Feather mite transmission in <i>Cuculus canorus</i> (Common Cuckoo): Age-related differences and implications for dispersal

Abstract Feather mites require effective dispersal strategies to colonize new hosts. In most birds, these mites are transmitted vertically from parents to offspring. However, in obligate brood parasites like the Cuculus canorus (common cuckoo), vertical transmission of cuckoo-specific feather mite species is unlikely because offspring have no contact with their biological parents. We hypothesized that cuckoo-specific feather mites disperse horizontally through physical contacts during the breeding season, such as copulation or male-male competition. If this is the case, yearling cuckoos—with limited breeding experience—should carry fewer mites than adults. To test this prediction, we examined age-related differences in feather mite load and prevalence in common cuckoos, and assessed how these patterns are influenced by host-related and external factors. For this purpose, we captured cuckoos during the breeding season and examined their wing and tail feathers for mites through visual inspection and photography. In addition, mites were collected from a subset of captured birds for identification. Of the 2 identified mite species, Coraciacarus cuculi was dominant, whereas Scutalges sp. was rare. As predicted, yearlings harbored significantly fewer mites than adults, supporting horizontal transmission as the primary dispersal mode, and mite load tended to increase gradually during the breeding season. In contrast, mite prevalence was similar between age groups but increased over the breeding season and was higher in male cuckoos than in females, possibly due to distinctive breeding strategies of the common cuckoo. These findings highlight an unconventional dispersal strategy in feather mites associated with brood parasitic birds and provide new insights into parasite transmission dynamics in avian systems.