Vou. II, Pr. II] LOOMIS—A REVIEW OF THE TUBINARES 25) 
fails. An extraordinary cold wave in a warm temperate region 
causes great destruction of bird life, suddenly contracting the 
food area to a degree unprovided for in the ordinary course of 
migration.* 
In brief, it is maintained that bird migration is the adjust- 
ment of the bird population of the world to the seasons, and 
that with the unfolding of the seasons came the unfolding of 
bird migration, the evolution of the seasons being the remote 
cause of bird migration.’ 
IMMEDIATE CAUSE OF MIGRATION 
Food 
Immediate failure of food plays an important part in the 
supplemental movements of exodus-migration.? These move- 
ments may be due to absolute shortage in the food store in the 
usual winter quarters, or they may be due to the normal fluc- 
tuations of the snow and ice line in the territory where winter 
and summer contend for the mastery, covering up the food- 
supply of birds that habitually feed on the ground or in the 
water, forcing them to retreat to a warmer area and to abide 
there until the snow and ice recede.* 
The northward migration of Black-vented Sree 
after breeding in the subtropics, can not be attributed to 
immediate failure of food, for species of similar feeding habits, 
like Townsend’s Shearwater and the Wedge-tailed Shear- 
water, find abundant food in the ocean area deserted by the 
Black-vented Shearwaters. Neither is the tropical migration 
of the Least Petrel to be explained on the score of immediate 
failure in the food-supply. 
Long ago, Gilbert White commented on the early exodus- 
migration in warm temperate regions. In the letter on the 
Swift, addressed to Daines Barrington, he says: “But in 
nothing are swifts more singular than in their early retreat. 
They retire, as to the main body of them, by the 10th of 
1Cf, Wayne, Auk, v. 16, pp. 197, 198; Clarke, Studies in Bird Migration, v. 1, 
p. 167 
2 Season is here used in an historical and not in an astronomical sense. A glacial 
period would be a phase of the evolution of the seasons. 
3 Variability in abundance due to deflection in the fly-lines of species that form 
colonies or are restricted in their distribution te small island-like areas is not to be 
confounded with supplemental migration—cf. Auk, v. 11, pp. 33-39. 
*Cold has not been emphasized as a factor in supplemental migration, for the 
species involved apparently thrive in spite of cold if proper food is plentiful. 
