CLASSIFICATION OF BIRDS. 345 



In cold and temperate climates many birds do not remain all 

 the year; these are called "migrants." Migrants pass at the 

 approach of winter to a warmer climate, and return again to 

 nest in the spring. Prior to migrating the birds usually collect 

 in flocks. These migrations generally take place about the same 

 date every year. Most migrants are insectivorous, and even if 

 the temperature were warm enough for them, they could not 

 subsist upon our scanty winter faima. 



There are other birds which move from place to place in an 

 erratic manner, the so-called " gipsy migrants," which leave one 

 district for another owing to scarcity of food, but have no fixed 

 place of migration. This form of movement we see in Tits 

 {Paridm) and "Woodpeckers (Picidce). 



Those birds, such as the blackbird and thrush, which are with 

 us throughout winter and summer are called " residents." 



Fossil birds present many peculiarities. Some (Odontornithes) 

 have teeth ; others of older date have distinct lizard - like 

 characters, such as the feather-tailed Archeoptei-yx. 



Classification of Birds. 



The following classification is that worked out by Dr Hans 

 Gadow, and is based upon characters from the various organic 

 systems. The whole class Aves is divided into two sub-classes, 

 the Archseornithes and Neornithes. The former are extinct 

 birds. The Neornithes are divided into two sections, the 

 Eatitse and Carinatse — the former being the wingless birds 

 with no keel to the sternum ; the latter, which contain the 

 majority of Aves, having a keeled sternum. Only those groups 

 represented in Great Britain are given, with one or two excep- 

 tions, and some only of the characters are enumerated, the 

 reader being referred to Dr Gadow's paper in the Proceedings 

 of the Zoological Society for March 1892, p. 229, for fuller 

 information, and also to his more recent book on ' A Classifica- 

 tion of Vertebrata, Eecent and Extinct.' 



