472 THE CUCKOO 



may be found in one nest, probably laid by different birds. The eggs 

 of the victim may be ejected or punctured either by the beak or claws 

 of the cowbird. As the eggs are found in hole-nests, the species 

 must, occasionally at least, lay the egg outside the nest and then 

 carry it in the beak. 1 



Some of the explanations of the cuckoo's parasitism that have 

 been offered are mainly interesting as illustrating the chief difficulty 

 which besets the question, that of distinguishing between cause and 

 effect. An example is supplied by Jenner's view that the short stay 

 of the cuckoo about three months prevents it from performing its 

 parental duties, but its short stay may be an effect and not a cause 

 of the parasitism. 2 The same difficulty applies to the view that 

 explains parasitism by polyandry. 3 



One might argue that our cuckoo has completed the stage through 

 which the American yellowbilled-cuckoo appears to be going. It has 

 found it more advantageous to use the nests of others than build an 

 unsafe nest of its own. This view overlooks two difficulties. All 

 cuckoos would not build equally badly, and it seems unreasonable 

 to assume that natural selection would lead to the extinction of the 

 individuals building nests that were adequate to their purpose in 

 favour of individuals compelled by their incapacity to lay in the nests 

 of other species ; it is difficult to see what advantage the offspring of 

 the latter would have over those of the former. Further, the habit 

 of occasionally laying eggs in the nests of other species surely comes 

 under the head of an acquired character one gained by the individual 

 during the course of its experience. Can such be inherited ? 



The cause of the parasitism of the cuckoo, whatever it may be, 

 must have been exceedingly potent in its operation, for not otherwise 

 could it have completely annihilated those deep-rooted parental 



1 Bendire, op. cit., p. 430. 



1 The statement that certain non-migratory species are parasitic cannot be used as an argu- 

 ment against Jenner's view till it is shown that these species were always non-migratory. They 

 may have become parasitic during a previous migratory period.- 



' The view that the somewhat peculiar position of the cuckoo's stomach makes the function 

 of incubation difficult has long been discredited, it having been shown that other species 

 which do incubate have the same peculiarity. 



