19, at eleven o'clock, I discovered a black-billed cuckoo in the act of brooding one young bird, a day old, and three eggs, in a small sapling pine, about three feet from the ground. The sitter, thus surprised, quietly retired and sounded her alarm from a distance. Twenty-four hours later, when I visited the scene, and when, as afterwards appeared, a second young bird had emerged, fear was more in abeyance, and the behavior different. I was allowed to approach as close as possible, and stood for twenty minutes, with the eye of the sitter not over twenty inches from my own, and only finally sent her off by trying to take her in the hand. Under these circumstances the cuckoo behaves much like the cedarbird, raising its head, though in a less marked degree, and remaining perfectly quiet, the only motions visible being those of breathing and the momentary flick of the third eyelid or nictitating membrane across the pupil.
V
The dawn of avian intelligence in the nestling, if one of the altrices, begins at about the third day, and in relation to the feeding reaction. Resting upon its huge pot-belly, as a central pillar, the little bird raises its trembling head, rather feebly at first, and supporting itself also, it may be, with its wings, and opening its mouth to the widest extent, thus displays its food-target. If the sign is unanswered, the head drops, the mouth closes, vibration ceases, and the bird lies prone, as if exhausted, the whole operation, which seems to call into play the entire body, lasting three seconds, more or less, according to the strength of the original stimulus.
This kind of behavior is a typical illustration of compound reflex action or instinct, and, when feeding follows, the reflexes assume more completely the chain form. When not due to hunger this response may be evoked at will by any suitable stimulus, whether tactile or auditory, whether the bird is in its nest or out of it, and regardless of the parent. The nestling now reminds one of an electrical toy, the action of which is purely automatic. Place the little bird on a piece of cloth or fold of your clothing and "press the button," that is scratch the cloth with the finger or a lead pencil, and behold this complex feeding response. When the nestling has not been fed to repletion, within the limits of fatigue, this reaction may be as automatic, uniform or stereotyped, and therefore predictable, as that of an electric bell.
Initial responses of this sort are relatively perfect. Consequently they can have nothing to do with past experience. They represent the hereditary powers of a hereditary mechanism. Now the point of greatest interest is that this inherited tendency to respond, in the course of a few days becomes replaced, as it were, by an acquired tendency. Instinct becomes "modified" by association. The mind or intelligence
