THE WEB OF LIFE 317 



union, searches for nests of suitable nurses, and when found, 

 watches them from the beginning of nest- building day by 

 day, in order to choose the one most suitable '. As Herrick 

 rightly points out, we must be careful in reading choice 

 and motive into the bird's behaviour. On the other hand, 

 we must not try to make an automaton of this remarkable 

 bird. 



The eggs of the cuckoo are relatively very small, they 

 have thick resistant shells, they show an extraordinary 

 variability in colouring, ranging from blue or blue-green, 

 through speckled blue, brown, mottled or marbled brown 

 and gray to nearly plain white. There is strong evidence 

 that the same cuckoo, for a season if not for life, lays the 

 same type of egg. In one particular good instance, among 

 others, in the fine Fenton collection of eggs in Aberdeen 

 University, eleven cuckoo's eggs taken in close proximity 

 (from five different kinds of nests) are indistinguishable. 

 In many cases the cuckoo places her egg in the nest of a 

 bird with eggs similar in size and colour to her own, but 

 in many cases it is quite otherwise. 



It is doubtful whether this resemblance of the cuckoo's 

 egg to that of the foster-parent is of any practical value. 

 Herrick writes : 



' We should like to know how many of the 119 potential 

 nurses of this bird would reject an egg of similar size, 

 whatever its colour. We know that many birds will 

 accept anything, especially after beginning to breed, while 

 others will not. Some will try to incubate stones or pota- 

 toes. . . . The uniformly speckled eggs of the cowbird 

 (Molothrus pecoria) fare only too well when contrasted 

 with the snow-white eggs of the mourning dove, and 

 the nearly white eggs of vireos, flycatchers, goldfinches 

 and bluebirds.' 



