SOCIAL PARASITISM IN BIRDS 



563 



larity in the egg-laying intervals." He 

 applied this idea to both the Cuckoos and 

 the Cowbirds and probably would have 

 extended it to cover the parasitic Weavers 

 and Honey-guides as well had he known 

 of them at the time. He writes that 

 "Parasitism could never succeed as a 

 general practice on a large scale, and the 

 fact that it is a specialty of two families of 

 birds shows that it is probably correlated 

 with a peculiarity which they possess in 

 common . This is to be found in a change in 

 the rhythms of the reproductive activities, 



leading to a change of instincts 



As to the 'why of this problem', that is, 

 why has the normal rhythm of the repro- 

 ductive cycle been disturbed .... 



nothing is certainly known " 



(Herrick, 30). 



The first writer to see that one explana- 

 tion would not serve for all the different 

 groups of parasitic birds was G. M. Allen. 

 In the chapter on parasitic birds in his 

 admirable book (1) he discusses all the 

 parasitic groups in a general way and 

 ends by saying that one must be prepared 

 to find that the parasitic habit has been 

 acquired in more than one way, and inde- 

 pendently in the different groups exhibit- 

 ing this habit. Wisely refraining from 

 offering an explanation of parasitism, he 

 suggests several ' 'possible ways of origin. ' ' 

 One of the possibilities is that parasitism 

 may have arisen from the occasional laying 

 of eggs in strange nests by birds that are 

 very sensitive to the ovarian stimulus pro- 

 vided by the sight of a nest with eggs re- 

 sembling their own. This is substantiated 

 by experimental evidence collected by 

 Craig, who found that in doves ovulation 

 could be induced by comparable stimuli. 

 In the case of the Flicker [Colaptes auratus] 

 . . . . the presence of a nest-egg seems 

 to encourage them to keep on laying as if 

 to attain a number whose contact stimulus 

 would satisfy the brooding instinct. It may 



be that in the case of those ducks whose eggs 

 seem so often to be laid promiscuously in 

 nests of their neighbors, the mere sight of 

 a nest with eggs resembling their own 

 may act as a stimulus inducing them to add 

 to the number" (1). Chance's field 

 observations on the European Cuckoo are 

 more or less in accord with this idea as he 

 believes that the sight of her victims 

 building their nests acts as a stimulus to 

 ovulation so that the female parasite has 

 an egg ready to be laid five or six days 

 later. This is also true of some of the 

 Cowbirds. 



However, I cannot agree with this 

 suggestion as a possible origin of the 

 parasitic habit unless it be accompanied or 

 preceded by a marked reduction in the 

 attachment of the bird to its own nest. 

 Even if the sight of eggs in strange nests 

 stimulated egg production in a bird that 

 was not parasitic, its natural instincts 

 would associate the resulting eggs with 

 its own nest and the bird would probably 

 lay them there, unless, as I said above, its 

 attachment to its nest were greatly 

 diminished. Then, too, after it has laid 

 the proper number of eggs, "whose 

 contact stimulus would satisfy" its 

 brooding instinct, it would normally 

 begin to incubate and stop laying. If its 

 nest-attachment were sub-normal in 

 strength, the bird might then wander 

 about to some extent and, on receiving 

 more visual stimuli might revert to egg- 

 laying. However in such a case, its own 

 eggs would have a lessened chance of 

 survival. 



Another possibility suggested is that if 

 a bird got into the habit of breeding in old 

 nests of other birds, it would be easy 



.... to imagine that the bird might not dis- 

 criminate between a newly completed nest and one 

 recently abandoned. The result would be that if the 

 intruder laid in the new nest, its rightful owners 

 would resent the intrusion and prevent the repetition 



