Nest attendance in two Finnish waterfowl species in relation to per- and polyfluoroalkyl substances and thyroid hormones.

Contamination of aquatic environments with per- and polyfluoroalkyl substances (PFAS) may disrupt wildlife reproduction by altering hormonal-behavioural pathways. In birds, nest attendance is a key incubation behaviour, mediating embryonic development and hatchability. Yet, the potential associations between PFAS and nest attendance remains underinvestigated. In 2022 and 2023, we quantified plasma PFAS concentrations in breeding females from two Finnish waterfowl species: common goldeneyes (Bucephala clangula), an income breeding cavity nester, and common eiders (Somateria mollissima), a predominantly capital breeding ground nester. Egg-turning and egg temperature were recorded using data loggers inside artificial eggs. We related these nest attendance variables to PFAS and circulating thyroid hormones (THs: triiodothyronine (T3), thyroxine (T4)), as previous studies showed that PFAS exposure is associated with altered levels of THs-key regulators of metabolism and thermoregulation. Mean PFAS concentrations were 17.4 ng/g in goldeneyes (n = 18) and 16.2 ng/g in eiders (n = 37). In eiders, higher perfluorooctanesulfonic acid (PFOS) concentrations were significantly associated with lower average egg temperature. Exploratory post hoc analyses (following marginal PFAS × species interaction trends) suggested positive associations between total PFAS (∑PFAS) and PFOS with egg-turning frequency in goldeneyes. No significant associations were found between PFAS and THs, and THs were not significantly associated with nest attendance. PFAS were not significantly related to hatching success. Further research is needed to determine if species-specific PFAS-nest attendance associations reflect trade-offs with other PFAS-related behavioural or physiological costs. Experimental studies may be necessary to clarify PFAS-thyroid and physiological stress pathways and their consequences for nest attendance and reproduction.