Hawaiʻi ʻamakihi are survivors. The tiny, yellow-green forest birds have persisted against an onslaught of challenges in recent centuries, from human-introduced predators and disease to habitat loss. However, new research suggests that the resilience of the ‘amakihi may not be without a price. The birds’ songs appear to have changed in areas subjected to the most upheaval, the legacy of dramatic population declines and isolation.
A songbird’s song can be a major factor influencing the species’ evolutionary trajectory. Because songs are often so involved in outlining territories and wooing mates, any variation in a song has the potential to split a songbird population into multiple song “cultures.” When these cultures cease mating with each other, this turns one population of intermingling birds into several, possibly giving rise to new species in the long term. But it’s not just the case that changes in a bird’s song can shape the species—changes in birds’ lives can shape their songs, too.
The Hawaiʻi ʻamakihi (Chlorodrepanis virens) provides a unique opportunity for scientists interested in how songs can be become locally distinct, given the bird’s fraught history. The ʻamakihi is a Hawaiian honeycreeper—a group of birds found only in the Hawaiian Islands. Most honeycreepers are now either extinct or endangered, devastated by human introductions of exotic predators to the islands, habitat loss, and the scourge of avian malaria.
Malaria—introduced to the islands in the 19th century via mosquitos—was, and continues to be, a plague for honeycreepers, nearly obliterating populations everywhere but in higher elevations too cool for mosquitoes to hang out. Hawaiʻi ʻamakihi, found only on the Big Island of Hawaiʻi and parts of Maui, have survived this threat, but rebounding populations of lower-elevation birds endure in a fragmented forest landscape, in isolated populations descended from a handful of malaria-resistant survivors.
Now, new research published in the journal Ecology and Evolution contends that the ʻamakihi rising in the wreckage do so with songs that differ from their brethren higher up in the mountains.
Researchers at the University of Hawaiʻi, Hilo recorded songs from male ʻamakihi in five different, disconnected forest populations across the Big Island. The locations varied in elevation (and thus malaria risk), and in what plants were around. They then categorized the structure of the songs by grouping them based on the alternating pattern of high and low notes. Once individual songs could be assigned to a song structure type, the team could compare how the songs differed across populations and habitats.
They found that songs varied more based on elevation than geographic distance between populations. Males on opposite ends of the island from each other could sing songs more alike than those sung by birds just uphill. There was also little influence of habitat type on song differences. This suggests that elevation—a proxy for malaria exposure—has largely set the stage for these song dissimilarities. As small pockets of malaria-resistant ʻamakihi grew after the culling in low elevations, they likely found themselves physically and culturally isolated from other enclaves, resulting in low elevation birds developing their own “accent,” one showcasing less musical complexity than that sung by the higher elevation birds.
It’s not known how these differences will influence the future evolution of the ʻamakihi, or if they will evaporate in the event of high and low elevation populations rejoining again. But, this discovery illustrates one of the many ways humans are indirectly—yet fundamentally—modifying wild animal populations by disturbing their ecosystems. These little birds are far more fortunate than the many honeycreepers that have disappeared from the Islands, but it’s clear that the impact of human-introduced disasters will stay with them for generations.