288 VERTEBRATE ANIMALS. 
even the Birds of Prey appear to seek their food mainly by 
the sight. The external nostrils are usually placed on the sides 
of the upper mandible, near its base, and form simple perfora- 
tions which sometimes communicate from side to side. In the 
curious Apiteryx of New Zealand, the nostrils are placed at the 
extreme end of the elongated beak. Sometimes the nostrils 
are defended by bristles, and sometimes by a cartilaginous 
scale. 
Before passing on to a consideration of the divisions of Birds, 
a few words may be said on the migrations of Birds. In tem- 
perate and cold climates, few birds remain constantly in the 
same region in which they were originally hatched. Those 
which do so are called “permanent birds” (aves manentes). 
Other birds, such as the Woodpeckers, migrate from place to 
place without following any very definite course, These are 
called “ wandering birds” (aves erratice), and their movements 
are chiefly conditioned by the scarcity or abundance of food in 
any particular locality. Other birds, however, at certain seasons 
of the year, undertake long journeys, usually uniting for this 
purpose into larger or smaller flocks. Such birds—of which 
the swallows are a familiar instance—are properly called “mi- 
gratory birds” (aves migratoriz); and their movements are 
conditioned by the necessity of having a certain average tem- 
perature, without which they cannot live. Thus the migra- 
tory birds of cold climates, when the cold season comes on, 
travel to warmer countries; but when the hot season of these 
regions approaches, they migrate back again to temperate 
zones. 
