I remember coming back to Algeria after a year or so in America at the age of six. I had completely forgotten the Arabic I had known, and relearning it was an incredibly difficult process that took years, made no easier by my frequent preference for books over playmates. Over the past seven months, I've found learning Siwi and Korandje far, far easier than learning Arabic was then, and I'm pretty sure I speak both of them, if not fluently, at least far better than I spoke Arabic after my first four months back. Yet the nativist theory of language acquisition that I remember from my linguistics courses says that kids should learn languages much more easily than adults. I don't expect anybody to pick theories based on anecdotal evidence from my childhood memories, but this has made me wonder again whether kids usually learning languages faster and better is due to a pre-programmed critical period for language learning, or simply to the big difference between the social contexts of adults and children. Coincidentally (being back in London), I found two works vaguely relevant to that question this weekend; neither offers an answer, but they are interesting background.
Kant's Sparrow confirms a claim originally reported by Kant - that sparrows brought up by canaries learn to sing like canaries. Apparently, they do - but not completely. Not only do their canary songs feature a detectable accent (they differ in several ways, notably in repeating the same syllable fewer times in a row), but their repertoire includes some song types ("two-voice syllables") which they rarely or never heard from the canaries raising them, and which the author attributes to sparrows' innate repertoire (22.214.171.124.) In other words, sparrow song, like human communication, combines innate and learned (arbitrary, if you like) elements.
Wolf Child and Human Child, by Arnold Gesell, is a short, not very helpful work on a very interesting case, apparently described more fully in Diary of the Wolf Children of Midnapore, by Rev. J. A. L. Singh - two children, later named Kamala and Amala, who were adopted into a wolf family, and raised for years alongside the mother wolf's own cubs. In 1920, in response to locals' reports of a "man-ghost", a party of men dug into the wolf's den, killed the mother wolf when it tried to fight back, and brought the two children back to be taken to an orphanage (and the two wolf cubs they lived with to be sold at a fair.) Kamala was about eight, and Amala substantially younger; however, Amala died only a year later Unsurprisingly, Kamala found language rather difficult to acquire; even without the wolf factor, I imagine losing your entire family and then your entire step-family before the age of nine might have a negative effect. At any rate, apparently, she spoke her first word two years after being captured, and her first two-word sentence after three and a half years. For later years the information gets a lot sparser, but it is claimed that by the time the poor kid died (from illness) nine years later, at the estimated age of seventeen, she "talked freely with full sense of words used." The report that after several years "her formerly rigid countenance took on more expression" suggests a similar gradual development in her body language. However, while at eight years old she knew little or nothing of how humans communicate, she seems to have learned at least some wolf methods - for months at the orphanage, she would howl every night, at 10 pm, 1 am, and 3 am, and when approached by someone she did not trust she would show her teeth. Unfortunately, the lack of detail makes it hard to say what this says about first language acquisition - how well could she really speak before she died? Perhaps Rev. Singh's diary offers some quotes.
NB: see comments; apparently there is serious doubt about the veracity of this account. Looks like I should have Googled first.. The original diary also turns out to be online.