CALIFORNIA WATER BIRDS. 9 



Provided there was sufficient time before the Auks, 

 Gulls, and Cormorants breed for the Shearwaters to bring 

 up their young on the islets along the middle coast of 

 California, there would not be room for the great hosts 

 that in all probability were winging their way southward 

 in September, October, and November. Many places too 

 that are tenantable during summer are not so in winter, 

 the sea breaking over them in stormy weather. The con- 

 gregating of hordes of Gulls upon the islets is a further 

 barrier. In the colder boreal regions means of sustenance 

 are lacking in winter, and off Lower California there is ap- 

 parently accommodation only for ' residents ' and perhaps 

 for some migrants from the tropics, for breeding begins 

 earlier than further north (for example, during the latter 

 part of December in the Man-o' -War Bird — Bryant, Proc. 

 Cal. Acad. Sci., 2d ser., vol. ii, p. 263), but south of the 

 Tropic of Capricorn summer has opened new territory 

 for habitation. As stated in the previous paper the vicinity 

 of Monterey affords subsistence in summer far in excess 

 of the requirements of the breeding population, owing 

 to the absence of suitable places for rookeries. It nat- 

 urally follows that birds come south and enjoy this 

 bountiful store after the cares of reproduction are over, 

 returning north when their breeding grounds become 

 habitable again. Hence it is not extraordinary if the 

 Shearwaters, knowing the way by previous experience, 

 resort to an inclement region having nesting accommoda- 

 tions, although available only for a brief period. In short, 

 it is held that the return of all birds to the region of their 

 birth is a physical necessity, there being no room for them 

 to rear their young anywhere else.* 



* Stragglers it is true might loiter by the way, but if the majority of 

 migrants were uncertain in their destination over-crowding must inevitably 

 •ensue. Perhaps the occupancy of rookeries affirmed to take place in ad- 

 vance of the breeding season prevents aliens from securing a foothold. 



