Worldwide, humans have altered ecosystems not only by reducing and changing the distribution of resources, but also by providing new foraging opportunities to wildlife. However, little is known about the early-life development and maintenance of new foraging behaviours, which are crucial for species to adapt to human-induced environmental changes. Using a longitudinal GPS tracking dataset from 71 adult and 147 juvenile white storks (Ciconia ciconia) tracked for up to six years, this study investigates shifts in the exploitation of landfill resources during ontogeny and explores whether selective survival, within-individual improvements, or both shape the emergence of this behaviour. Landfill use was found to increase with age. From their second year of life onwards, white storks visit landfills more often than in their first year, forage more in areas with abundant organic waste, and reduce their foraging energy expenditure. Overall, this study reveals that the age-related increase in the use of anthropogenic food sources is driven primarily by within-individual improvements operating most strongly in early life, rather than by selective survival of individuals that most frequently and proficiently use landfill sites. As more species rely on anthropogenic food subsidies, this work highlights how opportunistic species cope with and adapt to human-driven environmental change, influencing individual lifetime decisions and potentially impacting population dynamics.