Why did they die? Analysing the cause of death of grounded seabirds lodged at an avian rescue centre in Auckland, New Zealand

Procellariiform seabirds are vulnerable to numerous threats, including the growing issue of urban light pollution. Seabirds that are found grounded are often treated by avian/wildlife rehabilitation centres, but approximately 30% do not survive. Here, we necropsied 19 grounded Cook's petrels (titi, Pterodroma cookii) that did not survive and report the cause of death and injuries. We also investigate potential risk factors, including association with light pollution, seabird sex, age, and sensory features. We found that a 70% of Cook's petrels had head trauma, internal bleeding, and/or wounds as the main causes of death (p > 0.05). These injuries are consistent with collisions, likely due to disorientation from light pollution. Most Cook's petrels were not stressed or in poor body condition, suggesting Cook's petrels are typically healthy before being affected by lights. In the sample of Cook's petrels studied, mortality was significantly biased towards young and male seabirds. Despite this apparent sex difference in collision risk, there was no detectable sex difference in measured sensory features, e.g. males did not have significantly larger eyes than females. The potential sex bias in death suggests male seabirds could be more vulnerable to light pollution, which warrants further research. Further research is also required to determine whether individual differences in sensory features relate to grounding risk, as our study only included a subset of dead seabirds. We also recommend that all grounded seabirds are taken to rehabilitation centres rather than released immediately.