LIFE AND BEHAVIOR OF THE CUCKOO 
231 
we might say two of the stages, for observe that this is really 
a double structure. The cuckoo seized this robin's nest while still 
incomplete, and filled it with rootlets, that is, used it as a site on 
which to erect a nest of its ow^n. The robin held its ground long 
enough to deposit an egg. Then came the dove and stole the nest 
from the cuckoo, laid her eggs in it, without adding any new mate- 
rials, so far as known, and was trying to maintain herself in posses- 
sion when discovered. That the dove came last is shown by the 
fact that her eggs were the only ones in which incubation had not 
advanced. The addition of the rootlets by the cuckoo show^s how 
firmly instinct held the reins of action, and the visit of the dove, 
famed for her strong parental instincts, warns us of the folly of try- 
ing to fasten an incipient parasitic habit upon the cuckoo from 
such casual or sporadic acts alone. The fact that such a bird as the 
mourning dove, should steal a nest, and hold to it for the purpose 
of rearing its young, shows that failure to build a nest was with- 
out doubt due to a disturbance of the normal reproductive cycle, 
the eggs being forthcoming before the usual nest was ready. 
Under such circumstances it would seem that any nest provides 
a visual stinmlus to the act of laying in it, as in the case of the 
common cowbird of Argentina. In a similar manner the brown 
thrush, quail, and many other species of birds have been 
known to casually lay eggs in the nests of other birds. Probably in 
many such cases and in the others given in table 7, as in the known 
instance quoted above, the intruder is ready to fight for posses- 
sion, but commonly yields, and later builds a nest of its own. 
The development of the parasitic habit of the European cuckoo, 
has led to a gradual lengthening of the egg-laying interval, and 
its young has acquired the evicting reaction, together with other 
secondary changes. Finally, to bring this long section to a close, 
we have only to refer to the African and South American ostriches, 
in which the instincts of the female have undergone most pro- 
nounced changes, leading to a great waste of their eggs, which 
Darwin, Schillings, and other travellers speak of finding freely 
scattered over the plains. The adjustment seems to have been 
effected here by the strengthening of the parental instincts of 
the male, upon whom almost the entire duty of incubation and care 
of the young is said to fall. Moreover the neglected eggs are 
