When birds start their southward migration, the movement necessarily involves the full width of the breeding range. Later, there is a convergence of the lines of flight taken by individual birds, owing to the conformation of the land mass, and as the species proceeds southward the width of the occupied region becomes less and less. An example of this is provided by the common kingbird (_Tyrannus tyrannus_), which breeds from Newfoundland to British Columbia, a summer range 2,800 miles wide. On migration, however, its paths converge, until in the southern part of the United States the occupied area extends from Florida to the mouth of the Rio Grande, a distance of only 900 miles, and still farther south the migration path is further restricted. In the latitude of Yucatan it is not more than 400 miles wide, and it is probable that the great bulk of the species moves in a belt that is less than half that width. A migration route, therefore, may be anything from a narrow path that adheres closely to some definite geographical feature, such as a river valley or a coast line, to a broad boulevard that leads in the desired direction and follows only the general trend of the land mass. Also it is to be remembered that whatever main routes are described, there remain a multitude of tributary and separate minor routes. In fact, with the entire continent of North America crossed by migratory birds, the different groups or species frequently follow lines that may repeatedly intersect those taken by others of their own kind or by other species. The arterial routes, therefore, must be considered merely as indicating paths of migration on which the tendency to concentrate is particularly noticeable. In considering the width of migration lanes it will be obvious that certain species, as the knot (_Calidris canutus_) and the purple sandpiper (_Arquatella maritima_), which are normally found only along the coasts, must have extremely narrow routes of travel. They are limited on one side by the broad waters of the ocean and on the other by land and fresh water, both of which are unsuited to furnish the food that is desired and necessary to the well being of these species. Among land birds that have a definite migration, the Ipswich sparrow (_Passerculus princeps_) has what is probably the most rstricted migration range of any species. It is known to breed only on Sable Island, Nova Scotia, and it winters along the Atlantic coast south to Georgia. Living constantly within sound of the surf, it is rarely more than a quarter of a mile from the outer beach, and is entirely at home among the sand dunes and their sparse covering of coarse grass. [Illustration: B4630M