No. 429.] RELATION OF WIND TO BIRD MIGRATION. 745 
during the month a flight of many thousands of hawks will 
occur if the wind turns to the northwest quarter, the hawks 
appearing within several hours after the change of the wind 
takes place. This is shown by the following example : 
The morning of September 18, 1890, was warm and calm. 
In the most favorable station for observations near New Haven, 
between sunrise and nearly eight o’clock, only two hawks were 
observed. About eight o’clock a breeze started up from the 
northwest, and a great number of hawks soon appeared, over 
forty of which were shot. Again, on September 12 and 23, 
1895, no hawks were seen, but on the dates immediately 
following, September 13 and 24 respectively, hawks appeared 
in large numbers. 
The significance of these facts is that the wind not only 
changes the line of flight of the migrating hawks, but that it 
is also the immediate cause of their migratory movement. 
In further proof of this statement it has been observed that 
when a northwest wind blows for three days in September, on 
the third day hawks are not abundant ; for all of those hawks 
which are ready to fly southward start at once when the wind 
begins to blow in a favorable direction. 
A second wind from the northwest quarter, after a week of 
southerly winds or calm, will produce a second flight of hawks, 
usually of shorter duration than the first. 
A third favorable wind will seldom cause another large 
September hawk flight, although every wind from the north 
or northwest throughout the autumn produces a greater or less 
abundance of hawks along the Connecticut coast. . 
The expression *the immediate cause of migratory move- 
ment," used in the present paper, perhaps requires an 
explanation: If a favorable wind, acting as a physical agent, is 
used by birds as a means of migration, it is also an immediate 
cause of their migratory movement because it determines the 
time of their migration. 
By the expression “favorable wind ” is meant a wind which 
when resolved into components with respect to the migratory 
