Post-fledging survival of wedge-tailed shearwaters is linked to pre-fledge mass, which has decreased over 40 years

Seabird populations are declining across their global range due to a variety of threats, including shifting food webs from climate change. The impact of these threats is exacerbated in long-lived species with low reproductive output and high investment in a few offspring, where juvenile survival is of substantial importance to populations. Changes to post-fledging survival and recruitment of adults are difficult to detect, necessitating better information to forecast juvenile survival before fledglings take to sea. To achieve this goal, we test the hypothesis that there is an ideal mass range for fledglings, outside of which survival is limited, providing an early warning signal for future recruitment failures. Here we present a long-term study of chick banding, weighing and resighting of wedge-tailed shearwaters, Ardenna pacifica, a globally widespread but declining tropical/subtropical seabird in a sea-surface warming hotspot across a span of 43 years. We provide data on 1615 fledgling birds banded most years between 1977 and 2020, with 111 resighted as adults. We found that fledglings weighing 380–470 g have the best chance to survive to adulthood and those weighing 330–540 g have a possible chance of survival. We detected a gradual decline in masses since data collection began, with chicks fledging, on average, 1.6 g lighter each year. This decline has been sharpest since 1996, with fledging masses decreasing at an average rate of 3.8 g annually. Should this 1.6 g decline continue, the average fledgling will cross out of the ‘survivable’ mass range by 2047/2048. We contextualise these findings with observed declines reported in some populations across the species South Pacific range, adding to the conversation about challenges to seabirds in regions experiencing rapid change.