i8o 
THE CUCKOO’S SECRET 
reproduce her own species, will not the converse 
be true ? And when the cause — fosterers building 
their nests — is absent, will not the effect, that is the 
desire on the part of the female Cuckoo to reproduce 
herself, be checked, at any rate to some extent ? 
The fact that we decided in advance to try to 
stop Cuckoo A laying after she had laid fifteen 
eggs in 1921 is surely strong evidence, seeing that 
Cuckoo A did in fact lay only fifteen eggs in 
1921. 
Clearly, if it be true that the Cuckoo tends to 
place her eggs in the nests of that species by which 
she herself was reared, it stands to reason that each 
Cuckoo has to adapt its laying period to conform 
with that of its dupes. And since it has been so 
conclusively shown that at least very many Cuckoos 
confine their parasitism to one species, it does 
not affect the argument whether or no they 
themselves were reared by that species, although 
it is now generally accepted that they were so 
reared. 
Assuming it to be now established beyond the 
possibility of controversy that the normal interval 
between the laying of the eggs of a prolific Cuckoo’s 
series is a regular one of about forty-eight hours, it 
will probably be admitted by most ornithologists 
