THE MIGRATION OF NORTH AMERICAN BIRDS 17 
or crossing the face of the moon, have been observed to fly without 
hurry or evidence of straining to attain high speed. 
The speed of migration also is demonstrated by the dates of arrival, 
particularly during the spring movement. The Canada goose affords 
a typical example of regular, but slow, migration. Its advance north- 
ward at this season is at the same rate as the advance of the season 
(fig. 8). In fact, the isotherm of 35° F. appears to be a governing 
factor in the speed at which these geese move north, and over their 
entire trip the van- 
guard follows closely 
the advance of this 
isotherm. 
Few species perform 
such regular migra- 
tions, Many waiting in 
their winter homes 
until spring is well 
advanced and then 
moving rapidly to 
their breeding 
erounds. Sometimes 
thisadvanceisso rapid 
that the later mi- 
grants actually catch 
up with species that 
for a month or more 
may have been press- 
ing slowly but steadily 
northward. 
One of the best ex- 
amples of rapid mi- 
eration is found in the 
eray-cheeked thrush 
(Hylocichla minima 
alicvae). This bird 
winters in Colombia, 
Kceuador, Peru, Vene- 
zuela, and British 
Guiana and does not 
start its northward 
journey until many 
other species are well 
2 B213M 
on their way. Tt does  fieure 7.—Duck hawk, or peregrine falcon (Falco peregrinus): a day 
not appear in the migrant and one of the fastest and most graceful fliers of all North 
American birds. 
United Statesuntil the 
last of April—April 25, near the mouth of the Mississippi, and April 30 
m northern Florida (fig. 9). A month later, or by the last week in 
May, the bird is seen in northwestern Alaska, the 4,000-mile trip 
from Louisiana haying been made at an average speed of about 130 
miles a day. 
Another example of rapid migration is furnished by the yellow, or 
summer, warbler. Coming from the Tropics, the birds reach New 
134984°—35 
2 
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