The karoro Larus dominicanus in northern Aotearoa | New Zealand: diet and evidence of changing trophic position from regurgitated pellets and stable isotope analysis of contemporary and historic feathers and bones

Abstract

Coastal seabirds are valuable indicators of ecological change in nearshore marine systems impacted by human activities. This study examined how human population growth and urban expansion have influenced the long-term dietary patterns of karoro (southern black-backed gull Larus dominicanus) in Auckland, Aotearoa | New Zealand. Specifically, we assessed whether increasing urbanisation has led to a dietary shift from marine-based prey to greater reliance on terrestrial and anthropogenic food sources. Contemporary diet composition was analysed using regurgitated pellets from two island breeding sites (Rangitoto and Tiritiri Matangi) in the Hauraki Gulf—sites differing in proximity to urban influence—and an urban roosting site at Western Springs Park. To assess long-term dietary trends, we conducted nitrogen stable isotope analysis of feathers from museum specimens spanning 109 years and collagen from contemporary karoro bones and subfossil bones from Tokerau Beach predating human arrival in Aotearoa. Pellet analysis indicated a diverse diet comprising marine vertebrates, invertebrates, and terrestrial or anthropogenic food sources, with city proximity influencing dietary composition. Stable isotope analysis revealed significant changes in karoro trophic ecology over the past century with birds shifting from a predominantly marine-based diet to one more reliant on terrestrial food, likely due to declining marine prey and increased urban food availability. Stable isotope data from ancient bones indicated that pre-human karoro were obligate coastal marine predators and scavengers, occupying a higher trophic level than their modern counterparts. These findings provide insight into how urbanisation and ecological change have shaped karoro diets over time.