Apparently a pigeon hawk.

April 25. 

Horace Mann brings me apparently a pigeon hawk. The two middle tail-feathers are not tipped with white and are pointed almost as a woodpecker's.

Audubon's "Pigeon Hawk" [Merlin]
(Falco columbarius")
"Wings from two to three inches shorter than the tail, on the middle feathers of which are five, on the lateral six, broad whitish bands. Adult male with the cere greenish-yellow, the feet pale orange, the upper parts light bluish-grey, each feather with a black central line; lower parts reddish or yellowish-white, the breast and sides with large oblong brown spots; tibial feathers light red, streaked with blackish-brown. Female with the cere and legs greenish-yellow, the upper parts dark greyish-brown, the lower pale red, spotted as in the male. Young with the head light reddish-brown, streaked with dusky, the upper parts brownish-grey, the feathers margined and spotted with pale red, throat white, lower parts pale red, streaked with brown. The tail-bands vary from pale red to white."

H. D. Thoreau, Journal, April 25, 1861

Apparently a pigeon hawk. See  April 16, 1855 ("What I call a pigeon hawk, probably sharp-shinned.”); May 4, 1855 ("I think that what I have called the sparrow hawk falsely, and latterly pigeon hawk, is also the sharp-shinned , for the pigeon hawk’s tail is white-barred."); July 2, 1856 (“Looked at the birds in the Natural History Rooms in Boston. Observed no white spots on the sparrow hawk’s wing, or on the pigeon or sharp-shinned hawk’s.");  October 22, 1855 (“Suddenly a pigeon hawk dashes over the bank very low and within a rod of me . . . It sits a few moments, balancing itself and spreading its tail and wings, -- a chubby little fellow. Its back appears a sort of deep chocolate-brown.”); April 29, 1856 ("Scared a small dark-brown hawk from an apple tree. . .I saw with my glass that his tail was barred with white. Must it not be a pigeon hawk then? He looked a dark slate as he sat, with tawny-white thighs and under head, . . .I think I have not described this white-barred hawk before");  September 14, 1859 ("What kind of hawk is this? I can learn nothing from Wilson and Nuttall. The latter thinks that neither the pigeon nor sparrow hawk is found here !!"); April 27, 1860 ("I saw yesterday, and see to-day, a small hawk which I take to be a pigeon hawk. . . . I am decided by his size (as well as color) and his low, level skimming.") Compare April 25, 1860 ("The size more than anything made me think it a sharp-shin.") and  J. J. Audubon ("Every one knows the Sparrow-Hawk [Falco sparverius], the very mention of its name never fails to bring to mind some anecdote connected with its habits . . .”) See also  A Book of the Seasons by Henry Thoreau, The Pigeon Hawk (Merlin)