Affiliative social relationships and coccidian oocyst excretion in a cooperatively breeding bird species

In group living animals, behavioural interactions with conspecifics strongly modulate an individual’s physiological stress response. Stable social relationships may reduce an individual’s stress response, which in turn can affect the immune system and health. Ultimately, positive health effects of stable social bonds may contribute to maintain group living. We investigated whether, in cooperatively breeding carrion crows ( Corvus corone ), the quality of social relationships correlates with coccidian oocyst and nematode eggs excretion. We repeatably collected behavioural data on dyadic social interactions and individual droppings to quantify parasite eggs and oocysts from 36 individuals in a captive population of carrion crows in northern Spain. Individuals with strong social bonds, living with more relatives and in larger groups excreted a significantly smaller proportion of droppings containing coccidian oocysts. The probability to excrete droppings containing nematode eggs was not affected by social factors. The relationship between social interactions and coccidian oocyst excretion is consistent with the idea that high quality social relationships can positively affects individual’s health, setting the stage for the evolution of stable social living.