8 Introduction 



The mechanics of flight are, of course, far beyond 

 OTor scope, nor will we attempt to account for aU the 

 different modes of progression, but we may call atten- 

 tion to the fact that the power of locomotion does not 

 depend on length or strength of wing alone, while the 

 shape of a bird's body, which is often provided with 

 air-sacs, and the more or less hollow bones, are well 

 calculated to make progress in the air as easy as 

 possible. 



Consideration of flight naturally leads us to the 

 subject of Migration, which, indeed, we have already 

 been obliged to mention. From ancient times it has 

 been one of the marvels of bird-life, being referred to 

 in the book of Job, by Homer, and the later Greek 

 and Latin poets, as well as by countless subsequent 

 writers. Yet we seem to understand the phenomenon 

 only shghtly better than of old, though great efforts 

 have been made of late years to gain more definite 

 knowledge of the magnitude and direction of the move- 

 ments. 



In the first place it is necessary to mention three 

 classes of birds which are often confounded under the 

 name of migrants. The true or summer migrants, as 

 regards Britain, are those which, after breeding in our 

 country, leave it for the winter and rfcturn again in spring, 

 the times of their arrival and departure being more or less 

 variable quantities. The partial migrants on the other 

 hand are those which may be said to be more or less 

 resident in Britain as species, though many individuals 

 leave us on migration, and many that have not bred 

 with us visit us at the colder seasons. The birds of 

 passage are those which are only seen for a shorter or 

 longer period in autumn, pass on to more genial 



