usually a dark cream-color or a light buff. In their markings they vary considerably. Five from a nest in Maryland were covered throughout the entire surface with small blotches and dottings of a light brown, at times confluent, and, except in a single instance, not more frequent at the larger end than the smaller. The contents of a nest obtained by Mr. Audubon on the Yellowstone River had a ground-color of a light buff, nearly unspotted, except at the larger end, with only a few large blotches and splashes of a deep chocolate. In others, interspersed with the light-brown markings are a few of a much deeper shade. In some, the eggs are covered with fine markings of buff, nearly uniform in size and color; and others again are marked with lines and bolder dashes of brown, of a distinctly reddish shade, over their entire surface, and often so thickly as nearly to conceal the ground. The eggs are nearly spherical. The average length is 1.38 inches by a breadth of 1.13. They are subject to variation in size, but are uniform as to shape. They range in length from 1.48 to 1.32 inches, and in breadth from 1.08 to 1.20 inches. The eggs of _Tinnunculus sparveroides_, from Cuba, and of var. _cinnamominus_ from Chile, differ in size and markings from those of North American birds. Their ground-color is much whiter, is freer from markings which have hardly any tinge of rufous, but are more of a yellowish-brown. The Cuban egg measures 1.28 by 1.08 inches; the Chilian, 1.25 by 1.08. GENUS POLYBORUS, VIEILLOT. _Polyborus_, VIEILL. 1816. (Type, _Falco brasiliensis_, GMELIN. _P. tharus_, MOLINA.) _Caracara_, CUVIER, 1817. (Same type.)