xii CONTENTS 



CHAPTER V 



THE GENERAL HABITS AND FUNCTIONS OF BIRDS 



PAGES 



The flight of birds — Flightless birds— The fitness of 

 birds for an aerial existence — Various modes of 

 flight — Gliding flight : how performed — Flight by 

 active strokes of the wings : how performed — The 

 movement of the wings — Soaring or sailing flight : 

 how performed — Terrestrial and aquatic motion 

 of birds — Birds unable to progress upon the 

 land — Running, walking, and hopping move- 

 ments — Progress in water — The food of birds — 

 Modifications of the bill — Common to various 

 distantly related groups — Social, gregarious, or 

 solitary instincts of birds — Mimicry in birds 

 — Protective colours and resemblances of birds — 

 Forest birds — Desert birds — Marsh and swamp 

 birds — Moorland and mountain birds — Arctic 

 birds — Variation in birds — Of external structure 

 and colour — Dimorphism in birds — Sexual dimor- 

 phism — Dimorphism independent of sex — Struc- 

 tural dimorphism 1 78-211 



CHAPTER VI 



THE LOVE DISPLAYS OF BIRDS 



Bird music — Vocal music of birds — Variety in song — 

 Capriciousness of song — The origin and purpose 

 of avine song — Individual variation in the song 

 of birds — Imitation — Do birds sing by instinct 

 or imitation? — Vocal sounds of birds — The in- 

 strumental music of birds — Made by ordinary 

 appliances — Made by special modifications of 

 the plumage — The drumming of the Snipe — 

 Modifications in the wings of the PIumming-Birds 



