CONDITIONING IN BIRDS [Ref]
The classical experiment in conditioning and reflexes is that of Pavlov. It consisted of sounding a bell each time food was given to a dog. Finally the salivary response resulted even when the bell was rung, without the food being given to the dog. The dog was conditioned to the bell. First it had responded to the food, then to the food and the bell, and finally to the bell alone, by a flow of saliva. The beauty of this experiment is in its simplicity, dealing as it does with a single reflex.
Though much behavior is more complex, experiments have been worked out to show how the environment, in a broad sense, can influence inherited behavior. An illuminating example of this is the one I made dealing with young loggerhead shrikes and the duration of their infantile behavior. Young shrikes, as with young passerine birds in general, while in the nest are fed directly by the parents, who place food in their mouths. One of the earliest behavior patterns these young birds perform is to stretch up with widely opened mouth, fluttering wings, and buzzing calls, in anticipation of being fed. This we call begging. Though typically infantile behavior, it may reappear in courtship, but this latter we will not consider here.
Ordinarily this infantile begging behavior is discontinued shortly after the young birds leave the nest and become able to feed themselves. Observations indicate that in a state of nature this change is probably hastened in part by the young birds themselves, who come to avoid having food thrust down their gullets, and prefer to pick up the food for themselves, and in part by the waning interest of the parents in the young, which confers an advantage on the young who early become self-supporting.
CASE OF RETARDED DEVELOPMENT Certain observations made from time to time have indicated that though the age at which young birds changed from infantile begging for food to self-supporting independence was a fixed thing, started by instinct, certain external factors, notably the amount of care the young received, could affect the age at which this change occurred. Indeed there was a record of a young cedar waxwing raised by hand who never learned to feed itself.
When I secured a brood of four young loggerhead shrikes, or butcherbirds, the material was available to conduct a controlled experiment. The young birds were raised together by hand to the stage where they were ready to begin to pick up things, to feed themselves, and to begin to abandon their infantile behavior of begging for food. This was when they were twenty-one days old. They were then divided into two lots and housed separately. One couple had a supply of food kept in front of them, and hand feeding was gradually discontinued and stopped as soon as possible. At the age of twenty-eight days they fed themselves well, though they still begged freely when I approached. By the time they were thirty-nine days old they begged rarely, and after the age of forty-five days they were not seen to beg.
The other couple had no free food available at any time, and they were fed completely by hand, the food being placed in their mouths. At the age of twenty-eight days they had made no effort to feed themselves. By the time they were fifty-three days old they made efforts to feed themselves, trying to peck the food from the fingers instead of having it thrust into their mouths, and evidently would have changed quickly to independent self-feeding and abandoned their infantile begging behavior. But hand feeding was continued. At the age of seven and a half months, when the experiment was discontinued, though these birds were capable of feeding themselves, as was seen when food was accidentally dropped on the floor of their cage, they still begged for food from their human foster parent.
OBJECT LESSON FOR PARENTS These four birds used in this experiment were nestmates, and had similar heredity and early environment. The birds in the lot which received only enough care to ensure proper development became self-feeding, independent, and lost their infantile begging behavior when they were about a month and a half old. The other lot, which received an excessive amount of care in the latter part of infancy, and were hand fed without being allowed to develop the behavior that would have made them independent, retained the infantile behavior pattern of begging to be fed until the end of the experiment. They were then seven and a half months old, and their nestmates, under a different set of conditions, had lost their infantile behavior six months earlier.
With some birds it appears excessive care can be a conditioning factor. It can delay the loss of infantile behavior and the acquiring of the normal independence. Though instinctively the young shrikes tried to develop their independent behavior, when this was not possible they continued their dependent, conditioned behavior.