Alterations in characteristics of plastic ingestion and decreasing body condition in beachcast fledgling short-tailed shearwaters (Ardenna tenuirostris) at Phillip Island, Australia

Plastic pollution in the marine environment is a major concern for short-tailed shearwaters (Ardenna tenuirostris) through high levels of plastic ingestion. Using analysis of gastrointestinal tract contents, rates of plastic ingestion were found to remain relatively high in beachcast fledglings sampled at Phillip Island, Victoria, with 87.5% and 94.7% of birds examined in 2021 and 2022 respectively containing plastic. The two years were the extreme low and high for plastic mass and number of pieces compared to previous beachcast surveys at this site. Pooling 2021 and 2022, an index of body condition for the dead birds was lower for larger plastic loads by mass, but not for the number of pieces. We believe this indicates a link between decreased fledgling viability and retained plastic mass. Changes in proventriculus plastic had the greatest impact on total plastic loads. Using additional data from 2018, a longer timeline was established for plastic loads and intestinal fat scores in these birds, revealing that the mean and maximal dimensions of particles, as well as the proportion of birds ingesting large pieces (> 10.0 mm), increased between 2018 and 2022. Plastic was mostly light-coloured particles, although there was a shift towards darker hues since 2018. Proportions of industrial pellets declined while other plastics rose. Significant necropsy findings (both plastic and non-plastic related) are reported as an overview of pathological changes occurring in these birds. A decline in body condition since 2018 and the increasing size of retained plastics indicate a heightened risk to bird health.