Spalding found that if newborn birds were prevented from hearing their mother's call for five days, they would still recognize her; if prevented from hearing the mother for 10 days, they would never learn to recognize her. That is, he demonstrated the existence of what later was called.

critical periods

Explanation:

Critical period, in developmental biology or psychology, is described as a "maturational stage" in an organism's (animal, human) lifespan during which the organism's nervous system is specifically sensitive to a few environmental stimuli. However, the critical period is the time in which an individual develops language-based cues i.e, around five years to puberty after which it becomes difficult to develop language acquisition and the person is quite unsuccessful in that.

As per the question, the correct answer is critical periods.