THE LAYING OF CUCKOOS 179 
in 1921 with hides and cinema apparatus) had we 
observed a Cuckoo visit a nest without subsequently 
getting a Cuckoo’s egg from that nest. Is it not, 
then, clear that the Cuckoo watches her intended 
dupes building and at their nests for the sole 
purpose of providing a fosterer for her own off- 
spring ? 
I contend that the very sight of her natural 
fosterers preparing a home for their own young is 
sufficient to incite the female Cuckoo in occupation 
of that territory to reproduce her own species. It 
is at least significant, and pertinent to my theory, 
that the interval between the time at which the 
Cuckoo finds a nest and the time when she uses 
that nest for the deposition of her egg has been 
shown to correspond very much to the interval 
requisite to small birds before laying the first egg 
in another nest after the previous nest has been 
destroyed. And although it may not be generally 
realised, it is nevertheless a fact, and an interesting 
coincidence, that some of the eggs laid by a pen of 
hens will be found to be fertilised five days after 
the first introduction of a male bird. 
If, then, it be conceded that the sight of her 
natural fosterers preparing a home for their own 
young is sufficient to incite the female Cuckoo to 
