44 SECRETS OF ANIMAL LIFE 



They are adapted to their curious history in being 

 relatively small (though with considerable variabil- 

 ity in size) and in having thick resistant shells, but 

 still more strikingly in being, in many cases, like 

 facsimiles of the eggs of the selected foster-parent. 

 The fine " Fenton Collection " of birds' eggs in the 

 University of Aberdeen has over four hundred 

 cuckoo's eggs in the clutches of over fifty different 

 kinds of foster-parents, and the two immediate 

 impressions that one gets are, first, that the cuckoo's 

 egg is often a perfect copy of those of the foster- 

 parents; and, second, that it is often obtrusively 

 conspicuous. Now it seems to be practically certain 

 that the same cuckoo lays the same type of egg con- 

 sistently, and it is probable that Professor Newton's 

 theory is right, that having a blue egg, for instance, 

 may be hereditary in a given lineage, and that there 

 may also be in the same lineage a hereditary pre- 

 disposition to put the egg in a Redstart's blue 

 clutch. If the cuckoo is hurried or flurried, it may 

 put the egg in a clutch with which it is inharmoni- 

 ous, and as this often succeeds perfectly well, it 

 seems likely that some kinds of birds are much less 

 sensitive than others to the presence of an intruded 

 egg. Thus it is very rarely that a blue Cuckoo's 

 egg is found in the blue clutch of the Hedge- 

 Sparrow, and there is no "mimicry" between a 

 Cuckoo's egg and a Wren's. 



Another of the major cuckoo-puzzles has to do 

 with the behavior of the young bird in the nest. 

 What Jenner observed so carefully in 1788, several 



