52 HOW BIRDS MIGRATE 
The rate at which a bird migrates, however, is of course not to be 
considered its limit of speed. The migrating bird, like the long-distance 
runner, must adopt a pace which will enable it to reach the goal without 
danger of exhaustion by the way. Should necessity arise, it doubtless, 
for a time, could more than double the speed at which it normally 
travels. 
The rate of progress of the individual must not be confused with 
that at which the species advances. Nocturnal migrants probably 
cover 200 to 400 miles in a single night’s journey; but, as Cooke (03) 
has shown, ‘‘The average speed of migration from New Orleans to south- 
ern Minnesota for all species is close to 23 miles per day.” From this 
latitude northward, however, in response to the more rapid develop- 
ment of the season, the speed is constantly accelerated until the breed- 
ing-place is reached. Thus, the same author remarks, ‘Sixteen species 
maintain a daily average of forty miles from southern Minnesota to 
southern Manitoba, and from this point twelve species travel to Lake 
Athabasca at an average speed of seventy-two miles a day, five others 
to Great Slave Lake at 116 miles a day, and five more to Alaska at 
150 miles a day.” 
The slow rate at which a species moves, when compared with that 
at which the individuals composing it travel, is evidence that its mi- 
gration is not performed continuously, night after night, until the haven 
is reached, but that after a flight birds pause to rest, to await favorable 
weather conditions, and the further seasonal change which such con- 
ditions hasten. The observations of Wright (’09) in the Boston Public 
Garden, where, as in other city parks, local conditions are exceptionally 
favorable for the correct interpretation of migration phenomena, also 
confirm this view, if indeed further confirmation be required. 
The impelling motive being more powerful, the object more definite, 
and the seasonal influences more pronounced, the spring migration 
of birds is a more orderly and regular movement than the return in 
the fall. Clear nights and a rising thermometer are most likely to 
induce birds to travel, a ‘wave’ of migrants coming often on the crest 
of a ‘wave’ of warmer temperature. When such a movement is checked 
by cold or stormy weather, the result is an overflow of migrants which 
flood the woods. Ordinarily rare species may then become compara- 
tively common, and the impression is produced of an actual increase in 
bird-life. In the fall, when physiological factors incident to reproduc- 
tion are not potent and seasonal changes are less marked, birds travel 
more leisurely. Clear nights and a falling thermometer are then most 
favorable for a general movement. 
The high mortality in the ranks of migrating birds occasioned by 
the storms they encounter is evidence of their inability to anticipate 
changes in the weather. On the coast of Texas I have known birds to 
migrate northward in great numbers directly into the face of a ‘norther,’ 
with evidently no warning of the unfavorable conditions toward which 
they were hastening. Again, under the influence of exceptionally 
