Acoustic monitoring data used to assess ranges of cryptic species
Archived acoustic data unveils the true range of cryptic vireo species in Alberta, showing how sound can reshape our understanding of biodiversity.
Uncovering Hidden Voices
Two nearly identical songbirds have long confounded ornithologists in western Canada, but a new study has found a way to tease them apart using only their voices. Researchers have reanalysed hundreds of archived recordings to reveal a striking geographic divide between two forms of the Warbling Vireo - birds so similar in appearance that even experienced observers struggle to tell them apart.
The study, published in The Wilson Journal of Ornithology, shows how passive acoustic monitoring can retroactively determine species distributions, especially when taxonomy changes. Using recordings collected from Alberta over the past decade, researchers reclassified songs of the Eastern and Western Warbling Vireos, finding they occupy largely separate regions with only a narrow potential overlap.
The Power of Archived Sound
Passive acoustic monitoring - where autonomous recorders capture natural soundscapes - has rapidly become a powerful tool for bird research. But this study demonstrates a new use: revisiting past recordings in light of modern taxonomic insights. When one bird species is split into two, as often happens with advances in genetics and acoustics, figuring out where each species lives can be costly and labour-intensive. This research shows that long-forgotten audio can do much of the work.
“Acoustic data are like time capsules,” says lead author Samuelle Simard-Provençal. “They let us revisit past surveys with new questions and new knowledge.”
Species by Song
Although the Eastern and Western Warbling Vireos look virtually identical, their songs differ in pitch and structure. The team measured 11 acoustic features and found consistent differences in 10 of them - particularly in the frequency of notes and how quickly the birds varied their song. Using these traits, they trained a machine learning algorithm and human observers to classify recordings, achieving up to 97% accuracy.
Songs from east of the Rockies were mostly from Eastern Warbling Vireos, while the Western Warbling Vireo dominated in the west and in the distinctive Cypress Hills. This distribution confirmed earlier suspicions but with much finer resolution than previous field guides or checklists.
Implications for Conservation and Science
If the two forms of the Warbling Vireo are officially split into separate species - as some researchers have proposed - the implications for conservation could be substantial. The Western form appears to have a narrower range in Alberta, and a species’ range size is one of the most important criteria used in conservation assessments.
Importantly, the study underscores the long-term value of sound archives. “With global acoustic datasets growing rapidly, we’re gaining the ability to answer tomorrow’s questions with yesterday’s data,” says co-author Erin Bayne.
Next Steps: From Sound to Species Status
While this study focused on song, the researchers note that a full case for species status would require additional evidence from behaviour, plumage, and genetics. But their findings - alongside earlier genetic studies - bring us closer to resolving one of North America’s subtler bird mysteries.
In a time of rapid environmental change and shifting taxonomy, passive acoustic monitoring may be among the best ways to track - and even rediscover - biodiversity. Sometimes the clues have been there all along, waiting to be heard.
12 May 2025
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