CALIFORNIA WATER BIRDS. II 



Monterey ('No. i,' p. 180 et seq.), or of the Louisiana 

 Water-Thrush in the mountains of South Carolina ('Auk,' 

 vol. ix, p. 34). It is obvious that winter compels the 

 evacuation of Antarctica and that transmigration and early 

 summer displacement in regions of plenty, making room 

 for birds from further south, would prevent over-crowd- 

 ing on austral seas just as the general swaying of bird 

 life toward the equator does in the Northern Hemisphere 

 and on the continent of South America.* 



While there is great diversity in migratory movements, 

 the general effect of migration is that the bird population, 

 as a whole, is shifted southward when winter reigns in 

 the Northern Hemisphere and northward when it has its 

 domain in the Southern. (See my remarks on this point 

 in ' The Auk,' vol. xi, pp. 100-102.) 



In January, the near approach of the season of repro- 

 duction compelled the immediate return of the Black- 

 vented Shearwaters to their nesting haunts. f Desire for 



* If there proves to be migration of some birds breeding in the tropics 

 to the temperate zones during slimmer, the pressure from the hemisphere 

 having winter would reasonably account for it. 



It is not impossible that there exists a double migration in some pelagic 

 species, southern-born birds migrating north and spending the summer 

 in the home of the northern-born representatives of the species, breed- 

 ing birds of the Northern Hemisphere and those seeking refuge from 

 the scarcity occasioned by southern winter occurring at the same time 

 in the same region. A migration of this kind would account for birds 

 in latitudes higher than the breeding range independent of retrograde 

 movement. It should be added that Mr. Hudson explains the pres- 

 ence of boreal Sandpipers all the year round in Argentina by a similar, 

 though less protracted migration, summer birds coming from the Arctic 

 and winter ones from the Antarctic (1. c, vol. ii, pp. 187, 191). 



t The December and January Shearwaters and other late migrants, like 

 north-bound boreal Sandpipers on the Florida coast in June (Scott, Auk,' 

 vol. vi, pp. 156-159), not improbably penetrate into the higher latitudes, 

 and in consequence delay their journey until summer has become domi- 

 nant. Such migrants, however, are only stragglers, the vast hosts crowd- 

 ing closely upon the retreat of winter. 



