226 
THE CUCKOO’S SECRET 
Doubtless the real fact was that two or three 
Cuckoos were competing for the territory, no one 
of them having as yet acquired dominance. Our 
knowledge of the working of the pigmentary glands 
in a bird is incomplete, but we can be quite certain 
that no bird “ by taking thought ” can add to or 
alter the colour or markings of her egg. 
A subsidiary theory arises, as a corollary to the 
aforesaid conclusion that a hen Cuckoo prefers to 
lay in the nests of a particular species of fosterer, 
and that is that the given hen Cuckoo was herself 
fostered by a bird of that species. This must strictly 
be regarded at present as a theory, on account of 
the absence of indisputable proof. Such proof can 
only be obtained, so far as one can see, by some 
system of marking in one season a young hen 
Cuckoo, known to have been reared by a particular 
species of foster-parent, and of subsequently ob- 
serving the same hen Cuckoo, when mature, 
returning to victimise the same species of fosterer. 
This is “ a consummation devoutly to be wished,” 
despite the almost insuperable difficulties that will 
be obvious to any naturalist ; but at the same time, 
lacking such direct proof, it will doubtless be 
agreed that there is a strong prima facie probability 
in the theory. 
