Observations of marked individuals indicate that the Seaside Sparrow is monogamous and territorial, whereas the Sharp-tailed Sparrow is promiscuous, and at least the male is non-territorial. The male Seaside Sparrow defends its territory by chasing and singing. The male Sharp-tailed Sparrow confines itself to a breeding home range. This range is not a territory; it is inhabited by several males. Female Sharp-tailed Sparrows may be territorial; this is not certainly known. The Seaside Sparrow sings louder, more distinctly, more often, and from more exposed perches than does the Sharp-tailed Sparrow. These characteristics seem to be correlated with territorial habits. Other calls are described and their functions are discussed. The Seaside Sparrow nests in marsh-elder bushes, or in areas of mixed vegetation. The Sharp-tailed Sparrow prefers the inner, drier areas of a marsh, where black grass is dominant. The Seaside Sparrow places its nest farther above the ground than does the Sharp-tailed Sparrow. Both species used only black grass in constructing the nest. Copulation is described. The incubation period was not determined for either species. Three or four eggs seem to be a normal clutch. Females do all of the incubating. The young remained in the nests nine to ten days. These nests, of course were disturbed, for I visited them at least daily. The nestlings of the Seaside Sparrow are fed by both parents. Male Sharp-tailed Sparrows seem not to know the location of the nests and take no part in rearing the young at least up to time of fledging. The natal down of both species is described. Data on growth and behavior of the young are presented. Seaside Sparrows obtained most of their food from the shoreline of the marsh, in areas of open mud and smooth cord-grass. The plumage of the adult matches, in color, this mud. The Sharp-tailed Sparrow feeds everywhere in the marsh, but mostly in areas of dense and matted black grass. The plumage on the dorsum of this species is brown and streaked resembling the dead grass. Juvenal Seaside Sparrows, which spend most of their time concealed in the dense grass, resemble adult and juvenal Sharp-tailed Sparrows in plumage. Sharp-tailed Sparrows molt completely twice per year. The Seaside Sparrow molts but once per year. The difference in number of molts, too, is correlated with habitat preference, since the grassy forage habitat of the Sharp-tailed Sparrow must result in greater abrasion of the plumage than does the open feeding habitat of the Seaside Sparrow. LITERATURE CITED ALLEN, GLOVER M. 1925. Birds and their attributes. Marshall Jones Co., Boston, Massachusetts. xiii + 338 pp., frontispiece, 45 pls., 6 figs. CRUICKSHANK, ALLAN D. 1942. Birds around New York City, where and when to find them. The American Museum of Natural History, Handbook Series, No. 13, New York. xvii + 489 pp., frontispiece, 35 pls. DWIGHT, JONATHAN