338 THE STRUCTURE AND LIFE OF BIRDS CHAP. 
believe, at intervals of several days, so that if she were 
herself to undertake the incubation, she would have 
‘to leave one for some time unsat upon, or else have 
eggs and young in the nest together at the same time. 
A German naturalist, Karl Eimer, gives a rather 
different account ; the Cuckoo lays two eggs ina clutch ; 
that is, if she made a nest herself, she would lay only 
two eggs in it. And as she generally migrates 
before, August, she would not, if she herself nested, 
get many young ones reared in the course of the 
summer. In favour of the former view it may be 
urged that the American Cuckoo, who almost always 
builds a nest for herself, does have young birds and 
eggs in process of hatching in the nest at the same 
time. In any case we must look to the bird’s great 
laying powers for the explanation of the cuckoo 
instinct. The egg of the Cuckoo is wonderfully small 
considering the size of the bird. It is less than an 
inch long, and inch broad. A Hedge Sparrow’s egg 
is about 4 inch long and a little more than 4 inch 
broad. Thus there is no very great difference in bulk. 
But the Cuckoo is 12 inches long and the Hedge 
Sparrow only 54,a monstrous disparity even when we 
allow for the length of the Cuckoo’s tail. The dimin- 
utive size of the interloper’s egg no doubt deceives 
the toster mother, and is necessary if it is to hatch as 
carly as those of the rightful owner. Moreover, if it 
were not so small, how would the bird after laying it 
be able to take it in her beak and deposit it in the 
nest where it is tobeleft? The egg sometimes varies, 
approaching in colour those in the particular nest 
chosen, so that it is bluish when laid in a Hedge 
