The increasing noise pollution recorded in cities in recent decades has caused a collateral effect on birds that live in urban areas. To adapt to background noise, males have been forced to turn up the volume of their songs to make themselves heard by rivals and potential mates. But during the months of confinement, when we humans stayed home and the noise in the streets decreased drastically, something happened to the birds too.
Noise pollution affects bird’s songs
According to a study published in Science, the songs of the birds recovered the acoustic quality of their songs, reaching the sound level of 50 years ago.
The authors of the research, a group of ecologists from the University of Tennessee, have focused their research on white-crowned sparrows, a common species in urban areas in the United States. These birds have proven to be resilient, and able to survive the hustle and bustle of cities by increasing the volume of their songs.
Researchers have studied the behaviour of these sparrows in and around San Francisco for more than two decades. Part of the work has consisted of comparing their songs with recordings made in the 1970s. And in these twenty years, they have observed how, as the traffic became more intense, the lower frequencies of the songs of the sparrows also increased, so as not to be drowned out by the background hum of vehicles. But their peak frequencies remained largely the same, reducing the overall bandwidth of their communication.
Specifically, noise pollution, other than affecting individuals’ communication, induced increased pitch in urban birds or moved their singing activity to more “quiet” hours for instance, to nocturnal hours
For many bird species, songs degraded in this way are less effective at deterring rivals and attracting females. Birds sing louder in noisy environments, and research has shown that the resulting stress can accelerate ageing and alter their metabolism. Noise can also cause birds not to hear their own chicks or the warnings of other birds. This phenomenon could have a negative impact on the diversity of birds in many cities.
The main author of the study, Elizabeth Derryberry, remembers how at the beginning of the confinement, last March, she saw an image of the Golden Gate Bridge in San Francisco totally empty.
This is how this study began, which consisted of observing behavioural changes in the song of birds. The scientists recorded their songs and found that the sparrows sang 30% softer, on average than before the confinement. But not only that, they noticed that these birds sang songs with bandwidths typical of birds recorded in the 1970s.
The researchers report that sparrows in the latter group, exposed to greatly reduced background noise, exhibited drops in vocal amplitudes and reductions in vocal minimum frequencies, leading to increased vocal performance. These changes were much more noticeable for birds in urban areas, the authors say, likely giving these birds a much greater ability to compete for breeding territories.
“When the city was loud, they sang really loud,” said the study’s lead author, behavioural ecologist and ornithologist Elizabeth Derryberry, an associate professor at the University of Tennessee, Knoxville.
The authors say that the results reveal how quickly birds can adapt to changing environments and suggest that long-lasting remediation could lead to other promising outcomes, including increased species diversity.
Birds give up the complexity of song for volume in noisy places – so they sing louder but less attractive (to both humans & mates) songs in cities. When traffic noise in San Francisco fell due to COVID, birds began to sing more complex, quieter songs like they did 50 years ago!
Impact on the birds
According to the research, reduced bandwidth and throughput, not a higher minimum frequency, could be the reason why urban birds respond less to anthropogenic noise-adjusted songs. Ultimately, the lower vocal performance of birds in territories with high levels of anthropogenic noise could result in fewer mating opportunities and more challenges in defending their territory.
In an earlier study, Halfwerk found that certain male birds forced to compete with highway traffic often resorted to singing louder to be heard. However, it wasn’t so clear whether doing so had a positive or negative impact on the birds as a whole, so this time he and his team decided to take a closer look by studying not only the change in songs, but also finding out who it was. actually mate with whom as a result.
To find out what was going on, Halfwerk and his team chose to study the great tit, the same species studied in their earlier work; a bird that is capable of changing the frequency of its songs. They recorded the songs of 30 random male birds during mating season in the Netherlands. In doing so, they found that the males sang at their lowest level just before the females began to lay their eggs, indicating that it was the low-frequency songs that really made the difference for the females.
To further confirm their findings, the team also took a more direct approach; They played pre-recorded high-pitched and low-pitched male songs to females sitting on their nests to see which would make them emerge and found that low-pitched songs were more effective. However, this all changed when they pumped low-frequency background noise into the environment, making it difficult for the females to hear the songs; only then did they go for the males singing the higher frequency melodies.
Negative impact on bird populations
All of this shows, say the team, that noise pollution can have a negative impact on bird populations, seriously affecting the process of allowing females to be fertilized by the best available mates, which could ultimately put the species in danger. danger. risk.
The combination of less background noise and the better signal from a wider bandwidth meant that the males could probably be heard from twice as far away as before.
This improved communication has also helped rival males avoid each other, which means fewer fights.
By showing that sparrows can adjust their songs to their environment, the study suggests that species with more flexible behaviours can cope with aspects of changing environments. Noise abatement could allow other noise-sensitive bird species, such as California quail, to return to cities where they once roamed and sang.
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