22 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
with a rush, and during the height of the migration season in Saskat- 
chewan the ‘temperature in the southern part of the Mackenzie Valley 
just about equals that in the Lake Superior area, which is 700 miles 
farther south. Such conditions, coupled with the diagonal course of 
the birds across this region of fast-moving spring, exert a great influ- 
ence on migration and are the chief factors in the acceleration of speed 
of travel 
Variations in speed of migration, in different parts of the eon 
are illustrated also by the movements of the cliff swallow (fig. 5), 
which breeds from Mexico to Alaska and winters in Brazil aad 
Argentina. It would be expected in spring to appear in the United 
States first in Florida and Texas, then in the southern Rocky Moun- 
tain region, and finally on the Pacific coast. As a matter of fact, 
however, the earliest spring records come from north-central Cali- 
fornia, where the bird usually is common before the first arrivals are 
observed in Texas or Florida. The route taken, for many years a 
migration problem, was solved when it was found that these swallows 
went around the Gulf of Mexico rather than acrossit. The isochronal 
lines on the map show the more rapid advance along the Pacific coast. 
By March 20, when the vanguard has not quite reached the lower 
Rio Grande in Texas, the species is already north of San Francisco 
in California. 
ALTITUDES AT WHICH BIRDS TRAVEL 
At one time students of bird migration held firmly to the theory 
that normal migration takes place at heights above 15,000 feet, rea- 
soning (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 per- 
fect 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 
