Furthermore, this element of cognitive aspect of linguistic development is equally evident in other species with complex languages, such as in many species of birds (Edey & Johanson, 1999; Simonds, 2002). Typically among songbirds, if infant birds are not exposed to the songs of their parents (or other adults of their species), they lose the ability to communicate in that manner even if they are repeatedly exposed to those languages later in life (Simonds, 2002). Feral children demonstrate precisely the same phenomenon: they lose the ability to mimic any form of human speech because they lack exposure to it in infancy. They may possess normal levels of intelligence and that accounts for their ability to establish alternate forms of communication, such as through sign language based on the logical connection to the physical world and even to some abstract concepts. Even deaf children likely have more opportunity to absorb critical information about human speech during infancy, because they observe the mouth movement patterns...

Feral children have much greater capacity to create home signs but they usually cannot ever overcome the lack of exposure to human language in infancy any more than a songbird that is deprived of the opportunity to hear the songs of their species.
References

Brownlee, S. "Baby Talk" U.S. News & World Report; (June 15, 1998): 48-55.

Dennet, D. (2001). Consciousness Explained. New York: Little Brown & Co.

Edey, M. And Johanson, D. (1999). Blueprints: Solving the Mystery of Evolution.

New York: Penguin Books.

Gerrig, R. And Zimbardo, P. (2009) Psychology and Life. Boston: Allyn & Bacon.

Simonds, C. (2002). Private Lives of…