24 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4TH Ser. 
MIGRATION IN THE NORTHERN HEMISPHERE 
The migratory movements of the Northern Hemisphere 
species of albatrosses and petrels are counterparts of those 
occurring in the Southern Hemisphere species. Transequatorial 
migration is represented in Leach’s Petrel, and apparently in 
the Least Petrel, Storm Petrel, Kuhl’s Shearwater, and Manx 
Shearwater, and migration restricted to the Northern Hem- 
isphere is represented in the Fulmar, and probably in the 
Short-tailed Albatross. The Ashy Petrel, Black Petrel, and 
Fork-tailed Petrel have definite migratory movements, but 
their southern limits have not been reported. After breeding 
on Lower California islands, Black-vented Shearwaters 
migrate northward along the coast, some of them as far at 
least as Vancouver Island, their fly-line thus extending about 
fifteen hundred nautical miles. Black-footed Albatrosses have 
an eastward and westward exodus-migration as well as a 
_ northern one, the two movements covering the North Pacific 
area above the Tropic of Cancer. In the Southern Hemisphere 
the general dispersion of Wandering Albatrosses furnishes 
somewhat of a parallel. 
REMOTE CAUSE OF MIGRATION 
Bewildered by the varied migratory movements, the mind 
is prone to create complications where none exist, and to lose 
sight of the fact that in each bird, migration is simply an 
exodus, followed by a return movement to breeding grounds. 
Migration in one hemisphere is a complement of migration in 
the other. Exodus-migration in Southern Hemisphere species 
and return-migration in Northern Hemisphere ones adjust the 
bird population of the world to southern winter and northern - 
summer, and exodus-migration in Northern Hemisphere 
species and return-migration in Southern Hemisphere ones. 
adjust the bird population to northern winter and southern 
summer. 
The diversity of the migratory movements evidences that 
the adjustment of the bird population to the seasons was 
brought about under stress during a long period of time, the 
movements in each species being the outcome of a prolonged 
struggle. Even in the present age the adjustment sometimes: 
