192 THE SENSES OF AxNIMALS 



by the angle of attack of their large front paired fins acting 

 like the planes of an aircraft, can perceive only those sounds 

 that act upon the otoliths of the inner ear direct. They must 

 therefore have a much less efficient sense of hearing, for the 

 greater proportion of the sound waves reaching them must 

 pass through them with little resistance and travel on without 

 being perceived. 



Few kinds of birds make use of sonar, and those that do 

 appear to confine their sounds entirely to the audible part 

 of the sound spectrum (to human ears) and not to use ultra- 

 sonic vibrations. These are birds that nest far inside extensive 

 caves where no light penetrates ; no nocturnal bird is known 

 to make use of sonar in finding its way about or hunting its 

 food — though perhaps there are some as yet to be recognized. 

 Those that have been most closely studied are the pretty 

 little swiftlets of Borneo and Malaya, some of which make 

 the much prized edible birds' nests. These swiftlets nest in 

 enormous numbers in the huge caves of their native lands 

 and are able to find their way about, and to home on to 

 their own nests, in complete darkness by means of a sonar 

 system using sounds of a very low frequency when compared 

 with that of bats. A very convincing and simple experiment 

 was made on these birds by Lord Medway. He released some 

 of them in a room at night and they flew about in the dark 

 without colliding with the walls or each other, all the time 

 keeping up a continual twittering. The moment the electric 

 light was switched on the twittering stopped although the 

 birds cont'.nued their flight, reverting to sight instead of 

 sonar for finding their way. A tape recording made at the 

 time is a beautiful record — every time the light is switched 

 off" the twittering starts and every time the light is switched 

 on silence descends. Another bird that uses sonar is the 

 peculiar Oil bird, Guacharo, or Diablotin, of Venezuela and 

 some of the West Indian islands. The diablotin is about as 

 big as a crow and has a wide mouth beset with bristles some- 

 what like that of the nightjar; it is completely nocturnal 

 and sleeps all day in deep dark caves where it congregates 

 in large numbers. In the evening it awakes and "with croak- 

 ing and clattering that has been likened to that of castanets, 

 it approaches the exit of its retreat, whence at nightfall it 



