Since I began to study the habits of Gulls, and observe their changes of plumage, whether at the approach of the love season, or in autumn, I have thought that the dark tint of their hoods was in the first instance caused by the extremities of the feathers then gradually changing from white to black or brown, without the actual renewal of the feathers themselves, as happens in some species of land-birds. At Eastport, I had frequent opportunities of seeing the black-hooded males copulating with the brown-hooded females, so that the colour of the head in the summer season is really distinctive of the sexes. I found in London a pair of these birds, of which the sexes were distinguished by the colour of the head, and which had been brought from Greenland. They were forwarded by me to the EARL of DERBY, in whose aviaries they are probably still to be seen. This is certainly the species described in the Fauna Boreali-Americana under the same name; but it is there stated that the females agree precisely with the males, their hood being therefore “greyish-black;” which I have never found to be the case. As to the _Larus capistratus_ of Bonaparte’s Synopsis, I have nowhere met with a Brown-headed Gull having the tail “sub-emarginate;” and I infer that the bird described by him under that name is merely the female of the present species. LARUS BONAPARTII, BONAPARTIAN GULL, _Richards. and Swains._ Fauna Bor.-Amer. vol. ii. p. 425. BROWN-MASKED GULL, LARUS CAPISTRATUS, _Bonap._ Amer. Ornith., vol. iv. Female. LARUS CAPISTRATUS, _Ch. Bonaparte_, Synopsis of Birds of United States, p. 358. BONAPARTIAN GULL, _Nuttall_, Manual, vol. ii. p. 294. Adult Male in Spring Plumage. Plate CCCXXIV. Fig. 1. Bill shorter than the head, nearly straight, slender, compressed. Upper mandible with its dorsal line straight to the middle, then curved and declinate, the ridge narrow, the sides slightly convex, the edges sharp and a little inflected, the tips narrow but rather obtuse, with a slight notch on each side. Nasal groove rather long and narrow; nostrils in its fore part, longitudinal, submedial, linear, pervious. Lower mandible with a slight prominence at the end of the angle, which is long and narrow, the dorsal line then ascending and slightly concave, the ridge convex, the sides nearly erect and flattened.