SIGHT — FIELDWORK AND EXPERIMENTS 55 



British Adder, do not at once eat their prey when it has been 

 struck by the poison fangs. The object of the strike is to inject 

 sufficient poison into the victim and paralyse it so that it 

 cannot escape. I have had the good fortune to witness this 

 several times when voles and lizards have been the prey. The 

 adder strikes, and if it gets home it may wait quite two or 

 three minutes before following up its attack and proceeding 

 to swallow. Where the victim is a vole or mouse there is an 

 additional advantage in this waiting period, because if the 

 snake went in at once to try to engulf its prey it would stand 

 a chance of being bitten by its intended victim. I once saw 

 this happen when a venomous snake struck at a rat and 

 evidently either did not get its fangs into the rat, or perhaps 

 its poison was temporarily depleted. Whichever was the 

 reason, the rat bit the snake in the head and it subsequently 

 died — a case of the biter being bit ! 



Notwithstanding these accidents, which must often occur 

 in nature, the close-range vision of snakes is good. Nocturnal 

 snakes can, of course, see quite well in dim light and their 

 eyes are well adapted for this form of life. 



Birds 



Field observations and experiments designed to test the eye- 

 sight of birds are not so easy to suggest and carry out as one 

 might think. This kind of limitation must also occur in rela- 

 tion to certain aspects of other senses than sight, because the 

 very nature of a sense in a particular animal may make it 

 less susceptible to experimentation. 



The flight of birds, and their general activity when on the 

 ground, makes them less suitable for tests ; while field obser- 

 vations — except in respect of birds of prey — are not very 

 satisfactory. There are, however, some observations which 

 will at least serve to bring home to the student the extreme 

 acuity of sight possessed by birds. 



First we must remember that all diurnal birds are able to 

 see colours, and this ability helps them to select food ; while 

 in courtship colours sight plays its part. Of course the fact 

 that these birds can see colours does not play the main part 

 in courtship, as may be gathered from the fact that many 



