NESTLESS BIRDS AND ANNEXERS 55 



But to return to the question of the origin of this 

 parasitic habit. Parasitism in birds may have had its 

 origin either through the parent or the offspring. At 

 the present day instances are by no means rare not 

 only of birds laying an odd egg in the nest of some 

 other, and it may be very distantly related species, 

 but of birds taking possession of a nest and driving 

 away the rightful owners often when it contained eggs 

 which have been hatched in due course with those 

 of the invader. Partridge eggs, for instance, are 

 frequently found in the nest of the Pheasant, whilst 

 those of the latter are perhaps as commonly dis- 

 covered in the home of the Partridge. The eggs of 

 Gulls and Eider Ducks have also been found in each 

 other's nests ; whilst Stevenson records that he had 

 frequently known Moorhen's eggs to be laid in the 

 nests of Coots. A Pochard's egg has been found in 

 the nest of a Tufted Duck; and even more curious 

 still a Moorhen has been known to lay an egg 

 in the unfinished nest of a Blackbird ! Starlings 

 are known frequently to turn Woodpeckers out of 

 their nesting holes ; House Sparrows still more com- 

 monly take possession of nests of the Martin. Many 

 similar instances might be given, although it will be 

 remarked that perhaps the majority of them refer 

 to hole-building or domed-nest-building species. We 

 can now understand how the accidental dropping of 

 an egg into an alien nest might gradually become a 

 fixed habit, natural selection having a tendency to 



