The average size of the eggs of this bird is 1.56 by 1.94 inches. The color is usually a uniform dull white, but is occasionally tinged with as light bluish shade. They are nearly spherical, though not more so than the eggs of several species, and are equal at either end. Their surface is slightly granulated. The number of the eggs varies from three to four, though occasionally there are five in a nest. The maximum length of the egg of this species is 2.00 inches, the minimum 1.85; the maximum breadth 1.60, the minimum 1.50 inches. In occasional instances I have known the eggs of this species more or less distinctly marked, especially about the larger end, with blotches of a light yellowish-brown. Those most distinctly marked in this manner were taken and identified by Mr. Paine. A nest of this Hawk, found by Dr. J. W. Velie, was built on a poplar-tree, about forty feet from the ground, and was composed of sticks and lined with moss and leaves. There was a small cleared space of three or four rods in extent, in the middle of which the tree stood, and about a quarter of a mile from the main channel of the Mississippi River, on Rock Island. The Cooper Hawk was found on Mount Tom by Mr. William Street, nesting for the most part in pine or hemlock trees, usually choosing one in a thick clump. They begin to lay about the first of May, usually depositing four eggs. They are very shy, and it is almost impossible to get within shot of them, even when they have young. They rarely molest the poultry-yard, but seem to live chiefly on small birds and animals. They leave their nest at once whenever it is approached, and will not return until the intruder has gone. The var. _mexicanus_, originally described by Mr. Swainson from Mexican specimens obtained near Real del Monte, has been ascertained to cross our boundaries, and is found in all the territory between the Rocky Mountains and the Pacific, as far north as Washington Territory. Dr. Cooper has never met with this Hawk, but supposes its general habits, and especially those regulating its migrations, closely resemble those of _A. cooperi_, to which the bird itself, in all but size, is so similar. Dr. Coues speaks of it as a common resident species in Arizona. He states that he has seen young birds of this species, reared by the hand from the nest, become so thoroughly domesticated as to come to their master on being whistled for, and perch on his shoulder, or follow him when shooting small birds for their food. They were allowed their entire liberty. Their ordinary note was a shrill and harsh scream. A low, plaintive, lisping whistle was indicative of hunger. Dr. Suckley, who met with this bird on Puget Sound, where a specimen was shot on a salt marsh, states that, while soaring about, it resembled in its motions the common Marsh Hawk, or Hen Harrier (_Circus hudsonius_). SUBGENUS ASTUR, LACEPEDE.