What association seems to do for the young bird in the first instance is to eliminate a lot of useless reactions, by limiting its responses to those which count, for the amount of energy which goes to waste in this direction, especially up to the time of the later manifestations of fear, near the close of nest-life, must be very great. The pleasure of getting the food seems to lead to an association with the nest itself, the place where the food is received, and with the parent, the active agent, together with her calls and the vibrations of the nest or branches which attend her visits. Association with the nest seems to be strongest, for nestlings up to a late period respond freely to the call-notes of other birds, which happen to pass near, to sudden gusts of wind, at an early period especially, and to violent sounds of every description, like the distant whistle of a locomotive. A more curious sight, which illustrates the indefinite character of this association, is often witnessed up to the very last days of nest-life in many, if not all, of the common altricious birds of the country. Not only does the casual excitement of one bird arouse all the others into action, when, as it were, "the pot begins to boil," and then subsides, but the nestlings often respond to one of their mates, precisely as to the parent, for which it is plainly mistaken, crowding eagerly around it begging to be fed, and in their vain attempts to nestle under it, almost pitching it out of the nest.
In early nest-life, any sudden jarring of the nest or the branches about it will elicit the food-response as readily as when the vibration is imparted by the touch of the parental foot, but discrimination comes in this direction also, at any rate in certain species or individuals, and is often well-nigh perfect at the advent of the instinct of fear.
VI
The acquired habits or tendencies which have been described may in time become very uniform and more or less "stereotyped," but they are widely different from those instinctive characters which alone are inherited.
We have been obliged to speak frequently of fear, which in its origin must be regarded as an instinct in birds. It should not be forgotten, however, that if very young animals of any kind are separated from their parents, so that their natural environments and experiences are changed, we may look for a modification of this instinct, both as to the degree and manner of its expression, and as to the time of its appearance. In all the common altricious birds fear, in its later stages, is attuned with the instinct of flight, or, as in the case of the cuckoo, with the power of helping itself by entry upon a climbing stage, when seven days old. In a cedar waxwing, on the contrary, it is not well matured until the thirteenth or fourteenth day, when the entire brood, standing on their nests with heads upturned, suggest a gun, loaded,
