Brood parasites may facilitate horizontal transmission of feather mites: evidence from brown-headed cowbirds

Abstract For small symbiotic organisms, dispersal is a fundamental process that shapes their ecological and coevolutionary dynamics. Ectosymbiotic feather mites lack a dedicated dispersal stage and are primarily transmitted vertically (from parent to offspring) at the nest, resulting in high host specificity and limited opportunities for host-switching. However, feather mites switch hosts more often than expected, given their biology. One potential mechanism for this unexpected pattern is brood parasitism, which offers a unique opportunity for feather mites to disperse across host species. Here, we introduce the Brood Parasite Facilitation Hypothesis (BPFH), which proposes that brood parasites act as temporary dispersal vectors for symbionts, facilitating their transmission between host species. We demonstrate proof of concept of the initial step of the BPFH in feather mites: dispersal to a new host species. Using both morphological and molecular evidence, we show that brood parasitic Brown-headed Cowbirds ( Molothrus ater ) harbor feather mites acquired from their foster parents (Prothonotary Warblers; Protonotaria citrea ), both in the nest and after fledging. These data support the occurrence of nest-based horizontal transmission, but how frequently this occurs and whether foster-acquired mites persist into host adulthood and ultimately transmit between species remain open questions, in addition to identifying the factors that limit or promote long-term establishment on novel hosts. Overall, our findings highlight the role of dispersal in shaping host-symbiont associations and raise key questions about the ecological and evolutionary mechanisms driving host-switching events in symbiotic systems.