172, THE STRUCTURE AND LIFE OF BIRDS cu. 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. 
SOME OF THE BEST BOOKS ON THE SUBJECT. 
(1) Bronn’s 7hier-Rezch, vol. “ Aves.” ; 
(2) Milne-Edwards’ Phystologie et Anatomie compareée. 
(3) Max Fiirbringer’s Morphologie und Systematik der Vogel. 
(4) Various articles by Dr. Gadow in Newton’s Dictionary of 
Birds. 
(5) Michael Foster’s Text-bo0k 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. 
