56 THE SENSES OF ANIMALS 



birds are dullish in plumage ; but birds would not be brightly 

 coloured in so many species if these colours had no signi- 

 ficance. 



However, colour-sight alone is not the most outstanding 

 feature of the seeing abilities of birds. It is the sharpness of 

 sight and the way in which the eyes can accommodate them- 

 selves to swift changes of focus that excite one's interest. 



When a song-bird perceives the shape of some creature 

 which is an enemy, it will react at once, and will take certain 

 self-protecting steps. It will either freeze in order to lessen 

 the chance of capture, or it will resort to flight ; or, when 

 defending eggs or young, it may become aggressive and 

 threaten the enemy. In other instances several birds — often 

 of different species — will simultaneously mob the predator, 

 frequently driving it away. 



In all these varied actions the keen sight of the bird is all 

 important; for whatever defensive manoeuvre is put into 

 operation must be almost instantly carried out. To do this 

 it is necessary for the bird's eyes to accommodate themselves 

 quickly to differing circumstances. The threat may come 

 from the ground if a stoat or rat is hunting ; it may be an 

 owl in a tree that sets small birds off mobbing ; or a hawk 

 high up in the sky may be the cause of alarm. 



Now colour-sight must play an almost negligible part in 

 this kind of behaviour ; but the inborn instinct which enables 

 a bird to recognize shape as a threat is vital. For many years 

 it was thought that backwardly-curved wings showing up as 

 silhouettes against the sky were the danger signals that 

 alerted a bird to the presence of a hawk ; but recent experi- 

 ments are said to have proved that it is an outline which 

 shows a short neck which acts as a warning. Doubtless this is 

 so ; but I, myself, am not convinced that it is the shortened 

 neck that is the only feature which causes alarm. 



I have carried out some experiments on this subject which, 

 simple though they were, seem to me to have some weight. 

 I have cut out cardboard shapes which were nothing more 

 than crescents and have hung these high up over such plants 

 as peas, dwarf beans and young lettuces. I have then watched 

 the behaviour of sparrows, tits and similar birds to see what 

 happened. There has been alarm almost at once, and on one 



