LIFE AND BEHAVIOR OF THE CUCKOO 
227 
the nest seems to be built in anticipation of the eggs which are 
afterwards laid in it, and these eggs to be guarded and treated 
in view of the young which are to issue from them. The normal 
rythms are not only very precise and uniform but the correlated 
instincts of parent and child fit like key and lock. Yet perturba- 
tions, as we have seen, are liable to arise at every step, whether 
it be in migration, nest-building, egg-laying time or interval, or 
in the nurture of the young, and are more or less fatal according 
to circumstances. The young, moreover, sometimes fail in the 
development of their instincts, as when a thrush or warbler flees 
its nest before ready for flight, in consequence of the premature 
development of the sense of fear. 
The most common failure is, without doubt, in the adjustment 
of nest-building to the time of egg-laying, and I believe that at 
this point ''parasitism," as it is somewhat ambiguously called, 
in birds, took its rise. In many of these cuckoos and cowbirds 
there was a tendency then as now, not only to lay the eggs before 
there was a nest to receive them, but to lay them at slightly ir- 
regular intervals. 
Dropping eggs on the ground and taking no further interest 
in them is a form of behavior, which if generally indulged in would 
soon settle the question of survival most effectively, and although 
this rigorous form of selection is only sporadic in most species, it 
is reported to be common in the European cuckoo, for many of the 
cowbirds, and probably occurs more frequently in all birds than is 
commonly supposed. According to Hudson, whose work on the 
cowbirds of Argentina has been frequently quoted, the common 
species of that country (Molothrus bonariensis) frequently waste 
their eggs by dropping them on the ground, and they even laid 
in the old nests which he placed in trees for the purpose to testing 
them. While this species is now generally parasitic, it scatters 
its eggs in all directions, and on two or three occasions was ob- 
served by this naturalist to attempt to build a nest of its own, 
but without success. The North American cowbird (Molothrus 
pecoris) is also known to occasionally drop an egg on the ground, 
and when it is realized that a fresh egg is a prize to every preda- 
ceous animal which roams by day or night, it is not surprising 
that such habits are not more frequently reported. 
