Birds to humans: do not disturb

Bengalureans, in some areas, have been complaining about birds chirping in the night and depriving them of sleep. In Kolkata, the young, who have no issues about discotheque music, are exchanging copious notes on the social media about how to cut out the “annoying” koo-koo and kik-kik of the koel at night. In parts of Mumbai, old people are missing the bird songs at dawn that were once their morning alarm and, in the urbanised areas of smaller cities, people are wondering about the disappearance of the birds of their childhood.

Most common people are clueless about the changing bird behaviour, ranging from the shift in the timing of bird calls to the muting of the bird songs at dusk and dawn, but experts know the answer. The soundscapes of our world are changing and, with that, 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. They use sound to attract mates, defend territories, protect their young, sound alarms, give auditory cues to nestlings, and communicate other types of information, like finding proper habitat and locating 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 in June 2016, Fristrup said that “animals developed ears before vocal chords”.

The acoustic frequency of a bird’s song is related to its body size. The bigger the body, the longer the trachea, leading to a lower sound frequency, as in the case of treepie. A robin, on the other hand, being small in size has a smaller trachea and the frequency of its sound is higher.

According to Biological Sciences professor Kerry Rabenold of Indiana, US, quoted in a 2011 Oxford University research paper, large-bodied birds can produce sounds as low as one KHz, but most songs and calls produced by birds occur in the two to six KHz range. Sound frequency of birds, he says, can reach 10 to 12 KHz in places where geophonic sounds, like those of the wind, rain and rivers, are present.

The term Hertz as used in frequency measurement is named after the 19th century German physicist, Heinrich Hertz, who demonstrated the existence of electromagnetic waves.

Technically, wavelength is inversely proportional to wave frequency just as bird size and bird sound frequency are inversely proportional. The speed of the electromagnetic wave, expressed in metres per second, is equal to wavelength (in metres) multiplied by frequency (in oscillations per second or Hertz, abbreviated as Hz).

Since the evolution of vocalisation is influenced by the environment, large birds in noisy urban settings are forced to make louder sounds. Studies have shown great tits singing at higher frequencies in noisy urban areas, American robins singing at night rather than during the day and song sparrows’ lowest-frequency notes becoming higher in environment with high ambient noise.

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; their security could be threatened as a result of potential rivals getting a cue about their location, and, nestlings, exposed to noise, may not be able to figure out cues about an approaching predator.

Researchers are also studying how road noise impacts birds, and whether it is worse than other forms of pollution, such as chemical. In an experiment by Boise State University in Idaho in north-western 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 particular area by some species. The same thing did not happen with other factors associated with roads, 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 and, ultimately, survival. So the next time you hear birds talking at night or making unusual chatter during the day, you should know we humans are causing them distress, not the other way around. Only, birds can’t put up a ‘Do Not Disturb’ signboard on their homes.

(The writer is a senior journalist based in New Delhi)