Breeding stage impacts on chronic stress and physiological condition in northern gannets (Morus bassanus)

Physiological condition can affect survival and reproductive success in seabirds. However, seabirds rarely show outward signs of poor physiological condition, making it difficult to identify and address issues before they result in population level impacts. We investigate physiological condition of breeding northern gannets (Morus bassanus) between years and nesting stages. Blood smears were used to quantify blood cell profiles indicative of chronic stress, infection, disease, and immunocompetence. No blood parasites were observed, but elevated Heterophil to Lymphocyte (H:L) ratios, eosinophils, and monocytes suggest higher prevalence of infection in some years. Chronic stress levels, indicated by high H:L ratio, were elevated in incubation and early chick-rearing compared to late chick-rearing, which coincided with poorer body condition in breeding birds. This study highlights the value of haematology as a tool for identifying changing patterns of health that may serve as an early indicator of breeding failure, overwintering mortality, and population declines.