At one time students of bird migration held firmly to the theory
that normal migration takes place at heights above 15,000 feet,
reasoning (somewhat uncertainly) that flying becomes easier as
altitude is gained. Since the development of the airplane, however,
and with it man's exploration of the upper regions of the air, it
has become common knowledge that rarefied atmosphere adds greatly to
the difficulties of flight. This is due not only to the reduction
in oxygen (whether for gasoline engine or the lungs of a bird) but
also to the lack of buoyancy of rarefied air. Such birds as vultures,
pelicans, cranes, and some of the hawks feel this the least, since
compared with body weight the supporting surface of their wings is
very great, but for the smaller and shorter winged birds lack of
buoyancy at high altitudes presents a difficult obstacle in flight.
Even when flying close to the earth, small birds have to keep their
wings in rapid motion.
Another postulate favoring the high-altitude flying theory was
that the wonderful vision of birds was their sole guidance during
migratory flights; and to keep landmarks in view the birds were
obliged to fly high, particularly when crossing wide areas of water.
This will be considered in greater detail under Orientation (p. 23),
so here it will be sufficient to say that birds rely only in part
upon vision to guide them on migration. Also, it is to be remembered
that there are definite physical limitations to the range of
visibility even under perfect atmospheric conditions. Chief of these
is the curvature of the earth's surface. Thus, if birds flew over the
Gulf of Mexico to Louisiana and Florida at a height of 5 miles, they
would still be unable to see a third of the way across. And yet this
trip is made twice each year by thousands of thrushes, warblers, and
others.
Actual knowledge of the altitude of migratory flight is scanty,
though estimates obtained by means of the telescope, and still more
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
altitude. 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 greatest 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 conditions 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.