No. 429.] RELATION OF WIND TO BIRD MIGRATION. 753 
These conclusions apply to both the northward and south- 
ward migrations. Most of the effects of the winds stated are 
intensified when the wind is strong. 
It has been observed that after a period of adverse or light 
winds, during which no migratory movement was noticed, a 
moderate wind in a direction favorable to the migration causes 
a marked migratory movement, especially if such a wind is gen- 
eral and not local. It thus seems evident that birds wait for a 
favorable wind to carry them on their migrations. 
Changes of temperature, without doubt, affect the migratory 
movements of hawks, but, as has already been shown, a number 
of large flights have occurred in autumn on days when rises of 
temperature occurred near where the hawks started on their 
journeys. Moreover, flights of hawks often occur on cool days 
in spring, lower temperature usually accompanying west winds 
at that season of the year. 
Water birds, particularly the Limicolz, migrate in large num- 
bers from the north in July, before the hottest period of the 
summer has passed. It is not surprising, therefore, to find 
causes affecting the time of migration of birds other than 
changes of temperature. 
It is probable that many varieties of birds regularly make 
use of the wind as a physical agent in carrying them on their 
migratory journeys, the habit being well formed in the case 
of the Falconide. 
PHYSICAL LABORATORY, COLUMBIA UNIVERSITY, 
NEW York. 
