This interdisciplinary ethnography seeks to make a contribution to the practice of a centuries-old social custom, 'tamariki 'āngai' or 'informal island adoption,' as practised by the Mangaian Cook Islanders. Tamariki 'āngai refers to both the tradition of nurturing or 'feeding' children and the fostered children themselves.During preliminary observations and inquiry among my neighbours in Mangaia for several years, I discovered that their custom is widely practised in a variety of forms of social replacement that have many incentives and dilemmas. Herein I document what the tradition represents to the Mangaian people, what the protocol is in the custom and how it has evolved over time, and how the practice and the effects of the practice are managed by the people living in Mangaia and Aotearoa. Mangaian identity begins at birth and is associated with a connection through family to the land. The practice of tamariki 'āngai alters this 'umbilical' and all-important blood relationship, and while it is intended to strengthen relationships with extended family members, customary adoption compromises the eligibility of the feeding child' to acquire the birth right endowment of land that reaffirms a Mangaian's identity as being Mangaian.The literary context of my thesis is formed from early documentation by the colonising missionaries, court records, archived correspondence and news articles. Colonialism imposed alien concepts of record-keeping that were confusing for the Mangaians and had unforeseen consequences. The distinctions between registering births and registering adoptions were not understood by the people. Now the inaccuracies caused by this confusion are used against the tamariki 'āngai by their families to dissociate these 'feeding children' from their birth right land and thus, their identity.This study also examines the effects on the practice from the cultural bifurcation arising from the diaspora on the practice and the people 'back home' and in Aotearoa. Interviews with thirty-two informants provide rich descriptions that highlight agency and the management of contingencies, the tenacity of relationships and the search for identity. The voices and experiences of my sample population and early literary contributions attest to the durability of the practice and the Mangaian people. I seek to provide a relational analysis of the themes and patterns in the practice that convey individual and societal values that the families may want to consider as they shape their tradition in future generations. I hope to make a contribution that assists people to choose how to manage the practice to meet their contemporary goals.