226 
FRANCIS H. HERRICK 
general behavior, which we have elsewhere described as the 'rise 
of the brooding instinct.' 
The activities of nest-Hfe do not invariably follow in the single 
groove already described, for while the parents seldom visit 
their nest when it contains young, during daylight, without bring- 
ing food, they are quickly drawn to it by the presence of enemies, 
and occasionally merely to inspect or to brood. This was noticed 
only at nest no. 3, where out of 100 recorded visits by both 
birds, but 6 were made without food (table 5). In each case 
the female, and usually when brooding, would leave the nest 
quickly, presumably to forage on her own account, return, inspect, 
cleaning the nest if necessary, and then settle to brood. 
12. "WHY SOME BIRDS DO NOT BROOD," OR ORIGIN OF "PARASITIC 
HABIT" 
Although the question is often cautiously put in the form given 
above, it might better read, ''Why have certain cuckoos and cow- 
birds lost their nesting instincts," for it is perfectly safe to assume 
that every one of these birds is descended from ancestors which at 
one time built nests, and brooded like the majority of their kind. 
If, as Baldamus and others assert, the eggs of Cuculus canorus 
are laid at intervals of 6 or 7 days, it is evident that the success- 
ful rearing of young in this species would now be difficult if not 
impossible. It is equally clear that this unduly lengthened in- 
terval is a secondary condition, to which it is impossible to as- 
sign the lapse of the instincts to care for the young, as shown 
by the habits of our own cuckoos. The primary step which 
finally led to parasitism is to be sought farther back, and lay, as 
we believe, in that very lack of attunement between egg laying 
and nest building, to which all birds are casually subject, but 
which is especially characteristic of those cuckoos, cowbirds, 
and hangnests, among which alone in the great avian class this 
much discussed parasitism has been chiefly developed. 
In the analysis of the cyclical instincts (section 5) , we have seen 
that the reproductive cycle of birds is characterized by a series 
of discrete types of action, which may or may not be recurrent, 
and which follow in chain-like sequence, one kind of action being 
performed as if in anticipation of that which is to follow. Thus 
