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



entire time of my visit, while Black-footed Albatrosses were 

 rather common. Heermann's Gulls were rare in May, but 

 in June they appeared in force, becoming decidedly com- 

 mon. Specimens examined in June seemed to have recently 

 bred. These circumstances considered in connection with 

 the autumnal movements ^ of this Gull and its occurrence in 

 April on Isla Raza, Gulf of California,^ apparently indicate 

 that it moves northward after the breeding season, as is 

 believed to be the case in the Black-vented Shearwater and 

 Black-footed Albatross.^ Not improbably the movement 

 extends to other water birds breeding in the subtropics and 

 tropics,* there being an extensive overflow to northern lati- 

 tudes and great food-store after the tie to the nesting rocks 

 is loosed. 



Cause of Return Migration.^ 



' ' The day is passing when scientists seek to employ striking or extraor- 

 dinary phenomena in the solutions of their problems; rather are they looking 

 to that which appears insignificant and commonplace." 



The summer movements from breeding grounds in tem- 

 perate regions appear to be the key to the fundamental 

 causes of migration, for these movements occur without 

 procreative stimulus or direct pressure from winter,^ the 

 incentives to migration being therefore limited to narrow 

 bounds. In the previous paper'' facts have been presented 

 showing that the young are guided by the old, and that the 

 latter are directed by physical phenomena which repeated 



1 Calif. W. B. No. IV, pp. 294-299. 



2 Streets, Bull. U. S. Nat. Mus. No. 7, p. 26. (See also Nelson, N. A. Fauna No. 14, p. 23.) 



3 Calif. W. B. No. IV, pp. 305-307. 



4 There seems to be no reason for rejecting the old record of Creagrus fiircatus off Mon- 

 terey merely because of the remoteness of the nesting habitat. (Of. A. O. U. ' Check - 

 Mst,' 2nd E;d., p. 326; Rothschild and Hartert, ' Novitates Zoologicae,' Vol. VI, p. 190; 

 Salvin, Trans. Zool. Soc. Lond., Vol. IX, p. 506.) 



6 The term return migration is employed because the first migration in each bird is 

 from the place of its birth, the movement back to the breeding habitat being therefore 

 a return migration. 



^ Early departure after nesting, in temperate climes, is by no means a recent discov- 

 ery; it was known to Gilbert White more than a century aud a quarter ago. (See letter 

 to Daines Barrington on the Swift.) 



7 Calif W. B. No. IV, pp. 307-312. 



