191 7- J Breed') ii(j-habif.s of the Cuckoo. 191 



eggs, and have found five Golden Plover's eggs in a nest, 

 one o£ which was obviously laid by a different female, and 

 many other such instances are on record. 



Also, judging from the behaviour of domestic fowls, it 

 seems clear that the sight of a nest with eggs in it exercises 

 an attraction on a bird having none of its own, when it is 

 about to lay an egg, and if no opposition is met with. 

 Further, in the family of the Cow-birds, the members of 

 which seem in a curiously transitional state, we find in some 

 species that two females often lay in the same nest, whilst 

 one, M. rufoaxillaris, only selects M. badius — and no other 

 species — as foster- parent. 



If, then, we assume that, having by degrees ceased, more 

 or less entirely, to build a nest of its own, the Cuckoo was 

 unable to find suitable places in which to deposit its eggs, 

 we can easily imagine that, by degrees, and on the analogy 

 of the domestic fowl and other birds, it adopted the habit of 

 placing them in the nests of other species in which there 

 were already eggs, and also (and this is a point of con- 

 sideral)le importance in subsequent conclusions), it may be 

 presumed that preference would be given to such nests as 

 contained eggs more or less similar to its own. The females 

 adopting this habit would be freed from the exhausting 

 duties of incubation and rearing their young, and, as occurs 

 in those kinds of poultry in which the incubating instinct 

 has been suppressed, would probably lay more eggs in a 

 season, and consequently more j^oung would be reared. 

 The young so reared would have the parental instinct still 

 more suppressed, and thus the practice would become 

 hereditary. 



On the other hand, we can easily imagine that the families 

 of Cuckoos which still reared their own young would 

 decrease rapidly in numbers, given a careless mother and 

 nestlings requiring so much food for their rapid growth that 

 only the strongest of each brood survived, for it seems very 

 jiossible that the instinct of ejecting or starving out the 

 other inmates of ,ihe nest was an early, or even original, 

 trait. 



