LIFE AXD BEHAVIOR OF THE CUCKOO 
229 
guarding and concealment, out of which the instinct of incubation, 
has, in my opinion, grown. Without any doubt the act is as 
purely instinctive as any other by which the ends of reproduction 
are secured to the species. 
Parasitism could never succeed as a general practice on a large 
scale, and the fact that it is a specialty of two families of birds, 
shows that it is probabl}^ correlated with a peculiarity which they 
possess in common. This is to be found in a change in the rythms 
of the reproductive activities, leading to a change of instincts. 
This change in the American cuckoos is now chiefly expressed in 
the appearance of eggs at various intervals, or at intervals greater 
than one day and rarely exceeding two. The disturbance has 
in this case been completely met and adjusted by the develop- 
ment of a climbing stage in consequence of which the young can 
leave the nest in succession, and nest-life can proceed with eggs 
and young existing at the same time with prejudice to neither. 
As to the 'Svh}^'' of this problem, that is, why has the normal 
rhythm of the reproductive cycle been disturbed in the way de- 
scribed, nothing is certainly known; we can only surmise that the 
causes of all such changes concern the central nervous system, and 
that in the main they are independent of food-habits. 
The nests of these American cuckoos are slight, but adequate, 
and afford no more evidence of a decay of the nesting instincts 
than does the even cruder nest of the mourning dove, or many 
another wild species. However, in support of Darwin's views, it 
has been often pointed out that the yellow and black-billed 
cuckoos occasionally interchange eggs, and steal the nests of other 
birds. This is true, but since any bird is likely to steal a nest 
whenever its reproductive rhythms are upset at a certain point, 
it has no significance, unless it can be shown that such cuckoos 
as do appropriate nests, do not subsequently make nests or rear 
their own young. I have found recorded in the literature, so far 
as I have been able to review it (table 7) 24 cases in which these 
cuckoos have either interchanged eggs, or laid, in other birds' 
nests. One of the most interesting observations under this head 
is quoted by Bendire^^ from Mr. J. L. Davison, of Lockport, 
" Op. cit. 
