The effect of group size on ectoparasite load and physiological markers of health in a communally-roosting bird

Group living in animals can provide individuals with many fitness benefits, but also increases their exposure to parasites. However, the relationship between group size and parasite load both across and within species is highly variable, potentially due to selection acting on adaptations to reduce infection risks and costs, as well as species-specific variation in the type and frequency of social behaviours. Information about the risks and physiological costs of parasitic infection along a gradient of sociality and in different ecological settings is currently limited. Here, we explored how ectoparasite load and physiological markers of health are associated with group living in speckled mousebirds, Colius striatus. We found that group size had a non-linear effect on ectoparasite load: individuals in medium-sized groups were most infested. In addition, infested individuals in medium-sized groups showed the greatest signs of reduced health. We speculate that social immunity mechanisms such as allogrooming, and the physiological costs of group living might play important roles in mediating this relationship, where larger groups suffer increased risk of infection but also provide higher levels of anti-parasite behaviour or immunity. Our results suggest the existence of various mechanisms by which group-living animals can mediate increased ectoparasite transmission and the negative health consequences of infestation, and highlight the need for further research on mechanisms of social immunity in a broad range of taxa.