Yellow Rail is a notoriously difficult species to catch a glimpse of, but recording devices have revealed a previously unknown stronghold for the elusive rallid.
The boreal zone of North America is vast and relatively untouched, meaning it is a haven for birdlife but a challenge to survey. Marshes and bogs within this ecosystem are especially hard to monitor, but ornithologists have found a new way to map this tiny and inconspicuous wetland bird.
Until recently, the best way to survey Yellow Rails was to play recordings of calling males in suitable habitat and listen for any response, but this requires people out in the field and it has been unclear how the birds' behaviour may be affected by the playback.
Passive acoustic monitoring has revolutionised ornithology in recent years across many environments. The method involves leaving sound recording devices for an extended period of time then recovering them at a later date to begin analysing the audio files.
The Edéhzhíe Dehcho Protected Area in the Dehcho region of Canada's Northwest Territories lies some 150 km beyond the known limits of the range of Yellow Rail.
However, recent eBird reports from the Northwest Territories and swathes of suitable habitat prompted researchers to investigate.
Autonomous recording devices were placed at 199 positions within Edéhzhíe, where they gathered audio records for a week in mid-May 2016 and were retrieved two months later.
A machine-learning algorithm was trained to recognise the distinctive 'clicking' call of male Yellow Rails and helped the researchers to distill many hours of sound recordings and determine the number of birds at each survey position.
The results were staggering. Yellow Rails were recorded at 16% of the sampling positions during the survey, with an average of 1.3 birds at each position which had produced a positive result, redrawing the distribution map for the species.
Marshes had the highest densities of the birds, though rails were also detected in fen and bog habitats. The researchers estimated a population of 906 breeding pairs of Yellow Rail in Edéhzhíe, an area previously considered way beyond the range of the species.
Even the most northerly sites surveyed had Yellow Rails, pushing the known range north by around 350 km.
It is likely that Yellow Rails had merely gone undetected in the area prior to the work, with the distribution of most boreal birds thought to be underestimated, but the possibility remains that the results chart a significant range expansion.
Mills Lake, which turned out to be a key area in Edéhzhíe for Yellow Rail, is also important for other wetland birds like American Bittern and Nelson's Sparrow, so could act as important stepping stone for birds spreading from the south.
Acoustic monitoring studies like this could play a crucial part in anticipating and tracking avian responses to climate change and habitat loss in the future, particularly for unobtrusive species like Yellow Rail which aren't detected efficiently by traditional methods.
McLeod, L J T, Haché, S, Pankratz, R F, & Bayne, E M. High-density Yellow Rail (Coturnicops noveboracensis) Population Beyond Purported Range Limits in the Northwest Territories, Canada. Waterbirds 44(2): 175-184. DOI: https://doi.org/10.1675/063.044.0204