HEARING — GENERAL 73 



Sand Lizard, and the Slow Worm — can hear their prey 

 moving and are often directed to some insect or other food 

 item by the sounds made by them. Of course, as has already 

 been said, sight is the most regularly used sense employed 

 in feeding, but hearing undoubtedly acts as an additional 

 aid. 



In the fishes we meet the difficult point as to where the 

 detection of vibrations of various kinds comes near to real 

 hearing. It is probably true to say that fishes do not "hear" 

 sounds in the way in which sounds are heard and reacted to 

 in higher animals ; but nevertheless fishes are most sensitive 

 to vibrations in the water which may act as alarms and as 

 guides to navigation in streams and rivers, and also in the 

 finding of food in certain species. 



All birds have excellent hearing, though no bird has 

 external ears. Hearing is used in connection with song, alarm 

 calls, protection of young, and for the detection of food 

 items. The familiar thrushes and blackbirds and robins can 

 detect worms in their burrows by means of sound, and in 

 the next chapter the way in which this can be observed and 

 investigated will be explained. In the owls hearing plays a 

 special and most interesting part in their hunting behaviour, 

 and the particular way in which the ears of owls are adapted 

 for this purpose is one of the most fascinating aspects of the 

 sense of hearing in birds. 



In many species of owls this special adaptation takes the 

 form where the right and left ears of the birds are different 

 in shape and structure — they are asymmetrical. This would 

 seem to show that these owls use their ears as direction- 

 finders when conditions are unsuitable for hunting by 

 sight. 



Another interesting feature of the hearing abilities of birds 

 is the way in which many greatly varying species can hear 

 and respond to alarm calls — even when uttered by a dif- 

 ferent kind of bird from the one which calls. We see this 

 without difficulty in our gardens where the blackbirds seem 

 definitely to be the sentinels. The alarm note of a blackbird 

 when it sees a cat will not only act as a warning to its own 

 newly fledged young, it will also cause other birds — tits, 

 wrenSj hedge-sparrows and so on — to take cover or take 



