128 A HISTORY OF BIRDS 



relationships so close as is the case with the lower animals. 

 Commensalism and symbiosis are unknown among the birds, 

 while the parasitism of which instances have been given, and of 

 which we are presently to speak, is of a very different kind to 

 that which obtains among invertebrates. 



As may have been surmised, the parasitism of the Cuckoos 

 and some other birds is now to be discussed. In this form of 

 parasitism the female has adopted the practice of depositing 

 her eggs in the nests of other birds and there leaving them, 

 without further care, to be hatched by her dupes, who, un- 

 conscious of the trick which has been played, perform the 

 duties the immoral parent has contrived to shirk. 



First of all as to the Cuckoos. Though the broad facts of 

 the case, as illustrated by the Common Cuckoo {Cucnlus carionis), 

 are well known, they will be repeated here for the sake of the 

 bearing they have on less widely known facts which have 

 been brought to light during recent years, facts which are of 

 the highest importance. 



This bird then, which reaches the shores of Great Britain in 

 the month of April, deposits its eggs, singly, in the nests of the 

 smaller Passerines, though occasionally, by an error of judg- 

 ment, other species are selected. No less than eighty-four 

 different species of birds are known to have been victimised in 

 this way — including Eider-ducks and Grebes ! — but the majority 

 of such eggs are distributed among Robins, Hedge-sparrows, 

 Wagtails, Meadow-pipits, Tree-pipits, Warblers, Thrushes and 

 Red-backed Shrikes. 



The egg is duly brooded by the dupe with her own eggs, 

 and in due time is hatched. Within an hour or so of this 

 event a gruesome domestic tragedy is invariably enacted ; for, 

 prompted perchance by the overmastering instinct of self- 

 preservation, the foundling proceeds to eject from the nest the 

 offspring of its foster-parents, as though conscious that they 

 would be utterly unable to provide sufficient food to satisfy its 

 insatiable appetite while these competitors remained alive. 

 Accordingly, with a most diabolical persistence and ingenuity, 

 this blind and naked hooligan proceeds to the work of eviction, 

 one after another being pitched over the edge of the nest. This 

 feat is performed by burrowing under the victim until the body 

 thereof rests upon the middle of the little murderer's back, 



