REMOVAL OF EGGS 
205 
has always been responsible for removal when the 
number of eggs in a nest falls short of what it 
should be. I shall shortly bring forward evidence 
in support of the theory that in occasional instances 
the dupe will remove one or more of its own eggs. 
To the Ibis for January, 1918, C. F. M. Swyn- 
nerton contributed a very interesting and instructive 
paper on “ Rejections by Birds of Eggs unlike 
their own ; with remarks on some of the Cuckoo 
Problems.” In it the author describes his experi- 
ences of many experiments in interchanging and 
substituting eggs of several species breeding in 
South Africa. Some birds showed a very fine 
discrimination, others accepted almost anything. 
Herein we may see a reason why Cuckoos are 
parasitic mainly upon certain species ; for the less 
discrimination and resentment shown by the dupes, 
the greater is the chance of the embryonic young 
Cuckoo being hatched and reared to take its place 
in the perpetuation of the race. 
As pertinent to my present subject I quote one 
of Mr. Swynnerton’s results — 
“ On another occasion a Stonechat (Pratincole* 
torquata ) adopted a Shrike’s egg (Lanius collaris 
humeralis) given her in place of one of her own 
three. Several days later she was still sitting on 
