Variability in the health and condition of breeding northern gannets (Morus bassanus)

Abstract

Seabird populations are in global decline, with impacts from fisheries and invasive species on seabird mortality being well described. Physiological condition can affect survival and reproductive success at the individual level, but effects of physiological condition at the population level are less well understood. This study identifies patterns of variability in the physiological condition of a population of breeding northern gannets (Morus bassanus) between years, nesting stages, and fasting levels. Blood samples were collected from breeding birds attending chicks in 2018-2020, with blood smears used to quantify blood cell types indicative of chronic stress, infection, disease, and immunocompetence. While no blood parasites were observed, elevated Heterophil to Lymphocyte (H:L) ratios, eosinophils, and monocytes suggest that there was a higher prevalence of infection in 2018 and 2020 than in 2019. Standardised body condition was lowest during early chick-rearing, and chronic stress levels, indicated by high H:L ratio, were elevated in both incubation and early chick-rearing compared to the late chick-rearing stages. Interestingly, birds returning from foraging trips were less stressed than those departing, suggesting an effect of fasting on stress levels. However, sample size was too small to determine whether this difference was statistically significant. This study highlights the value of haematology as a tool for identifying patterns of variability in the health and condition of seabird populations; however further research is needed in order to better understand the processes driving this variation and its effect on populations.