Ecological Context Constrains Host-Associated Genetic Structure in the Common Cuckoo.

Host specialization is often expected to promote population genetic structure by restricting gene flow among individuals associated with different hosts. However, whether behavioral host specificity consistently generates spatial genetic differentiation remains unclear. The common cuckoo (Cuculus canorus), an obligate brood parasite, exhibits host specialization at the individual level, and several behavioral mechanisms-including natal philopatry, habitat imprinting, and host imprinting-have been proposed to explain its origin. In South Korea, cuckoos show a pronounced regional shift in host use, parasitizing the vinous-throated parrotbill (Suthora webbiana) on the mainland and the meadow bunting (Emberiza cioides) on Jeju Island. We examined whether population genetic structure was associated with geographic distance, habitat composition, or host distribution in this system. Genetic structure was exceptionally weak across the mainland, and genetic relatedness showed no meaningful association with geographic distance, habitat composition, or predicted host distribution. Detectable differentiation was largely restricted to the geographically isolated Jeju population, with an additional deviation at Ansan that may reflect sampling heterogeneity rather than stable population structure. Despite ecological heterogeneity and the presence of multiple potential host species on the mainland, genetic variation was largely homogenized across the continuous mainland landscape. This pattern is consistent with extensive gene flow across the mainland, where S. webbiana is widespread and constitutes the predominant host. Our findings show that host specialization alone does not necessarily generate spatial genetic structure and highlight how ecological context and host dominance can constrain host-associated genetic differentiation in highly mobile brood parasites.