232 
FRANCIS H. HERRICK 
not wholly wasted, for those strewn about the vicinity of the nests 
are said to be largely devoured by the young. 
13. CONCLUSIONS 
1. Cuckoos do not display more intelligence than many other 
species of birds, the extraordinary acts which many of them per- 
form being sufficiently accounted for by the possession of modified 
and highly specialized instincts. 
2. The origin of the parasitism in many of the old World 
cuckoos and American cowbirds is to be sought in the disturbance 
of the cyclical instincts, to which it has been shown that these 
families of birds, are especially subject and in particular in the 
attunement of egg-laying to nest-building. Sporadic cases of 
this sort occur in all birds, when they either drop their eggs on the 
ground and eventually abandon them, or lay in other birds' 
nests, when they will sometimes fight for possession. We may 
assume that through the action of inheritance and selection the 
practice has become established more or less completely in the 
present parasitic species, but while we can indicate the steps of 
the process, the causes which have led to each in succession, can 
only be surmised. 
3. American black- and yellow-bill cuckoos show a tendency 
to produce eggs at irregular intervals of one to two or three days, 
which accounts for the presence of eggs and young in their nests 
for a longer time than is usual, -but here the comparison ends. 
Any disadvantage which might arise from such a condition has 
been completely allayed by an early division of the young, each 
one of which (in the black-bill) leaves the nest in succession on 
the seventh day from birth, and spends about cwo weeks in a 
climbing stage preparatory for flight. Special powers and instincts 
have arisen in the young in adaptation to this condition. 
4. The evicting instinct of certain Old World cuckoos has 
apparently arisen as a response to a contact stimulus of a dis- 
agreeable kind, which would be more irritating in a living and 
moving nestling than in a dead one. It is transitory, beginning 
to rise on the first to third days, and to wane in the tenth to the 
fourteenth. 
