Feather Mites on Group-Living Red-Billed Choughs: A Non-Parasitic Interaction?

Abstract

Most reports of interactions between feather mites and their avian hosts have assumed that mites have detrimental effects on their hosts, i.e. that they behave as parasites. We investigate the effects of feather mites Gabucinia delibata on the body condition of Red-billed Choughs Pyrrhocorax pyrrhocorax, a highly social, mediumsized corvid species. Feather mites were absent in fledglings at the nest and were probably acquired by Choughs from 1 to 5 months after fledging, when they joined communal roosts. The abundance of feather mites on both wing and tail increased with age and development of social habits in non-breeding Choughs, but decreased when they reached breeding status. Mated Choughs had similar abundances of feather mites. The abundance of mites correlated positively with body condition (computed as the residuals of mass on a size factor) in both males and females, and body condition was in general better for Choughs holding mites than for those lacking mites. The conclusion that feather mites do not have detrimental effects on Choughs was reinforced by the fact that feather mite abundance did not differ between Choughs with normal and crossed bills, respectively, suggesting that preening does not remove mites from the feathers. This particular association thus could be labelled as non-parasitic, suggesting at least a commensal, and possibly a mutualistic relationship. In the latter case, the hypothesised benefits accrued to Chough hosts from their association with mites may derive from an improved feather cleaning, and from the supposed protection against pathogenic organisms achieved primarily by preemption of resources by more benign species, such as feather mites.