Real Tweets

Recent fascinating scientific investigations reported in the renowned magazine Science reveal that songbirds are adapting to the reduced noise of the lockdown period by modifying their sounds. Due to a reduction in traffic-noise and machine noise and other ambient sound pollution in the months since the lockdown ground our unsustainable economies to a halt, birds are literally changing the way they sing. They are even modulating their bird-song to be expressed at lower frequencies, which can travel over greater distances. There is seen to be new variation in “trills,” which is an expressive device in birdsong. This enhances communication among bird populations, and can even lessen conflict between or inside bird communities because territorial boundaries can be communicated more clearly, and reproductive and feeding opportunities can also be better exploited — the dull grey static and throb of human activity having dropped away, birds are actually producing song such as has not been recorded since the 1950s!

I don’t usually read scientific studies. But this was so fascinating, simply for what it pointed to in our lived experience. Buddhist teaching emphasises the interdependence of all life — of all matter, really; all substance, at the sub-atomic level — and so I could not resist reading through this study. It was so painful to feel how the everyday background noise that we take for granted is, in fact, a mega suppressor of the voices and the communication patterns and frequencies of other beings.

For this reason, rather than just offer a link that some might not open easily on a smartphone, it seemed more helpful to pass on as many chunks of the study itself. I have included the sections most salient for us non-scientific types to grasp some of the material implications of our behavior — of our very lifestyle itself — for the way other beings can carry on their lives, their “life, liberty, and pursuit of happiness.”

Actions taken to mitigate the threats of coronavirus disease 2019 (COVID-19) to human life and welfare have inadvertently resulted in a natural experiment offering unanticipated insight into how human behavior affects animal behavior (1). Worldwide, elective quarantine and stay-at-home orders have reduced use of public spaces and transportation networks, especially in cities. Anecdotal media accounts suggest that restricted movement has elicited rarely observed behaviors in commensal and peri-urban animals (2). Though not all of the reports have proven to be accurate (3), widely publicized observations like coyotes crossing the normally heavily trafficked Golden Gate Bridge in the San Francisco (SF) Bay Area (California, USA) have provoked widespread fascination with the prospect that animals rapidly move back into landscapes recently vacated by humans.

Reports also indicate that animals have been exploiting newly emptied soundscapes. A number of media outlets have noted people becoming newly aware of more conspicuous animal sounds, such as bird songs, particularly in normally noisy areas (4). While people staying home may simply be paying closer attention to the animals around them, it is possible that restricted human movement reduced use of motorized vehicles, effectively unmasking bird songs otherwise obscured by associated noise pollution. Theory also suggests animals should respond to reduced background noise by altering their acoustic signals to optimize the transmission of information (56). Resolving this uncertainty presents an unprecedented opportunity to address enduring questions about how human behavior alters soundscapes and animal acoustic behaviors (7), while offering vital insight into biotic resilience to long-standing anthropogenic pressures.

…The inference that the observed shifts are due to a reduction in the high energy, low frequency sound generated by motor vehicles is supported by traffic flow data from the Golden Gate Bridge; whereas vehicle crossings have progressively increased since the bridge opened in 1937, vehicle crossings in April—May 2020 returned to levels not seen since 1954 (Fig. 2C). Although noise recordings are not available from the 1950s, this benchmark indicates that a relatively brief but dramatic change in human behavior effectively erased more than a half-century of urban noise pollution and concomitant soundscape divergence between urban and nearby rural areas. In other words, the COVID-19 shutdown created a proverbial silent spring across the SF Bay Area.

We found clear evidence that birds responded to the reduction in noise pollution during the COVID-19 shutdown. Consistent with prior studies (1122), we found that birds sang more softly when noise levels were lower (β = 0.27 dB ± 0.04; t281 = 7.0, p < 0.0001, e.g., the Lombard effect) and at shorter recording distances (β = 0.43 dB/m ± 0.08; t281 = 5.3, p < 0.0001) before and during the shutdown. Notably, birds produced songs at even lower amplitudes during the shutdown (β = -4.08 dB ± 1.4; t87 = -3, p < 0.004; Fig. 3, fig. S3, and table S3), well beyond what would be expected from the Lombard effect alone. This departure reveals that prevailing theories of animal communication do not capture the potential magnitude of vocal responses to noise abatement beyond the Lombard effect. Despite a reduction in song amplitude, communication distance more than doubled during the shutdown (β = 8.4 dB ± 1.9; t87 = 4.4, p < 0.0001; fig. S4 and table S4), further indicating the impact of noise pollution on communication during normal conditions. This doubling in communication distance could elevate fitness by reducing territorial conflicts (23) and increasing mating potential. In addition, the signal-to-noise ratio (SNR) doubled in relative energy (β = 6.5 dB ± 2; t95 = 3.3, p < 0.002; table S5), which helps explain media reports suggesting that bird songs sounded louder during the shutdown (4). A doubling would allow people to hear birds at twice the previous distance, or effectively four times more birds than usual (21).

Because the same individuals were not sampled at each time point (mean longevity of white-crowned sparrows is 13 months (24)), we cannot determine if the observed shift in vocal performance was due to immediate flexibility (25) or because males with higher performance (but typically more masked) songs outcompeted males with lower performance (but less masked) songs for breeding territories during the COVID-19 shutdown. It is nonetheless possible to infer that, on average, birds in urban areas exhibited significantly greater capacity to compete for breeding territories. This highlights the intriguing possibility that more juveniles preferentially copied higher performance songs during the shutdown. If so, then the shutdown may have altered the trajectory of cultural evolution within and among populations in the study region. Re-evaluating the same birds following the resumption of human activity would clarify what behavior(s) gave rise to the observed population-level shift in vocal performance and potential evolutionary outcomes of the COVID-19 shutdown.

Like the half-century soundscape reversion that occurred in more urban areas of the study region, some bird songs exhibited traits, such as trill minimum frequency, during the shutdown that have not been heard in decades (fig. S5). Comparisons of historical recordings illustrate that minimum frequencies have tracked a progressive half-century rise in background noise levels in urban songs. Notably, at the Richmond site in Contra Costa County (Fig. 1), the minimum frequency of the Berkeley dialect recorded during the COVID-19 shutdown approached lows not recorded since the spring of 1971 (Fig. 2D) (26).

“Singing in a silent spring: Birds respond to a half-century soundscape reversion during the COVID-19 shutdown” (Science Magazine, 24 Sep. 2020)