THE MIGRATION OF NORTH AMERICAN BIRDS Ba 
accurate data resulting from altimeter observation from airplanes, 
are slowly accumulating. It is, of course, obvious that some birds 
that cross mountain ranges during migration must attain a great alti- 
tude. Observers at an altitude of 14,000 feet in the Himalayas have 
recorded storks and cranes flying so high that they could be seen only 
through field glasses. Being beyond the range of unaided vision they 
must have been at least 6,000 feet above the observers, or at an actual 
altitude of 20,000 feet above sea level. Such cases, however, are 
exceptional, as aviators have reported that they rarely meet birds 
above an altitude of 5,000 feet. 
It is now known that migration in general is performed below an 
altitude of 3,000 feet. Some proof of this statement is available. 
Observations made from lighthouses and other points of vantage 
indicate that migrants commonly travel at altitudes of a very few feet 
to a few hundred feet above sea or land. Sandpipers, sanderlings 
(Crocethia alba), and northern phalaropes (Lobipes lobatus), observed 
in migration on the Pacific oceanic route, have been noted to fly so 
low that they were visible only as they topped a wave. Observers 
stationed at lighthouses and lightships off the English coast have 
similarly recorded the passage of land birds, which sometimes flew 
just above the surface of the water, and rarely above 200 feet. During 
the World War broad areas in the air were under constant close 
surveillance, and among the airplane pilots and observers many took 
more than a casual interest in birds. Of the several hundred records 
resulting from their observations only 36 were of birds flying above 
5,000 feet, and only 7 above 8,500 feet. Cranes were once recorded 
at an altitude of 15,000 feet, while the lapwing (Vanellus vanellus) 
was the bird most frequently. seen at high levels, 8,500 feet being its 
ereatest recorded altitude. 
These observations naturally relate only to daytime travelers, but 
there is no reason to believe that nocturnal migration is performed at 
higher altitudes. The fact that many birds are killed each year by 
striking the lanterns at lighthouses, or other man-made obstructions, 
does not, however, furnish conclusive proof that low altitudes are 
generally used. during nocturnal flight, for it should be recalled that 
these accidents occur chiefly in foggy or unsettled weather, and also, 
that powerful lights have a great attraction for many species of birds. 
The altitude at which birds travel is affected by other weather con- 
ditions also. For example, flight at the higher elevations is facilitated 
on clear, warm days by the currents of warm air that ascend from 
broad areas. 
ORIENTATION 
There probably is no single aspect of the entire subject of bird 
migration that challenges our admiration for birds so much as the 
unerring certainty with which they cover thousands of miles of land 
and water to come to rest in exactly the same spot where they spent the 
previous summer or winter. The records of birds marked with 
numbered bands afford abundant proof that the same individuals of 
many species will return again and again to their identical nesting 
sites. These data show also that many individuals migrate in fall 
over the same route, year after year, making the same stops, and 
finally arriving at the precise thicket that served them in previous 
winters. 
