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 which follows only the general trend of the land mass. Also it is to be remembered that whatever main routes are described, there remains 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 and the purple sandpiper, 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. [Illustration: _Figure 9._--Breeding and wintering ranges and migration of Harris's sparrow, an example of a narrow migration route through the interior of the country. The heavy broken lines enclose the region traversed by the majority of these finches; the light broken line encloses the country where they occur with more or less regularity; while the spots indicate records of accidental or sporadic occurrence.] Among land birds that have a definite migration, the Ipswich sparrow has what is probably the most restricted 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: _Figure 10._--Distribution and migration of the scarlet tanager. During the breeding season individual scarlet tanagers may be 1,900 miles apart in an east-and-west line across the breeding range. In migration, however, the lines converge until in southern Central America they are not more than 100 miles apart. For migration paths of other widths see figures 9, 11, and 12.]