16 INTRODUCTION. 



The resident native birds of countries where the heat of 

 the year is comparatively uniform, moult gradually ; and the 

 same may be said of those that have their haunts in regions 

 that are always cold, and where the food is comparatively 

 limited. Such birds are seldom so denuded of feathers as to 

 be unfit for pretty vigorous flight. Birds which migrate 

 from region to region, moult more periodically ; and in places 

 where the migration is extensive, it will perhaps be found, 

 upon further examination, that the bird moults twice in the 

 year: though in most instances the Spring moult is less 

 general than the autumnal one, being, in many birds, the 

 males especially, rather a change of colour than of all the 

 feathers. Birds which migrate polarly, or for the purpose of 

 breeding, generally receive their nuptial colours, if not their 

 plumage, after they arrive; but when they migrate equa- 

 torially, they change their plumage before they begin their 

 journey. 



The vernal change in the plumage of birds is owing to the 

 same cause as the change of their voices from the chirp or 

 cry to song ; and in a state of nature the two cease together. 

 This change bears some analogy to the blossoming of plants, 

 while the autumnal moult more resembles the fall of the 

 leaf. 



That the migration depends much upon the state of the 

 atmosphere is proved by the fact that the number is aug- 

 mented with an augmentation of that state which we sup- 

 pose makes them leave the country from which they came. 

 Our winter migrants are always most numerous when the 

 season is most severe in the countries from which they come : 

 and there are some rare species which reach our shores only 

 in very severe winters. 



With our summer migrants the state of things is just the 

 reverse. Warm weather in the places whence they come 

 sends them to us sooner and in great numbers; and cold 



