176 THE SENSES OF ANIMALS 



Birds have no ear pinnae, for they would interfere with 

 the streamHned shape so necessary to them in rapid flight ; 

 the tuft of small feathers that covers and protects the entrance 

 to the ear-tube leading to the drum is faired into the general 

 surface so that it is practically invisible as a separate part 

 of the plumage. In the owls the entrances of the ear-tubes 

 are modified so that the two sides are asymmetrical, the 

 degree of asymmetry being greater in some species than 

 others; in the extreme cases the distortion is so great that 

 the shape of the skull itself is asymmetrical in this region. The 

 effect of these arrangements is to make the "polar diagram" 

 for the reception of sound different on the two sides ; in some 

 owls the direction of greatest sensitivity is greatest above the 

 horizontal axis on one side, and below it on the other. The 

 polar diagrams of symmetrical ears produce an ambiguity 

 in locating sound sources in the vertical plane, although they 

 have none in the horizontal plane. The asymmetrical ears 

 of owls resolve the ambiguity by giving polar diagrams some- 

 what resembling those that would characterize ears set one 

 above the other, which would have no ambiguity in the 

 vertical plane but would have one in the horizontal. The 

 asymmetry gives the owls the best of both systems, for as the 

 ears are not symmetrical in either horizontal or vertical 

 planes they have no ambiguity in either of them. Owls are 

 thus able instantly to locate the direction of the source of a 

 transient sound, and to pounce with astonishing accuracy 

 on the prey which they cannot see. There is a further point 

 associated with the great sensitivity and accuracy of the 

 owl's hearing; they would be vitiated by any sounds arising 

 from the bird's flight. The wings of most birds make plenty 

 of noise when they fly, for example the hum of humming- 

 birds, the whirr of the wings of small perching birds and 

 game birds, the swish of falcons and eagles, and the music of 

 the swan's pinions. The plumage of owls, however, is so 

 soft that no sound comes from their wing feathers when they 

 fly, nor is there any sound from the body feathers as the 

 stream of air flows over them. Owls are not particularly 

 swift or strong fliers, and no doubt they have sacrificed some 

 flight efficiency in exchange for the silence essential for their 

 method of hunting by ear. 



