182 
THE CUCKOO’S SECRET 
To sum up the foregoing remarks : I think 
that it is only reasonable to assume that a Cuckoo 
does not lay until the incentive to do so has first of 
all been provided by the sight of a pair, or probably 
more than one pair, of her particular dupes actively 
engaged in the preparation of their nests. The eggs 
thus “ conceived ” are subsequently fertilised, and 
ready to be laid at a time when her intended dupes 
have arrived at a period in their domestic affairs 
suitable to their reception. In the meanwhile the 
Cuckoo usually pays one or more visits to the 
destined fosterers’ nests, probably in order to 
locate them precisely, and to assure herself of their 
satisfactory progress. From this can be deduced 
the reason for eggs of even dominating Cuckoos 
appearing in the nests of rare and uncommon 
fosterers, and for Cuckoos normally parasitic 
upon one species to make occasional use of a 
nest of another species. The reason is that 
some abnormal delay has occurred in the com- 
pletion of the nest by the pair of dupes on which 
the Cuckoo had “ conceived ” her egg, and when 
the time comes that she is forced to lay it, she 
resorts to a nest of some other species that has 
come within her ken. 
This is strikingly borne out in the case of 
