BIRD STUDY 39 
occasional migrations. A noted instance of this kind occurred 
in 1904, when heavy snows in the northern regions brought the 
Lapland Longspurs to South Dakota on the wings of a storm 
in great numbers. The writer of this chapter was returning 
home about 10 o’clock at night. A keen north wind was blow- 
ing, and the air was filled with snow as fine as sifted flour. It 
was the beginning of a vicious storm. Several times he stopped 
to listen. The storm seemed filled with birds moving south- 
ward. Their sweet calls were heard in every direction. The 
next morning Vermillion was filled with dead and wounded 
birds. Becoming bewildered in the storm, they had been hurled 
against buildings, telephone poles and wires. Such migrations 
are not uncommon and seem to be occasioned by lack of food, 
or by the covering of food by snow in the birds’ usual winter 
habitat. 
The great majority of birds migrate at night; noted ex- 
ceptions are the birds of prey, such as Hawks and Vultures, and 
birds that gather their food on the wing, such as Swallows and 
Swifts. Other birds, such as Ducks and Geese, seem to migrate 
partly by day and partly by night. 
It has generally been thought that birds migrate at night 
to avoid attack by their enemies. It seems reasonable, however, 
to suppose that the food problem is a larger element than the 
safety problem. Swallows and Swifts must migrate by day to 
supply themselves with food. Hawks and Vultures have better 
opportunity for securing food in the course of the day’s migra- 
tion than they would have by migrating at night and resting 
in some particular place during the day. The food of the great 
host of small birds is such that they must spend practically the 
whole day seeking,it. If they migrated by day they would not 
be able to secure sufficient food, for they cannot search for it by 
night. Ducks and Geese, of course, need only to descend into 
any corn or wheat field for a few hours, morning and evening, in 
the course of flight. This is their regular method of feeding 
when they remain in one locality. 
Bird flights are preferably upon still, clear nights, and 
are thought to reach at times an altitude of at least three miles. If 
a storm is raging, the birds cannot hear one another’s calls and, 
