Hidden rainforest birds tracked in 3D with acoustic arrays

Hidden rainforest birds tracked in 3D with acoustic arrays

By Rex Graham

Bird lovers from Beethoven to ornithology legend Luis Baptista thrilled to the vocalizations of rainforest birds.

Now, scientists with high-tech listening technology are gaining a far deeper understanding of bird behavior from their songs.

Birding with acoustics and algorithms

Low-cost wireless microphone arrays and sophisticated acoustic analysis and modeling algorithms can be used to keep track of individual birds in time and 3D space – all in stunning detail. Scientists can triangulate the position of acoustic signatures of individual birds amid the forest cacophony.

The technological proof of principle was reported in 2013 in Signal and Information Processing by an interdisciplinary group of UCLA researchers led by ornithologist Charles Taylor.

Charles Taylor, birdsnews.com

UCLA Biology Professor Charles Taylor has pioneered the use of technology in studying rainforest birds, including acoustic arrays and signal processing algorithms that identify individual birds in time and 3D space whether the birds are hidden in the jungle or not. (Photo: Agnijita Kumar, UCLA Daily Bruin)

Nine years ago, Taylor and and Martin Cody, a UCLA professor emeritus of ecology and evolutionary biology, carried earlier versions of sound recording equipment to Mexican jungles in wheelbarrows, avoiding crocodiles, poisonous snakes and other critters.

Science and adrenaline

“There are moments when my adrenaline got going … not just by seeing the sight of a beautiful bird,” Taylor told the Daily Bruin.

Zac Harlow, one of Taylor’s students, demonstrated the utility of acoustic array technology platform by recording the White-breasted Wood Wren (Henicorhina leocosticta) in the Montes Azules Biosphere Reserve in Chiapas, Mexico. The region is a popular destination for international travelers on  birding tours.

The Montes Azules reserve is home to monkeys, jaguars, sloths and other mammals and a huge number of noisy birds. There Wood Wren is a vocal prima donna amid other prima donna birds.

Each wood wren sings its own version of cheer oweet oweet cheery weather.

The wood wren’s song reminded Baptista of the opening of Beethoven’s Fifth Symphony.

“Luis was an amazing guy and could reproduce any bird song,” Taylor said. “When he whistled it probably did sound like Beethoven’s 5th.”

No-see-ums, no problem

Taylor didn’t whistle to the wood wrens. Instead he and his colleagues played tape-recorded wood wren songs in the reserve, and territorial males and females wood responded, but in slightly different ways. Without seeing the birds, the researchers found that females took up positions in high branches. The males were “more directly confrontational in approaching the playback closely,” they wrote in the Signal and Information Processing paper.

The wood wrens tended to give one song type several times in succession before moving to a different branch or patch of leaf litter on the forest floor.

Improvements in acoustic analysis and modeling simulations now can provide ornithologists with near real-time movements of birds with stationary arrays of microphones. In the July 2014 issue of Unmanned Systems, a Taylor group study, led by UCLA Distinguished Professor of Engineering Kung Yao validated an improved algorithm that allows a laptop to pinpoint an individual bird’s location less than 10 seconds after it vocalizes.

Music to birdsong community

“Kung Yao’s group has made a real contribution that might percolate through the birdsong community,” Taylor said.

Yoa’s team tested the technology on recordings of Berwick’s Wren, Cassin’s Vireo, California Thrasher and Black-headed Grosbeak broadcast from speakers.

In an earlier study conducted 2005-2009 at the Montes Azules Biosphere Reserve, the largest expanse of lowland tropical rainforest in Mexico, Taylor’s team used acoustic arrays to identify individual Mexican Ant-thrushes (Formicarius moniliger)and track them. The team recorded 1,121 songs of 19 birds over three years to correctly identify 89 percent of the songs from the correct bird. (Each bird had been captured and color-banded, and many behaviors revealed by the arrays were confirmed with “ground-truth” observations.

In a 2011 paper in IBIS, Taylor’s team reported that the songs recorded by their microphone arrays revealed high site fidelity of male ant-thrushes. Some males recorded and tracked in 2005 were still singing the same tune on the same territories in 2006-2009. The same could not be said for mate selection. The males formed monogamous pairs during each season, but change partners within two to three years.

Rainforest birds’ songs decoded

In an amazing confirmation of the technology, a map of the territories of 12 male ant-thrushes based on acoustic recordings matched ground-truth observations. However, the territory maps based on the acoustic recordings were bigger than ground-truth observations suggested, and the acoustic arrays revealed how each individual territory abutted the territories of neighbors in fine detail.

Each male’s territory boundaries remained fixed over the years as well, with minor modifications when a new male took over an existing territory.

Mexican Ant-thrushes apparently remain on their home territories longer than the closely related Dusky Antbird in Panama, where males and females frequently switched territories. In the case of the antbird, a team led by Eugene Morton, now a biology professor at York University, said, “Territory switching may be an overlooked but common tropical form of territoriality that increases individual survivorship during periods of low food abundance (dry season).”

Noisy neighbor face-offs

Food abundance is more favorable for the ant-thrushes at the reserve in Mexico. By continually tracking individual birds for years, territories and their male occupants remained stable for years. Ant-thrush nests were often positioned near territorial boundaries, possibly to keep an eye on the neighbors.

Territorial boundaries were occasionally noisy. On either side, neighbors engaged in fierce back-and-forth vocal disputes. The birds sang, buzzed and squeaked at one another.

Mexican Ant-thrush territories, birdsnews.com

Maps of Mexican Ant-thrush territories in May 2009 based on (a) locations of acoustic array recordings assigned to individuals, and (b) ground-truth data from visual sightings of color rings and locations where individuals were mist-netted and ringed. The rainforest birds were studied by UCLA researchers. (Illustration: Alexander Kirschel, IBIS)

Trysts with female ‘floaters’

However, on occasion female “floaters” traveled through the males’ established territories, suggesting to the researchers that some hanky-panky may occur.

The researchers were able to make discoveries of territorial dynamics and other behaviors, based on inferences from the various locations of each secretive singer. Ground-truth observations, where possible, confirmed inferences made from song recordings.

Labor-intensive field work was greatly reduced. Changes in territory occupant or boundary – within 50 cm – quickly revealed itself in song. “Such accuracies are obtained despite high levels of ambient noise typical of tropical rainforest habitats,” wrote Taylor’s team in their 2011 paper in IBIS.

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