DIVISIONS OF BIRDS. 44! 



divides into the two bronchi, and not in a true larynx situated 

 at the summit of the windpipe. Lastly, the trachea of birds 

 is always of considerable proportionate length, and it is often 

 twisted or dilated at intervals, this structure, doubtless, having 

 something to do with the production of vocal sounds.* 



Before passing on to the consideration of the divisions of 

 Birds, a few words may be said as to the migration of birds. 

 In temperate and cold climates comparatively few birds remain 

 constantly in the same region in which they were hatched. 

 Those which do so remain, are called "permanent birds " (aves 

 manentes). Other birds, such as the Woodpeckers, wander 

 about from place to place, without having any fixed direction. 

 These are called "wandering birds" (aves erratica), and their 

 irregular 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 large flocks. 

 These birds such as the swallows, for instance are properly 

 called " migratory birds " (aves migratoritz). The movements 

 of these birds are conditioned by the necessity of having a 

 certain mean temperature, and consequently they leave the 

 cold regions at the approach of winter, and return again for the 

 warmer season. 



CHAPTER LXVII. 

 DIVISIONS OF BIRDS. 



i. GENERAL DIVISIONS OF AVES. 2. NATATORES. 

 3. GRALLATORES. 



OWING to the extreme compactness and homogeneity of the 

 entire class Aves, conditioned mainly by their adaptation to an 

 aerial mode of life, the subject of their classification has been 

 one of the greatest difficulties of the systematic Zoologist. 



By Professor Huxley the Birds are divided into the following 

 three orders : 



i. SAURUR^E. In this order the caudal vertebrae are 

 numerous, and there is no ploughshare -bone. The tail is 



* The student desirous of fuller information as to the anatomy of Birds 

 should consult the masterly article by Owen on "Aves" in the 'Cyclo- 

 paedia of Anatomy and Physiology,' or the second volume of the 

 ' Vertebrata ' of the same author, from which the preceding summary has 

 been chiefly derived. 



