Feather mites are positively associated with daily survival in cliff swallows

Feather mites (Acari: Astigmata) have been reported to be parasitic, commensal, and even mutualistic on the birds that serve as their hosts. We investigated whether there was a relationship between number of feather mites (Pteronyssoides obscurus (Berlese, 1885)) on the wing and daily survival of cliff swallows (Petrochelidon pyrrhonota (Vieillot, 1817)) during the breeding season at 12 nesting colonies in Nebraska in 2005. Survival of birds with known mite loads was monitored by mark-recapture, and survival models with and without a linear effect of mites were compared with the program MARK. For adult swallows, mites were positively associated with daily survival at six colonies, negatively associated at two colonies, and there was no relationship at four colonies. For recently fledged juveniles studied at two colonies, survival varied positively with mite load at one, while the other showed no relationship. Feather mites may provide direct benefits to cliff swallows by consuming old oil, pollen, fungi, and harmful bacteria on the feathers or by pre-empting resources used by deleterious fungi or bacteria. The data do not support a truly parasitic relationship in which mites are costly to cliff swallows; these particular feather mites may be beneficial mutualists.