34 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
many perching birds en route from the Mississippi Valley to the 
Rocky Mountain region, some of the hawks and many ducks and 
shore birds pay the valley scant attention. They may arrest their 
fall journey to feed among cottonwoods or along sand bars, but when 
ready to resume their flight they leave the river and fly directly 
south over the more or less arid region that lies between the Arkansas 
and the Rio Grande. 
WIDE AND NARROW MIGRATION LANES 
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 migra- 
tory 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 sand- 
piper (Arquatella maritima), which are normally found only along the 
coasts, must have extremely narrow routes of travel. They are lim- 
ited 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 spar- 
row (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. 
Harris’s sparrow (Zonotrichia querula) supplies an interesting exam- 
ple of a narrow migration route in the interior of the country (fig. 16). 
This fine, large finch is known to breed only in the region from Fort 
Churchill, on the west shore of Hudson Bay, northwest to the shores 
of Great Bear Lake. Very few actual breeding records of the species 
