MIGRATION. 
371 
The line of migration is usually in a south-westerly 
direction, deviating sometimes to the westward and 
sometimes to the southward; the line of flight generally 
coincides with the course of such rivers, streams, woods 
and valleys as take a similar direction; deep gorges in 
the mountains, especially when opening into a still larger 
valley, are much frequented by migratory flocks. If a 
river or valley changes its direction from south-west or 
north-east, it is followed until the latter direction has 
deviated about ninety degrees from the earlier line. The 
passes, no matter what direction they take, are always 
sought, if they form a break through a range of moun¬ 
tains lying directly across the line of flight. Tschudi 
says, that in Switzerland those roads which occupy the 
deepest passes are always used by birds of passage; the 
same may be asserted of the Pyrenees. The great 
number of these breaks in the mountains will not allow 
of our enumerating them here; the routes over the flat 
country may be easier named, though a glance at the 
map, and a reference to what has just been stated, will, 
of itself, be sufficient. In Germany, of all rivers the 
Rhine is one whose valley is the main track followed 
by these feathered armies; after the Rhine come the 
Danube, the Elbe, and the Oder; all other rivers and 
brooks are, however, more or less frequented by 
migrating flocks; in France, the valleys of the Rhone 
and the Garonne; in Spain, the Guadalquiver and 
Guadiana; in Russia, the Vistula, Dnieper, Don, and, 
above all, the Volga ; in North-eastern Africa, the Nile ; 
in Asia Minor, the Euphrates and the Tigris; in the 
rest of Asia, those larger rivers, which run north 
and south; in North America, the rivers Connecti¬ 
cut, Hudson, Delaware, Susquehannah, and the mighty 
