Parrots are famous for their mimicry of human speech in captivity, but have been less studied in nature. This dissertation addresses the development and function of vocal signatures in a free-ranging population of Green-rumped Parrotlets (Forpus passerinus) in Venezuela. I first showed that the most ubiquitous component of the adult repertoire, the contact call, functions as a vocal signature used to identify mates when visually separated. To understand how this uniqueness originates, I used audio-video-rigged nests and cross-fostering experiments to show that nestlings of both sexes learned at least part of the signature attributes of their contact calls from both primary caregivers. Using the same experimental design, I then tested whether developmental precursors of the contact call might also be learned. Male nestlings showed significant evidence of learning food solicitation calls from mothers, while begging calls of nestling females appear to have a significant genetic component transferred from biological mothers. My fourth and final chapter asked to what degree vocal ontogeny is related to physical growth and maturation, and how functionally-discrete signals might be related ontogenetically. Data mining analysis of a suite of spectrographic attributes revealed three well-defined stages to nestling vocal ontogeny. In a functional sense, the first two stages were always given in food-solicitation contexts. In contrast, the third stage was comprised of functionally discrete contact calls given when visually separated from parents. Contact calls were like the adults’ and contained significantly more individual information that begging calls. This represented a developmental metamorphosis in both a functional and structural sense, and a departure from “experienced-based” signals typical of the food solicitation stage-nestlings used contact calls to engage individuals that were absent, bur soon to be present. I speculate that the need to establish individuality forms the developmental basis of vocal learning in parrots.