262 FLEAS, FLUKES AND CUCKOOS 



Africa the foster parents continue to feed it for a further period of about 

 two weeks. This is an extremely arduous task which also incidentally 

 exposes them to various enemies in a manner which would not occur 

 with their own unobtrusive young. 



Attention has already been drawn to the fact that sometimes the 

 female cuckoo appears to mesmerise her victims. As a nestling the 

 young cuckoo also produces an unusually exciting effect and not only do 

 the fosterers exert themselves madly to satisfy its hunger, but even strange 

 birds feel drawn to come and feed it. This also occurs during the period 

 after the young cuckoo has left the nest, but still requires feeding. 

 Pliny noticed this peculiar effect on the foster mother: " She joyeth to 

 see so goodly a bird toward : and wonders at herself that she hath 

 hatched and reared so trim a chick." This power of psychological 

 stimulation is probably yet another important biological adaptation, 

 which, like the extraordinary characteristics already mentioned, has 

 become necessaiy owing to the cuckoo's peculiar and difficult mode of 

 life. 



Adaptation of the Cuckoo's Eggs 



We have already mentioned that the colour and markings of the 

 cuckoo's eggs resemble, to a greater or lesser degree, those of the foster 

 birds chosen to rear their young. This fact has always aroused interest 

 and also considerable controversy. For a long time it was thought that 

 the female cuckoo knew the colour of her own eggs and could select a 

 clutch which they matched, or could, through some physiological reflex, 

 even control the colour of her own eggs. It has also been maintained 

 that the female cuckoo mates with the fosterer — a fact which, it was 

 claimed, explains not only the colour of her eggs but their small size, and 

 the derivation of the middle English term "cuckold "! 



Recently, investigations, particularly painstaking observations in 

 the field, have thrown considerable fight on the whole problem, but it 

 must be admitted that uncertainty still exists concerning much of the 

 cuckoo's private life. Great progress was made in unravelfing the tale 

 when it was discovered that individual female cuckoos always lay the 

 same coloured eggs with the same characteristic design. Another dis- 

 covery which showed how selection worked was the proof obtained from 

 innumerable careful field studies and many experiments, that the small 



