Sociability Linked to Reproductive Status Affects Intestinal Parasite Infections in the Red-Billed Chough

Social behaviour plays a crucial role in the dynamics of parasitic infections in wild bird populations. The red-billed chough (Pyrrhocorax pyrrhocorax), a corvid undergoing notable population declines, shows contrasting social structures linked to reproductive status: non-breeding individuals aggregate in communal roosts during winter, whereas breeding pairs often maintain territorial pair-bonds and roost at nesting sites. This study tested whether differences in sociality (communal roosting vs. territorial pairs) affect intestinal parasite infections. Fresh faecal samples were collected during winter in central Spain and analysed using flotation and McMaster techniques to detect and quantify coccidian oocysts and helminth eggs. The results revealed a relatively high positive rate of Isospora (36.2%, n = 116) and a low positive rate of helminths (9.5%, n = 116) among communally roosting non-breeders, while no parasites were detected in samples from territorial pairs. One communal roost in the Southern Plateau showed higher coccidian positive rate, possibly influenced by structural features that facilitate faecal contact. Although sample size for pairs was limited, the absence of parasites in this group suggests reduced infection risk, likely reflecting superior condition and immune defences rather than differences in exposure alone. These findings highlight the value of non-invasive parasite monitoring as an early-warning tool in wildlife health assessments and stress the importance of considering social behaviour and environmental heterogeneity into conservation strategies for threatened species.