THE CUCKOO AND HIS KINDRED. 129 



the young birds themselves, especially such species 

 as Nightjars, Pigeons, Petrels, Gannets, and Puffins. 

 Of course many apparent exceptions may promptly 

 suggest themselves to the reader, but we must be 

 very careful in treating them as exceptions when 

 we bear in mind how little we really know of the 

 subject of the delicate and assuredly certain relation 

 of Food to Reproduction. What may appear 

 difficulties and exceptions might really support our 

 case if the facts and conditions were more clearly 

 understood, or we had a more thorough knowledge 

 of them. It therefore seems not altogether im- 

 probable that the above-mentioned facts bear very 

 closely on the curious phenomenon of parasitism 

 amongst the Cuckoos. The young of the European 

 Cuckoo is notoriously voracious ; its food is of a 

 kind that is not readily obtainable, namely, cater- 

 pillars and insects, each individual one usually having 

 to be sought for and conveyed to the nest singly. 

 It is obvious, therefore, that the parent Cuckoos 

 would find the task of rearing from five to eight 

 nestlings a difficult if not impossible one, and that 

 they have gradually acquired the habit of dis- 

 tributing the labour of rearing each young bird 

 amongst such species as are best able to bring it 

 to maturity. We might reasonably presume that a 



