Comparing temporal trends of mercury contamination in feathers and eggs of female tawny owls (Strix aluco) in central Norway

Mercury (Hg) is a global pollutant with potentially severe health impacts on humans and wildlife, especially raptors, which face high exposure due to trophic magnification and long-term environmental integration. Raptors are widely used as Hg environmental biomonitors, with feathers and eggs favored for their easy, minimally invasive collection, transport, and storage. While raptor feathers and eggs have been commonly used to retrospectively analyze Hg time series, few studies have examined physiological and ecological processes driving matrix-specific patterns. This study compared 34-year (1986-2019) time series of Hg in tail feathers and eggs of tawny owls (Strix aluco) in central Norway, accounting for plasticity in their dietary sources (i.e., adjusted temporal trends), proxied by stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C; corrected for the Suess effect). Generalized Additive Models assessed temporal trends and potential drivers. Unadjusted temporal trends (Year only) differed between feathers and eggs, whereas unadjusted and adjusted (for dietary proxies) trends did not differ within each matrix. Differences between matrices may reflect varying Hg sequestration mechanisms or integration windows. δ¹³C appeared as the main driver of Hg variability in both matrices, likely indicating shifts in carbon sources in tawny owl diets over time. Future studies should focus on obtaining more accurate information on environmental and biological sources and pathways over time, to better understand their influence on Hg time series in this established biomonitoring species.