This species appears to nest almost exclusively on cliffs, and rarely, if ever, to make any nests in other situations. In a few rare and exceptional cases this Falcon has been known to construct a nest in trees. Mr. Ord speaks of its thus nesting among the cedar swamps of New Jersey; but this fact has been discredited, and there has been no recent evidence of its thus breeding in that State. Mr. Dall found its nest in a tree in Alaska, but makes no mention of its peculiarities. The eggs of this species are of a rounded-oval shape, and range from 2.00 to 2.22 inches in length, and from 1.60 to 1.90 in width. Five eggs, from Anderson River, have an average size of 2.09 by 1.65 inches. An egg from Mount Tom, Mass., is larger than any other I have seen, measuring 2.22 inches in length by 1.70 in breadth, and differs in the brighter coloring and a larger proportion of red in its markings. The ground is a deep cream-color, but is rarely visible, being generally so entirely overlaid by markings as nowhere to appear. In many the ground-color appears to have a reddish tinge, probably due to the brown markings which so nearly conceal it. In others, nothing appears but a deep coating of dark ferruginous or chocolate-brown, not homogeneous, but of varying depth of coloring, and here and there deepening into almost blackness. In one egg, from Anderson River, the cream-colored ground is very apparent, and only sparingly marked with blotches of a light brown, with a shading of bronze. An egg from the cabinet of Mr. Dickinson, of Springfield, taken on Mount Tom, Massachusetts, is boldly blotched with markings of a bright chestnut-brown, varying greatly in its shadings. _Subgenus_ ÆSALON, KAUP. _Æsalon_, KAUP, 1829. (Type, _Falco æsalon_, GMELIN, = _F. lithofalco_, GM.) _Hypotriorchis_, AUCT. _nec_ BOIE, 1826, the type of which is _Falco subbuteo_, LINN. _Dendrofalco_, GRAY, 1840. (Type, _F. æsalon_, GMEL.) This subgenus contains, apparently, but the single species _F. lithofalco_, which is found nearly throughout the Northern Hemisphere, and in different climatic regions is modified into geographical races. Of these, North America possesses three, and Europe one; they may be distinguished as follows:— Species and Races.