314 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



California coast, ^ the Louisiana Water-Thrush of the South 

 Carolina highlands,^ and the Scarlet Tyrant of Argentina^ 

 moving toward the equator as early as birds on the shores 

 of the polar sea. 



In all this diversity there is unity — these varied move- 

 ments and the return ones, as a whole, constituting the 

 great movement that sways the bird population southward or 

 northward as winter is dominant in either hemisphere. The 

 complexity of this adjustment suggests that it has been 

 evolved, as exigencies have arisen, through a long period of 

 time. However, it is not evident that any movements sur- 

 vive that the immediate urgency does not demand.* So nice 

 is the adjustment that the territory is occupied where winter 

 and summer contend for the mastery, Horned Larks, Long- 

 spurs, Ducks, Geese, etc., retreating or advancing as the 

 snow and ice advance or recede. Moreover, great destruc- 

 tion of bird life takes place when exceptional severity of 

 season suddenly contracts the food areas to a degree un- 

 provided for by the ordinary course of migration.^ Also, 

 the loitering of birds in mild autumns® apparently indicates 

 that partial relief from winter would result in a decline in 

 migration. In fine, it is maintained that winter, past and 

 present, with its failure of food is the outward cause of all 

 migratory movement.^ 



1 Calif. W. B. No. I, p. 180 et seq. 



2 'Auk,' IX, p. 34. 



3 Cf. Hudson, 'Argentine Ornithology,' Vol. I, p. 154. 



4 The departure of birds early in summer, the replacing of breeding ones by individ- 

 uals of the same species from higher life zones, the evacuation of winter quarters in 

 tropical and warm temperate regions, are a necessity, it is contended, under existing 

 conditions. Migration must be a gradual depopulation, and hence the movement begins 

 when birds have finished the cares of reproduction. Independent of other considera- 

 tions, pressure of population, because of winter, necessitates, it is held, the displacing 

 of breeding birds by winter representatives of the species, and pressure of population, 

 because of winter in the opposite hemisphere, necessitates the removal of winter so- 

 journers ill temperate and tropical regions to higher life zones; in the grand scheme of 

 migration there being no room for birds without the bounds of their present habitats. 

 Local variation is not lost sight of (cf. 'Auk,' Vol. XI, pp. 26-39). The fact of its being 

 confined within narrow limits apparently evidences that it cannot be more extended 

 without causing disaster. 



5 A recent instance is reported by Mr. Wayne from the Low Country in South Caro- 

 lina, ' Auk,' Vol. XVI, p. 197. 



6 See ' Auk,' Vol. IX, p. 39; Cooke, Rep. Bird Migr. Miss. Vail., p. 26. 



' This view is not intended to conflict with the theories of the origin of migration in 

 a waning ice age and in secular refrigeration. 



I have referred to physical cause in greater detail in ' The Auk,' Vol. XI, pp. 94-112. 



