SIGHT — FIELDWORK AND EXPERIMENTS 59 



the luck to see it suddenly drop like a plummet down to 

 earth. Transfer your gaze to the spot where the bird landed 

 and, if you can see where it is, you should be able to tell 

 whether it has made a capture or not. If the kestrel takes 

 wing again almost at once, it will have missed its prey ; but 

 if it has seized the vole which it sighted from its position in 

 the air, it will stay where it is for a while. 



What is remarkable in this behaviour is that the keen eye 

 of the kestrel not only had to be able to detect the vole far 

 below it, it must have been able to keep its intended prey in 

 focus during the whole of its long drop. Naturally kestrels 

 and other birds of prey do not always succeed in their efforts at 

 capture, but if you carry out enough of these observations, you 

 will find out that far more victims are caught than missed. 



It may seem a big step to go from the small and beautiful 

 kestrels to the huge and ugly vultures ; but no commentary 

 on the eyesight of diurnal birds would be complete without 

 some mention of these great carrion-eating birds. 



It is not uncommon to hear the view expressed that 

 vultures must smell the odour of decaying flesh in order that 

 they may find the corpse from which the scent reaches them 

 when they are circling round at very great heights. Now first 

 of all it must be understood that there is little evidence that 

 any but a few species of birds have an appreciable sense of 

 smell ; furthermore, a little thought or some field observations 

 by those lucky enough to be in countries where vultures are 

 to be found, would soon convince anyone that sight is the 

 sense employed by these birds when searching for food. 



Vultures often give evidence of anticipating a "kill" as 

 when a lion kills an antelope. They arrive on the scene 

 almost as soon as the kill is made, and as they will normally 

 have been flying above the area, surveying it as they fly 

 round and round, it is obvious that no scent will have reached 

 them ; but their eyes will have sufficed to indicate that in due 

 course a meal will be there. These large gatherings of vultures 

 in the neighbourhood of the kill have not in most instances 

 perceived the happening simultaneously. What occurs is that 

 one vulture will see what has occurred and will start its 

 downward flight. This action will trigger off the remainder 

 of the flock, which will follow suit — an interesting example 



