60 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
Though the insistent crescendo note of the ovenbird is ordinarily 
associated with the full verdure of May woods, this bird has been 
known to reach its breeding grounds in a snowstorm and the records 
of its arrival in southern Minnesota show a temperature variation 
from near freezing to full summer warmth. Temperatures recorded 
at the time of arrival of several other common birds show variations 
of 14° to 37° F., the average variation being about 24°. 
It should be remembered that North American species spending 
the winter months in tropical latitudes experience no marked changes 
in climatic conditions from November to March or April, yet fre- 
quently they will start the northward movement in January or 
February. This is in obedience to physiological promptings and has 
no relation to the pr evailing weather conditions. For migratory 
birds the winter season is a period of rest, a time when they have no 
cares other than those associated with the daily search for food or 
escape from their natural enemies. Their migrations, however, are 
a vital part of their life cycles, which have become so well adjusted 
that the seasons of travel correspond in general with the major 
seasonal changes on their breeding grounds. With the approach of 
spring, therefore, the reproductive impulse awakens, and each 
individual bird is ‘irresistibly impelled to start the journey that ends 
in its summer home. 
Through their influence upon the food supply, weather conditions 
in the breeding areas do, of course, figure prominently in the picture. 
In fact, they are the major factors in determining the average dates 
of arrival of the different species. The word ‘‘average’’ must he 
emphasized, for the migrations of birds have so evolved that in 
general they synchronize with average climatic conditions. In other 
words, migration is so ingrained that each species moves north in 
spring w hen the average weather that will be encountered is not 
unendurable. The hardy birds travel early, fearless of the blasts of 
retreating winter. The more delicate species come later, when there 
is less danger of encountering prolonged periods of inclement weather. 
Some of these hardy birds pause and allow the spring season to 
advance, and then by rapid travel again overtake it, or in some cases 
actually outstrip it. At times this results in some hardship and 
occasionally in the destruction of large numbers of individuals, us 
has happened when early migrating bluebirds have been overwhelmed 
by a late winter storm. Unless such conditions are prolonged, how- 
ever, no serious effect on the species is noted. Nevertheless, the 
soundness of the bird’s instincts is evidenced by the fact that natural 
catastrophes, great though they may be, do not permanently diminish 
the avian population. 
As has been pointed out, the advance of average temperature lines, 
known as isotherms, is found to correspond closely with the north- 
ward movements of certain species. For example, the northward 
travels of the Canada ome are found to coincide with the advance 
of the isotherm of 35° F. (fig. 8). 
The spring flight of migrants, if interrupted for any reason, is 
resumed when weather conditions again become favorable, and it is 
probable that all instances of arrival of birds in stormy weather can 
be explained on the theory that the flight was begun while the weather 
was auspicious. The state of the w eather when a flight starts at any 
southern point, the relation of that place to the average position of 
