io 4 THE CUCKOO’S SECRET 
learnt in 1921. So amazingly quickly is the egg 
laid that I failed during 1920 to discover that the 
Cuckoo actually sat on, and laid the egg in, the 
nest. Is it likely, then, for those who claim to have 
accidentally witnessed a Cuckoo in the act of 
laying to be strictly accurate in their impressions ? 
She invariably removed one, and only one, of the 
fosterer’s eggs, no matter whether that was the 
only fosterer’s egg in the nest or one of several. 
On no occasion, either in this or in the following 
season, did this Cuckoo lay in a nest wdiere there 
was not already at least one egg of the fosterer. If 
this be typical — which in all probability it is — it 
points to the conclusion that usually when a Cuckoo’s 
egg is found alone in a nest it was not placed there 
before the dupe had begun to lay but exchanged 
for one egg already laid. This would, of course, 
not apply to cases where all eggs have been removed 
from a nest upon a day on which it was a Cuckoo’s 
intention to lay in that nest ; for example* see the 
preceding notes for June 12 and 13 on pp. 80-83. 
But I incline to Dr. Rey’s opinion that — 
“ there is an extraordinary individual variation 
in the biology of the Cuckoo which may become 
locally fixed through the influence of heredity. 
Many an unedifying dispute regarding the life 
