AVES. 289 



" Permanent Birds" {aves manentes). Other Birds, such as the Wood- 

 peckers, migrate from place to place without following any very 

 definite course. These are called " Wandering Birds " {aves erraticoe), 

 and their movements are chiefly conditioned by the scarcity or 

 abundance of food iu any pax-ticular 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 

 " Migratory Birds " {aves migratorice) ; and their movements are con- 

 ditioned by the necessity of having a certain average temperature, 

 without which they cannot live, and also by the necessity of finding 

 a climate suitable for the bringing up of the young. Thus the 

 Migratory Birds of cold regions, 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. 



