172 THE STRUCTURE AND LIFE OF BIRDS CH. VI 



are in danger of drifting to the bank and falling 

 victims to any beast of prey. To prevent this ducks 

 and others have the habit of sleeping with one 

 leg tucked under the wing, while with the other they 

 keep gently paddling so that they revolve in a circle. 

 In summer time, when they have had a long day, 

 they will begin this early when there is still some 

 light, and then is the time to watch them. This 

 remarkable habit is a kind of sleep-walking turned to 

 good account, and is, no doubt, perfectly compatible 

 with complete unconsciousness. 1 



Some of the best Books on the Subject. 



(i) Bronn's Thier- Reich, vol. "Aves." 



(2) Milne-Edwards' Physiologic ct Anatomie cojnparce. 



(3) Max Furbringer's Morphologie wid Systematik der Vogel. 



(4) Various articles by Dr. Gadovv in Newton's Dictionary of 

 Birds. 



(5) Michael Foster's Text-book of Physiology. 



(6) Huxley's Elementary Physiology. 



(7) Coues' Field and General Ornithology. 



[See references given in footnotes.] 



1 My attention was first called to this interesting point by 

 Mr. Thompson, head keeper at the Zoological Gardens. 



