OCTOBER, 1902. 
THE MIGRATION OF BIRDS. 
195 
phenomenon and operates between faunal areas. In general the 
two phenomena are thus of such an entirely different order that 
they will permit of no just analogy. 
Among birds there are two classes of migrations that are local. 
In the tropics and the warmer portions of the temperate zones 
we have a movement into the mountains in spring, and back 
again in fall. These movements, although local, are obviously 
movements between faunal areas, and hence are similar to the 
general migrations. The other form of local migration is found 
among birds that spend most of their time roving over the sea. 
These birds breed in favored localities upon rocky islands. We 
have here a case somewhat analogous to the migrations of fishes. 
It is not a phenomenon operating between faunal areas. These 
cases Prof. Brooks has used as furnishing an argument against 
the failure of food in the south. He has failed to see, however, 
that the food supply of these birds and those of our more migra- 
tory species is found under entirely different conditions. The 
food of the former consists of material that is abundant about 
these rocky islands throughout the year. It is not affected by the 
dry season, and hence it is not necessary that these birds should 
perform such extended migrations. 
In so far as migration is concerned in the extreme northern 
portion of the geographical distribution of a species, there is prob- 
ably an analogy beween the movements of birds and those of 
fishes. When birds arrive in the north their movements are af- 
fected by local conditions. It is obvious that they will return to 
their old homes if these have been favorable ; if not, they must seek 
new areas. It has been shown that the failure of food in the south 
will cause birds to move northward in spring. Investigations con- 
cerning migratory routes show that birds will be led back to the 
immediate vicinity of their old homes, where they seek breeding 
places that offer the best facilities. This latter portion of the move- 
ment may be analagous to that of fishes. The desire to secure 
■ safe breeding then becomes an important factor in migration, but 
only a modifving factor, a factor toward wdiich we may look as 
a cause for the diversity and vastness which the phenomenon has 
attained. It may be one of the main factors concerned in the ulti- 
mate dispersion, and hence the increase of geographical distribu- 
tion and extension of the migratory routes. 
It has been shown that love of home and nesting ground, and 
the dangers of rearing young in the south, cannot be considered 
as principal causes for the spring migration. It has furthermore 
been shown that birds are quite dependent upon a particular kind 
