This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Birds calling back and forth in the morning make up one of the defining sounds of tropical forests. We're likely to assume that hearing or seeing more types of birds indicates a healthy forest.
But counting species isn't the only way to measure a forest's health. Evolutionary biology asks another question that matters just as much: How closely related are the bird species that live together there? Higher species diversity points to a forest rich in varied plants and other food sources, creating the ideal conditions for a wider range of species—and unique evolutionary lineages—to persist.
In a study of 422 bird species across 1,410 sites on seven islands in Borneo and the Wallacea region, my team and I found that densely forested areas do not necessarily harbor a greater diversity of bird species. Our findings show that assessing bird diversity requires more than simply counting species. To conserve these communities and their diversity, we also need to look at their "family tree." Imagine all the bird species on Earth connected within one vast family tree.
Even species whose ancestors diverged millions of years ago still belong to the same evolutionary tree, although they occupy more distant branches. At each of our observation points, we compared bird communities with forest conditions. We measured forest cover within a radius of 250 meters (820 feet) and tracked changes in forest cover from 2010 to 2022.
We used three "yardsticks" to measure the evolutionary relatedness among the species we recorded: On Buru Island in Maluku, where the forests remain relatively undisturbed, different bird species live alongside one another, including the Buru flowerpecker (Dicaeum erythrothorax), Buru racket-tail (Prioniturus mada) and red lory (Eos bornea). The three species show an intriguing phylogenetic relationship: The Buru racket-tail and red lory share the characteristic hooked-bill morphology of parrots (Psittaculidae), while the Buru flowerpecker (Passeriformes) is connected to them at a much deeper evolutionary level through the Psittacopasseres clade. But evolutionary ties only tell part of the story.
These species survived to live alongside one another due to stable forest conditions. In the Sangihe and Talaud islands of North Sulawesi, bird communities are less diverse. Both island groups face higher levels of threat because forest cover has declined since 2010.
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