- A sweeping analysis of nearly a million recordings from 48 national parks in the U.S. shows that birds vocalize more in the presence of airplane noise.
- The data collected by scientists at the U.S. National Park Service will help answer questions about how birds respond to airplane noise.
- With air travel increasing, bird communities across the world are likely to be exposed to the drone of airplanes, and not just near airports.
- Bioacoustics, the study of how sound is produced, distributed and perceived in the natural world, is already helping shape human understanding of animal communication.
It’s sometimes easy to forget that birds have ears, craftily concealed as they are. Which is why the thunderous passage of airplanes sends ripples of disquiet in their ranks. Now, a sweeping analysis of nearly a million samples of bird vocalizations collected by scientists at the U.S. National Park Service (NPS) is offering surprising insights into how birds process this noise.
Instead of subduing bird vocalization, a plane passing by leads to an uptick in bird chatter, according to the research presented at the 178th meeting of the Acoustical Society of America on Dec. 4. The chances of hearing a bird increased by 2 percent when a plane is also heard in a 10-second audio clip. “The extensive presence of aircraft noise exposure in time and space implies that birds are chronically producing more sound in many locations,” Kurt Fristrup from the NPS, who presented the findings, said in a statement.
It might seem like a trivial matter that birds become more boisterous as flights roar past, but it speaks to a broader question: what does an increase in air travel mean for the feathered fliers? Air travel has ballooned in the past two decades as flying costs have declined, and shows no sign of slowing down. According to an estimate from the European plane maker Airbus, the number of commercial planes will more than double in the next twenty years.
The sound associated with jet planes measures somewhere between 120 and 140 decibels, varying with distance from the source. At 25 meters (82 feet) the thrust from a jet taking off can reach 150 decibels and potentially rupture the human ear drum. At 300 meters (984 feet), about three football fields away, a takeoff can sound as loud as a lawn mower at close range (around 100 decibels). For humans, regular exposure to sounds above 85 decibels can cause hearing loss.
Humans are only beginning to explore the animal world using sound. Bioacoustics is the study of how sounds are produced, distributed and perceived in the natural world, and is shaping a novel understanding of animal communication. It is also informing concerns about the impact of pervasive anthropogenic sources of sound like ships and planes.
Usually, birds rely more on visual cues than sound; their sense of sight is more developed than that of humans, while their hearing is just about as good. However, sound is still vital for monitoring and mapping surroundings. The tremendous diversity of bird species makes it difficult to generalize about how they react to aircraft noise.
The NPS research is a rare large-scale survey, which used data collected from 48 national parks across the U.S. between 2011 and 2017. Most of the existing work focuses on specific bird species at a particular site, usually near airports. The NPS data show that as flights crisscross across the airspace, a large part of the country is exposed to the drone of airplanes.
Allison S. Injaian, a researcher with the Bioacoustics Research Program at Cornell University, New York, said the results were consistent with what she found among wood thrushes (Hylocichla mustelina) breeding near a local airport. “We looked at the effects of the Ithaca Regional Airport on vocal behavior in wood thrush breeding in a bird sanctuary approximately one mile from the airport,” Injaian said. “In this specific context, our results show that wood thrush breeding close to the airport increase their vocal behavior during dawn chorus in response to morning flights.” She said that one possible explanation of the “surprising” result was that the birds were compensating for diminished ability to communicate.
“We have similar observations on a much smaller scale that some bird species seem to increase their singing when aircrafts take off and land,” Selvino R. de Kort, a behavioral ecologist at the Manchester Metropolitan University, U.K., told Mongabay in an email. “It is possible that sound in itself, irrespective of its source, stimulates birds to sing,” he said.
In the absence of human-generated noise, greater vocalization would be a response to natural sounds like the songs of other birds, he explained. If you consider airplane noise as an acoustic stimulus it is “probably adaptive” for birds to sing in their presence. But what is a good strategy for natural stimuli may not be as helpful in dealing with human-made noises.
De Kort co-authored a study published recently in the Journal of Animal Ecology that showed that sound from planes taking off was linked to hearing impairment in a population of chiffchaffs (Phylloscopus collybita) living near Manchester Airport. The birds were also found to be more aggressive.
Responding to airplanes could be costing birds in other ways. A change in vocal behavior could diminish ability to attract mates or safeguard territory. Birds that spend more energy singing may have to cut back on important tasks like foraging. The plane itself could be viewed as a looming predator to a bird, something to fly away or seek cover from. Any reaction requires a redistribution of a bird’s limited energy reserves, whether it is flying off or more vigorous vocalization. Even a rapidly beating heart has an energy price tag.
A study of brent geese (Branta bernicla) found that birds that are disturbed every half hour by aircraft or people take 30 percent more time to feed compared with brent geese in relatively less disturbed areas. Given the diverse ways in which different species respond to the noise, the NPS scientists will be digging deeper into their data to distinguish the kinds of bird sounds and the species that produce them.
Whether frequent exposure means birds get used to the rumble of airplanes and respond accordingly is debatable. “It is likely that birds habituate to noise as do humans,” de Kort said, “however, habituation does not mean that individuals are not under a substantial level of stress.” It is a wonder then, that bird populations continue to settle in noisy places and even near airports. “This can probably be explained by the fact that in evolutionary terms, anthropogenic noise is relatively novel and the birds have not evolved yet to deal with it,” he said.
Wolfenden, A. D., Slabbekoorn, H., Kluk, K., & de Kort, S. R. (2019). Aircraft sound exposure leads to song frequency decline and elevated aggression in wild chiffchaffs. Journal of Animal Ecology, 88(11), 1720–1731. doi:10.1111/1365-2656.13059
(Banner Image: The shadow of a plane in Botswana. Image courtesy Tiffany Roufs and Jeremy Hance/Mongabay)
Malavika Vyawahare is a staff writer for Mongabay. Find her on Twitter: @MalavikaVy
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