Immune-challenged wild mallards move less and spend more time in sheltered habitats

Background Sickness behaviour is an important component of the response to disease and can influence how animals move through and use their environment. Such changes in behaviour may have consequences for animal ecology and, in species that serve as important hosts for infectious diseases, potentially affect disease dynamics. However, studying sickness behaviour in free-ranging animals remains challenging because it requires identifying when animals are sick while simultaneously tracking their behaviour in their natural environment. Methods To improve our understanding of sickness behaviour in wildlife, we investigated sickness-related behavioural responses in wild mallards ( Anas platyrhynchos ). For this purpose, we experimentally induced an acute immune response in free-ranging mallards using lipopolysaccharide (LPS) encapsulated into biodegradable microspheres, and monitored movement patterns and habitat use for one week using GPS and accelerometry bio-loggers. Results We show that LPS-treated birds reduced their speed, flew shorter distances, and decreased their overall activity levels compared to control individuals, consistent with lethargy-like sickness behaviour. Furthermore, LPS-treated birds spent more time in sheltered areas and less time on open water during the peak of the acute phase response, with a potential shift in the opposite direction towards the end of the study. Discussion These findings improve our understanding of sickness behaviour in free-ranging wildlife and provide a basis for future studies investigating how sickness-related changes in movement and habitat use may affect ecological interactions and disease dynamics under natural host–pathogen contexts.