科学家们窃听了南美的一片森林，寻找一只失踪的鸟
你怎么去找一种你都不知道存在的动物?
人们最后一次看到紫翅地鸽(Paraclaravis geoffroyi)是在1985年，这是一种喜爱竹子的小鸽子，原产于巴西、阿根廷和巴拉圭的南美大西洋森林。但是，研究人员想知道，是否有可能在野外捕捉到这种难以捉摸的鸟的声音，以发现是否有任何个体留下?
这不是一个闻所未闻的想法。生物声学是生态学的一个分支，依靠声音来进行环境分析。从记录海豚的交流模式到从远处研究蝙蝠以避免病毒从人类传播，科学家们已经使用了生物声学。SN: 10/23/22)。有了人工智能，现在有可能使用大型音频数据集来训练算法，以便在嘈杂的自然背景中识别不同的动物声音。
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But the problem is that recordings of the purple-winged ground dove singing are as rare as the bird itself.
“I came across [the bird’s song] watching a 1985 interview with Carlos Keller, a former bird breeder in S?o Paulo state, who had a few individuals of the dove,” says Carlos Araújo, an ecologist at the Instituto de Biología Subtropical at the Universidad Nacional de Misiones in Argentina. “And they sang while he spoke.”
With Keller’s help, Araujo and colleagues accessed the decades-old recording and isolated the bird’s song.
The next challenge was to see if it was even possible to identify individual bird songs amidst the sounds of other birds chirping, leaves rustling, rain falling, insects whirring and gnawing and larger animals moving through the forest.
“We took a step back and did some analyses with other birds that are critically endangered but there are known individuals,” Araújo says. The team focused on three species found in Foz do Igua?u, a national park that straddles the border of Brazil and Argentina: the cherry-throated tanager (Nemosia rourei), the Alagoas antwren (Myrmotherula snowi) and the blue-eyed ground-dove (Columbina cyanopis). These birds live in the same environments as the purple-winged ground dove. And the blue-eyed ground dove’s story inspires hope: The species went missing in 1941 and was rediscovered in 2016.
The researchers installed 30 recorders in strategic spots along green areas in the Brazilian part of Foz do Igua?u and recorded from July 2021 to April 2022. They also used data from another 100 recorders on the Argentinian side of Foz.
“We went looking for the Guadua trinii bamboo to place the recorders,” says Benjamin Phalan, Head of Conservation at Parque das Aves, a private institution in Foz do Igua?u focused on the conservation of Atlantic Forest birds. Like the purple-winged ground dove, the three bird species follow the flowering season of the G. trinii bamboo, which happens about once every 30 years.
The team pushed through thickets of bamboo, braved ticks and biting flies, and watched out for venomous snakes such as jacaracas pit vipers. Bumping into these snakes is “rare but can happen. So we use galoshes or gaiters to protect us in case anyone steps on a snake or near it,” Phalan says.
The recorders captured one minute of landscape sound every 10 minutes and generated about 3,000 days’ worth of recordings. “A lot of data to sift through,” says Araújo.
Readily available analysis software wouldn’t work. These software, Araújo says, “need a lot of data input. With such rare species, we just don’t have that much data to train the identification algorithm.”
So the team started from scratch, working with the little data they had for the three endangered birds. First, Araújo created a signal template — exactly like the birds’ singing — based on just a few recordings. The algorithm then compares that template with the soundscape recordings, separating signal from noise. If it spots a sound that is similar to the template, chances are that it is the bird that the researchers are looking for.
The method relies on a statistical model “that is not new, but was used in a very clever and unusual way,” says David Donoso, an ecosystem ecology researcher at the Technische Universit?t Darmstadt in Germany. Donoso and colleagues recently used bioacoustics to investigate the recovery of Choco, a biodiversity hot spot in Ecuador that had been transformed in an agricultural area.
There are different approaches to bioacoustics depending on what you’re looking for, Donoso says. “You can either use fewer recordings to map a whole animal soundscape to tell what species are there, like we did, or you can use lots of recordings to look for a single sound pattern,” he says. The study at Foz do Igua?u “shows that you can use a relatively simple model to answer a complex question — and it works.”
The tool worked reasonably well to identify the cherry-throated tanager and blue-eyed ground-dove singing, but not so much for the Alagoas antwren, Araújo’s team reports October 23 in Bioacoustics. “We’re trying to understand what happened, but we know that the algorithm works,” he says.
The next step, Araújo says, is to refine the algorithm’s precision to find the Alagoas antwren and train it to look for the purple-winged ground dove. And they will do so at the same time. “We’re aiming at both goals at once because we’re running against the clock to find these birds,” Araújo says. “In the end, we are looking for a ghost.” But not a silent one, he hopes.