306 ANIMAL INTELLIGENCE. 



she occasionally laid her egg in another bird's nest. If the old 

 bird profited by this occasional habit through being able ta 

 migrate earlier, or through any other cause; or if the young 

 weie made more vigorous by advantage being taken of the mis- 

 taken instinct of another species than when reared by their own 

 mother, encumbered as she could hardly fail to be by having 

 eggs and young at the same time ; ^ then the old birds or the 

 fostered young would gain an advantage.^ 



The instinct would seem to be a very old one, for there 

 are two great changes of structure in the European cuckoo 

 which are manifestly correlated with the instinct. Thus,, 

 the shape of the young bird's back has already been noted ;. 

 and not less remarkable than this is the small size of the 

 egg from which the young bird is hatched. For the egg 

 of the cuckoo is not any larger than that of the skylark,, 

 although an adult cuckoo is four times the size of an adult 

 skylark. And ' that the small size of the egg is a real 

 case of adaptation (in order to deceive the small birds in 

 whose nests it is laid), we may infer from the fact of the 

 non-parasitic American cuckoo laying full-sized eggs.' 

 Yet, although the instinct in question is doubtless of high 

 antiquity, there have been occasional instances observed 

 in cuckoos of reversion to the ancestral instinct of nidifica- 

 tion ; for, according to Adolf Miiller, ' the cuckoo occasion- 



* Allusion is here made to the fact that the cuckoo lays her eggs at 

 intervals of two or three days, and therefore that if all were incubated 

 by the mother, they would hatch out at diff f^rent times — a state of things 

 which actually obtains in the case of the American cuckoo, whose 

 nest contains eggs and young at the same time. 



2 It is worth while to observe, as bearing on this theory of the origin 

 of this parasitic habit, that even non-parasitic birds occasionally deposit 

 their eggs in nests of other birds. Thus, Professor' A. Newton writes in 

 his admirable essay on ' Birds ' in the Encyclopasdia Britannica, ' Cer- 

 tain it is that some birds, whether by mistake or stupidity, do not un- 

 frequently lay their eggs in the nests of others. It is within the know- 

 ledge of many that pheasants' eggs and partridges' eggs are often laid 

 in the same nest ; and it is within the knowledge of the writer that 

 gulls' eggs have been found in the nests of eider-ducks, and vice versa ; 

 that a redstart and a pied flycatcher will lay their eggs in the same con- 

 venient hole — the forest being rather deficient in such accommodation : 

 that an owl and a duck will resort to the same nest-hole, set up by the 

 scheming woodman for his own advantage ; and that the starling, which 

 constantly dispossesses the green woodpecker, sometimes discovers that 

 the rightful heir of the domicile has to be brought up by the intruding 

 tenant.' 



