42 SECRETS OF ANIMAL LIFE 
at first sight. (1) To begin with, it is not an iso- 
lated phenomenon. It occurs in many other cuckoos 
and in the quite unrelated Cow-Birds. Some kinds 
of Old World Cuckoo follow the usual routine of 
nesting and brooding; the American Black-Billed 
Cuckoo, though usually a normal nesting and brood- 
ing parent, occasionally puts an egg in the nest of 
another bird; at least one species of Oriental 
Cuckoo is parasitic in one part of the country and 
nests in another; and there are many instances of 
diverse kinds of birds casually laying in the nests 
of their neighbors. Thus the cuckoo’s evasion of 
the normal parental duties is not an isolated 
phenomenon, and it is also instructive to remember 
that the parasitic instinct is not always perfect. 
Many cases are known of a cuckoo’s egg in an 
altogether unsuitable nest, for instance, in that of 
a bird which does not feed its young on insects. 
(2) A second consideration is that the mother- 
cuckoo’s behavior is congruent with some other 
peculiarities in the bird’s constitution and ways. 
Thus there are far more males than females (some- 
times perhaps five to one), and polyandry is the 
natural result, as has happened also with the Cow- 
Birds. And while it is probably going too far 
to call the polyandry the cause of the parasitism, 
it will tend to a slackening of parental ties, and in 
any case the parasitism is adapted to the polyandry. 
Similarly, the great fertility of the cuckoo (some- 
times credited with a score of eggs, though a dozen 
is probably nearer the truth) is as likely to have 
