begins to take the reins, though feebly at first, into its own hands. Out of many experiments made upon the young of our common wild birds I select by way of illustration the brown thrush and black-billed cuckoo. In July 24, a nest of this thrush which I had been watching contained three birds, the youngest of which barely had its eyes open, and was from twenty to thirty hours old. When taken from its nest, at 3:15 p.m., and tested in the way suggested above, I obtained one hundred food reactions in thirteen minutes, each representing the entire complex of movements already analyzed, and each lasting from three to fifteen seconds, according to the strength of the stimulus received. A test immediately following gave fifteen reactions to the minute. The reaction time, which was very rapid at first, seemed to slow down only as a result of fatigue, but there was not a single failure to react upon the given stimulus. The same reaction was produced by clapping the hands, touching the bird, or holding its head between the fingers. On this day the two older birds, which were from two to three days old, would react while in the nest, but not when out of it. Now, upon the next day, about twenty-four hours later, as well as upon subsequent days, when the same tests were made with the same birds none of them would react when removed from the nest, with the exception that one feeble response was obtained from thrush No. 3, on July 25.
The same result was obtained with cuckoos. I was unable to get a single food-response from a black-bill, four or five days old, during the twelve hours or more that it was held a captive away from its nest. It would have starved rather than open its mouth of its own accord, and it even regurgitated the food which was pressed into its throat and gullet. Its whole conduct showed conclusively that the sense of fear had not yet appeared, but the moment this hungry bird was returned to its nest, and its feet touched the familiar twigs, it seemed to expand, as by magic, into a new creature, for standing erect, with every feather tube raised, and with vibrating wings, the neck trembling like a tuning fork, it opened its mouth and gave the food-reaction with all the vigor of which it was capable, and gave it repeatedly, loudly calling.
Edinger speaks of the modification of behavior described above, as an acquired habit, by which nestlings assure themselves against impending danger, and compares their attitude with that of an old bird coming hesitatingly to the nest, and looking about as if in anticipation of trouble. A simple experiment, however, like that just given upon the cuckoo, shows that this modification has nothing to do with assurance, for it begins long before there is any decided evidence of fear, and the little bird does not begin to look about in a suspicious manner until it is six or seven days old, when fear is arising. Moreover, all the attitudes, expressive of this instinct, both in young and adult, must, I believe, be considered as instinctive and not acquired.
