INSTINCT AND INTELLIGENCE IN BIRDS 95 
occasion, or build one of its own. It even laid eggs in artificial nests, 
which Hudson placed in trees to test its propensities in this direction. 
(3) The common practise of stealing nests of other birds, but of hold- 
ing them, as a rule, only for laying its own eggs, as illustrated to-day 
by the North American cowbird (Molothrus pecoris). The instincts of 
the intruder seem to be satisfied by “concealing” its eggs, or simply 
laying them against the wall of another bird’s nest, and leaving them. 
At this stage the European cuckoo, we may suppose, not only frequently 
dropped its eggs on the ground, but occasionally tried to incubate them, 
and may have even attempted a rough nest. At this stage also the 
normal tendency to lay eggs at daily intervals was possibly disturbed, 
and the interval became irregular, with the gradual establishment of a 
longer rhythm. 
At this point several roads would seem to be open, for the resources 
of nature are not limited to one course. Parasitic or non-brooding 
cuckoos have “ chosen ” one, so to speak, the brooding American species 
another, and if we are to accept the accounts, certain owls, which breed 
in the far north, successfully rear young in the short Arctic summer, 
with an interval of a week or more between each egg. Yet there can 
be little doubt that an undue lengthening of this interval would seri- 
ously interfere with nest-life in many species, and break the tendency 
to guard the egg. All would seem to depend upon the correlated 
instincts of parent and child. With an interval of from five to seven 
days, which has been credited to Cuculus canorus, self-brooding would 
be impracticable without a change in its instincts, for it migrates in 
July. While it is certain that the egg-laying interval was gradually 
extended in this bird, it is not known at what corresponding point the 
parasitic practise was finally established. Certain it is, however, that 
then as now, the egg, whether laid direct in a nest or dropped on the 
ground and subsequently conveyed to one, was abandoned. The Amer- 
ican brooding cuckoos (Coccygus erythropthalmus and C. americanus) , 
although suffering a similar disturbance in the brooding interval (of 
one to three days), have adjusted these differences by another course. 
The young which are hatched in succession, also leave the nest in suc- 
cession, when one week old, and enter upon a climbing stage which lasts 
a fortnight. In this way the brood is divided into two groups, and any 
untoward effects which might result from a marked difference in age 
of the nestlings, is avoided. The greatest disadvantage of such a mix- 
ture, in the nest of this species, would seem to lie in the fact that the 
oldest and strongest usually succeed in holding up most of the food. 
We may add that the American cuckoos have never advanced far be- 
yond the first stage, as designated above, although they have suffered a 
disturbance in the normal rhythm of egg-production, and that the 
parental instincts are as strong with them as in passerine birds. The 
