Long-term stable isotope analysis shows restricted foraging behaviour of the Critically Endangered African penguin

African penguins Spheniscus demersus are endemic to the Benguela upwelling region off southern Africa. They have been in significant decline since ca. 1950, and in 2024 were listed as Critically Endangered, with the scarcity of quality food believed to be the main driver of their decrease. The decline in penguin prey has been linked to competition with commercial fisheries and environmental changes. Breeding adults predominantly forage on small pelagic fish within 40 km of their colony, but there is little information on the foraging ecology of immature and non-breeding penguins. We used stable isotope values (δ 13 C and δ 15 N) of feathers from adult and juvenile African penguins and blood from adult African penguins from colonies across their breeding range to infer their diet during the pre-moult and breeding periods. Feathers from museum skins were also analysed to assess whether the diet has changed since the advent of industrial fisheries. Newly grown feathers of juvenile penguins were comparably more depleted in 15 N and 13 C than those of adults, likely due to their limited foraging skills and wider dispersion. Isotope values differed markedly between colonies, and pre-moult (feather isotopes) and breeding penguins (blood isotopes) differed in their δ 15 N values, indicating foraging on different prey and possibly at different locations. There were no consistent differences among isotopic values of penguin feathers from current and historical samples, indicating that penguins have maintained similar diets through time and may not have the capacity to shift to alternative prey species in southern Africa. This supports the hypothesis that reduced food availability and quality are significant factors in the collapse of African penguin numbers.