Asymmetric aggressive response towards heterospecifics following secondary contact between hybridizing island birds

Because islands are geographically isolated and harbor unique ecological conditions, oceanic archipelagos facilitate population divergence and the evolution of new species. However, these geographic barriers are often not absolute, with insular populations coming into contact after a period of spatial isolation. Such cases of secondary contact provide an opportunity to explore how the completion of speciation proceeds. In the Solomon Islands, two species of birds experienced secondary contact after a period of geographic isolation (> 1 million years): the red Cardinal Honeyeater (Myzomela cardinalis) colonized the island of Makira several hundred years ago, coming into contact with the Makira-endemic and melanic Sooty Honeyeater (M. tristrami). The two species hybridize and use the same nectar resources, resulting in competition for mates and food. We explored species interactions and the maintenance of species boundaries through field experiments simulating territorial intrusions at both allopatric and sympatric sites. We found that territorial males of both species reacted more aggressively to intruders while in sympatry compared to allopatry, possibly due to increased resource competition at the contact zone. Importantly, both species also acted more aggressively to the colorful and larger Cardinal Honeyeater over the black and smaller Sooty Honeyeater stimuli. As the two species overlap in a specialized feeding niche, elevated resource competition in sympatry may favor a stronger response to the larger and more dominant species, which are likely more effective in excluding other birds from accessing limited nectar. Our work provides general insights on species interactions when diverged island populations come into contact after a period of isolation.