Cities are no safe havens for avian life. Birds collide into buildings very often because windows reflect the surrounding environment or offer avenues that look open. Birds that have nocturnal migratory patterns (i.e. migrate by the night) are particularly adversely affected by night lights of the city.
“Artificial lights can cause them to migrate too early or too late and miss ideal climate conditions for nesting, foraging and other behaviours,” the International Dark Sky Association states.
A recent study on the metropolis of Chicago, US, finds strong support for the relationship between bird mortality and artificial lighting. Of all American cities, Chicago presents the greatest light pollution risk to migratory birds. The study was conducted using a bird collision dataset collected at McCormick Centre, one of the major cultural centres of the city, over a 21 year long period.
Collisions kill hundreds of millions of birds each year in North America alone—but turning off lights and darkening windows can make a huge difference. Our latest research in @PNASNews describes how: https://t.co/ljqvClujou pic.twitter.com/4DuTce1np2
— Benjamin Van Doren (@bvdbirds) June 8, 2021
“Chicago poses the greatest potential risk from light pollution to migrating birds of all cities in the United States and over 40,000 dead birds have been recovered from McCormick Place alone since 1978,” the paper states.
The data revealed that collision driven mortality bears a strong correlation with migration traffic, the area of lighted windows in a building and local weather conditions.
In spring and autumn, collision rates were 11 and 6 times higher when windows were lit, as opposed to when they were darkened. Whether or not a particular window was lit was the most important factor in deciding whether a bird collided into it. Darkening a window even reduced collision incidences on nearby windows.
New research shows that #lightpollution has increased by at least 49% over 25 years globally. This figure only accounts for light visible via satellites. So, the true increase may be much higher – up to 270% globally, & 400% in some regions.
Learn more: https://t.co/uZ7rYLz7ru
— IDA Dark-Sky (@IDADarkSky) September 14, 2021
The study predicts a 53% (autumn) to 59% (spring) decrease in collisions upon reduction of lighting to minimum levels that have been recorded historically. They also predict an increase of 46% (autumn) to 116% (spring) if all the windows were to be lit.
Naturally, switching off all the lights on all nights might not be commercially feasible, so it is suggested that reducing light pollution for the largest 25% of migration events, for even that leads to a reduction in collisions in no small measure.
Nocturnal bird behaviour
A 2017 study observed that changes in the behaviour of nocturnally migrating birds in response to light stimuli disappeared immediately after lights were switched off. Contrary to what is usually believed, building height may not be that important a factor in bird collision compared to the lit window-area.
Reports of birds colliding with buildings go back to as early as the nineteenth century. While referring to Porzana carolina, a small waterbird, an 1884/5 report on bird migration in the Mississippi Valley noted how “in 1884, an electric stood in their path and lured them to destruction’ and ‘were killed or wounded by striking the-light tower”.
In the wake of recent research on the subject, citizen-driven initiatives to switch off outdoor lights at night have gained traction in the recent past. Authorities have now started to issue advisories during peak migration season and when weather conditions are favourable for avian movement. Population declines in general notwithstanding, the number of bird collisions at the McCormick Centre plummeted quite abruptly once light out programmes commenced in 1999.
India and light pollution
Light pollution and other ‘by-products’ of rapid urbanisation threaten birds – and other animals – in the Indian subcontinent as well, as a review rightly notes. Some migratory birds that are particularly vulnerable are those whose migratory routes pass through India, include common crane, bar-headed goose, falcon, northern wheatear, Amur falcon etc.
Adverse effects of artificial lights on nocturnal ecology are observed on other species like bats, loris and insects. Light also deters sea turtles from moving to the beach at night to lay eggs. Hatchlings use light-cues from the horizon to move towards the ocean – however, artificial sources draw them away from the ocean, leading to their death.
A study on the tammar wallaby showed how artificial lights create a perception of a change in day length and lead to delayed births and the suppression of melatonin (a hormone responsible for the sleep-wake cycle).
Like birds, species of migratory fish too bear the brunt of the untoward consequences of anthropogenic lights. Artificial lighting has also altered predator-prey relations.
– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)