8 CALIFORNIA ACADEMY OF SCIENCES. 



avoid over-crowding. Upon the ocean opposite condi- 

 tions seem to prevail. The necessity for migration ap- 

 pears to be greater on southern than northern seas, for 

 the frigid waters of the Antarctic encroach upon the 

 South Temperate Zone. It is not remarkable, therefore, 

 if Petrels in their flight from the dearth of winter pene- 

 trate into the Northern Hemisphere as far as Pomarine 

 and Parasitic Jaegers and North American Limicoke pen- 

 etrate into the Southern. 



That the rear guard was present in December and Jan- 

 uary does not disprove that the Shearwaters came north 

 to escape from winter famine. If the end of their journey 

 was in the remote southern regions, the mild climate of 

 the California coast would be exchanged for the inhos- 

 pitable Antarctic summer. Further, if the interval occu- 

 pied by reproduction be the limit of the sojourn in the 

 breeding habitat, as is true in numerous species visiting 

 boreal climes, most of the year would be spent on warmer 

 seas to the northward. The vexed question, why some 

 birds migrate from tropical and temperate to colder lati- 

 tudes to rear their young, arises when inquiry is made 

 why the Shearwaters do not breed on California shores. 

 That migration is not satisfactorily explained on the score 

 that the conditions of temperature essential to reproduc- 

 tion are wanting in the winter abode, is evinced by cir- 

 cumstances like the periodic return of the American 

 Golden Plover from the Pampas to the nesting home on 

 the tundras, and the breeding of Tufted Puffins on the 

 Santa Barbara Islands and of Yellow Warblers on the 

 shores of the Frozen Ocean.* 



* Bearing upon this point, is the extension of the breeding range of 

 certain species further south on the Pacific than upon the Atlantic coast, 

 Tardus ustidatus being a notable example. On favored slopes in the Santa 

 Clara Valley oranges thrive, and on the same ground this Thrush breeds ■ 

 abundantly. Specimens may be obtained that cannot be distinguished 

 from examples of swainsonii from the East. 



