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 " {uves erratvxi;), 

 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, undeitake 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 migratorke) ; 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 tVie hot season of these regions 

 approaches, they migrate back again to temperate zones. 



