IO CALIFORNIA ACADEMY OF SCIENCES. 



It is the custom of writers upon the migration of birds 

 to speak of the tropics as a region of superabundance of 

 food capable of permanently supporting the migrants 

 that come from colder climes. If this was really the 

 case, after the departure of these birds to their nesting 

 grounds there would exist a vacuum that would remain 

 unfilled until their return. That such a void occurs any- 

 where in nature is not sustained by fact. " Living space 

 is always at a premium," life seeking an entrance wher- 

 ever there is a possibility. What compensation there is 

 from the Southern Hemisphere for the departure of north- 

 ern birds is not definitely known, for no trained students 

 of migration have been on the ground to enquire fully 

 into the matter. This much, however, has been ascer- 

 tained, that some birds of the South Temperate Zone 

 extend their northward flight into the tropics, and that 

 there is some migratory movement in species restricted to 

 tropical regions. (Cf. Hudson, ' Argentine Ornithology;" 

 Chapman, Bull. Am. Mus. Nat. Hist., vol. iv, p. 284, 

 vol. vi, p. 14; Ridgway, Rep. U. S. Nat. Mus. for 1890, 

 p. 267; Nuttall, Man. Orn., 2d ed., Land Birds, p. 27.) 

 Below the Tropic of Capricorn migration toward the 

 southern pole and summer seemingly prevent deficiency 

 of food through the presence of North American birds. 



The birds of southern oceans doubtless vary in the 

 extent of their migrations like those of the Northern 

 Hemisphere, some species not reaching warm temperate 

 regions, others extending to the tropics, etc. The Slen- 

 der-billed Fulmar and Black-tailed Shearwater apparently 

 exemplify migration that is usually not protracted into the 

 North Temperate Zone. There probably exists early 

 summer movement toward the equator in temperate cli- 

 mates similar to that of the Scarlet Tyrant in Argen- 

 tina (1. c, vol. i, p. 154), of the California Murre off 



