Vanishing bird songs and haphazard, noisy urban sprawl

People in parts of the National Capital and Bengaluru have been complaining about birds chirping in the night, depriving them of sleep.

In Kolkata, the young, who have no issues about loud discotheque music, are exchanging copious notes on social media about how to cut out the “annoying” singing of the koel at night.

In Mumbai, old people m bird songs at dawn that were once like their morning alarm, while in the recently-urbanised areas of smaller cities, people wonder about the disappearance of birds that they heard so lovingly in their childhood.

Most common people are clueless about changing bird behaviour, ranging from the shift in the timing of bird calls to the muting of their songs at dusk and dawn.

But experts know why. The soundscape of our world is changing and with that, so is changing wildlife behaviour and communication.

Sounds generated by humans in the form of road and rail traffic, aircraft noise, the blasts of mining, military activity, air conditioners, firecrackers and festival drums are all affecting the soundscape.

Unlike the urban sounds related to human activity, which are automatic and polluting, the sounds produced by birds are intentional, specific and meaningful.

Birds use sound to attract mates, defend territory, protect their young, sound alarms, give auditory cues to nestlings, find proper habitat and locate food.

“Hearing is the universal alerting cue, the essential sense that’s never disappeared in any species. From a zoological, evolutionary perspective, hearing is far more universal than vision,” according to Kurt Fristrup, chief scientist at the US National Park Services’ Natural Sounds and Night Skies Division. In an interview to Enasia.com three years ago, Fristrup said that “animals developed ears before vocal chords”.

The acoustic frequency of a bird’s song is related to its body size. A big body would have a long trachea and a lower sound frequency, as in the case of tree pie. A small robin, on the other hand, has a smaller trachea and higher sound frequency.

According to Biological Sciences professor Kerry Rabenold of Indiana, US, quoted in an Oxford University research paper, large birds can produce sounds as low as one KHz, but most of their songs and calls are in the two to six KHz range. The frequency can reach 10 to 12 KHz where geophonic sounds of wind, rain and rivers are present.

Since vocalisation is influenced by the environment, large birds in noisy urban settings are forced to go against their nature and make louder sounds. Studies have shown great tits singing at higher frequencies in congested urban areas, American robins singing at night rather than during the day and song sparrows’ lowest-frequency notes becoming higher in a noisy environment.

The changes in the timing and frequency of bird signals/songs both carry risks. Males, forced to sing at a higher pitch to avoid the masking of their songs by the urban din, could be perceived as less desirable by females, and nestlings exposed to noise may miss the cues about an approaching predator.

Researchers are also studying how road noise impacts birds, and whether noise pollution affects them more than other forms of pollution.

In an experiment by Boise State University in Idaho in US, a phantom road was created to study the impact of road noise on the migrating birds in Glacier National Park. Pre-recorded road traffic noise was broadcast on speakers attached to trees in the forested area.

The study found a decline of more than one quarter in bird population along the phantom road and almost complete avoidance of the area by some species.

The same thing did not happen with other adverse factors such as visual disturbance, collisions, habitat destruction and chemical pollution.

Experiments like this have established the vital link between acoustic communication and bird behaviour. Over time, a disturbed acoustic ecology affects bird-bonding as well as survival.

So, the next time you hear birds talking or singing at odd hours, you should know that we humans are causing them distress, not the other way around. Only birds can’t put up a ‘Do Not Disturb’ sign on their homes!

The writer is a journalist