[Illustration]
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.