Strong assortative mating with nest site partitioning may facilitate species coexistence during secondary contact in island birds.

Species coexistence is critical to the assembly of biodiverse communities. When closely related species come into secondary contact, competitive and reproductive interactions can both influence the potential for coexistence. We tested whether ecological and/or evolutionary factors affect coexistence dynamics in a pair of closely related honeyeaters that have come into secondary contact in the Solomon Islands. Myzomela cardinalis colonized the island of Makira and established a population in sympatry with the endemic M. tristrami. These taxa are ecologically similar and hybridize. If ecological factors are promoting coexistence, we expect species to partition where they forage for resources or defend breeding territories. In addition, evolutionary pressures may facilitate coexistence if species mate assortatively, avoiding production of potentially low-fitness hybrid offspring. Using data on foraging locations, nest locations, and mate pairing, we find support for partitioning of space use, specifically in the context of nest placement. Phenotypic pairs of the recently arrived species nested significantly closer to the coastline than did endemic species pairs. We also find strong evidence of assortative mating based on phenotype. Thus, both ecological and evolutionary factors appear to contribute to the coexistence of these closely related hybridizing species.