What City Lights Do to Birds Flying Over India at Night

Most songbirds, waders, and raptors that migrate over South Asia do it at night. Darkness gives them cooler, denser air to fly through, fewer predators to dodge, and a sky full of navigational cues, stellar constellations, the faint polarised light at dusk, and the planet's magnetic field sensed through iron-rich cells in their beaks. The bar-tailed godwit, the common swift, the Amur falcon crossing northeastern India toward Africa: all of them read the night sky the way a sailor reads a chart.

Artificial light at night scrambles that chart. When a city emits enough upward-scattered light to create a luminous dome, migrating birds can lose their magnetic compass orientation entirely. Research on nocturnally migrating European robins, a species closely related to migrants that pass through India, found that weak, broadband artificial light at intensities found in urban areas is enough to disrupt magnetic compass orientation. The birds did not fly randomly; they flew in the wrong direction with complete confidence. That is the insidious part: the disruption does not look like confusion. It looks like a bird that knows exactly where it is going, headed somewhere it should not be.





Light pollution kills birds through two distinct mechanisms, and a city produces both simultaneously. The first is disorientation at altitude. Birds flying at height get pulled toward bright urban cores, circling illuminated towers and communication masts until they exhaust themselves or collide. The second is window strike. A bird drawn down into the city at low altitude by artificial light then encounters glass, transparent or reflective surfaces that do not register as obstacles. Window collisions kill an estimated one billion birds annually in North America alone; comparable systematic data for India does not yet exist, but the physics and the architecture are identical.


The problem intensifies during peak migration windows. In India, the post-monsoon period from late September through November carries enormous southward movement: Amur falcons funnel through Nagaland, flamingos shift between wetlands, and countless warblers, flycatchers, and shrikes move through the Gangetic plain. October, the month this story runs, sits inside that window. A city like Mumbai, Delhi, Bengaluru, or Hyderabad, seen from altitude on a clear October night, is a bright smear across what should be a navigable dark corridor. Birds that have flown this route for thousands of generations are now flying over a landscape their biology was never designed to read.

Not all artificial light is equally harmful. Short-wavelength blue and white light, the spectrum emitted by LED streetlights and most commercial building illumination, is the most disorienting for birds. It suppresses melatonin, disrupts the circadian rhythms that tell a migrant when to fly and when to stop, and scatters furthest in the atmosphere, contributing most to skyglow. Longer-wavelength amber and red light causes less disruption, which is why some cities in Europe and North America have begun switching to amber LEDs in areas near known migration corridors or bird sanctuaries.

India's rapid LED rollout under the UJALA scheme replaced hundreds of millions of inefficient bulbs, a genuine energy win, but the replacement spectrum was predominantly cool white. The energy savings are real. So is the trade-off that was never publicly discussed. A warbler crossing the Aravalli ridge at 2 a.m. does not care about a utility company's carbon accounting. It cares about finding a dark patch of sky that points south.



The birds that make it through a lit city do not arrive unscathed. Studies on migratory birds caught at banding stations show that individuals passing through heavily light-polluted areas arrive at stopover sites with lower fat reserves, elevated stress hormones, and disrupted sleep architecture, meaning they rest less efficiently even when they do stop. A bird that burns extra energy circling a lit tower has less fuel for the next leg of its journey. Across a flyway that might stretch from Siberia to Sri Lanka, those deficits compound.

India has significant dark-sky assets that migration corridors already use: the Rann of Kutch, the high-altitude deserts of Ladakh and Spiti, the forests of the Western Ghats away from resort clusters, the Sundarbans at night. These places function as refuge not only because of habitat but because of darkness. They are getting rarer. Satellite data tracking nighttime luminosity over India shows consistent brightening across all major states over the past two decades, with the rate of increase outpacing population growth in several urban corridors.

Migration is not a single bird's problem. It is a population-level event, and light pollution operates on populations. A corridor that disorients even five percent of migrants crossing it, across millions of individual journeys each year, removes birds from breeding populations, disrupts seed dispersal and insect control across two continents, and quietly thins the flocks that the next generation of migrants will join. The Amur falcon, which stages in Nagaland before crossing the Arabian Sea, is one of the longest-distance migrants on Earth. Its route runs directly over some of India's fastest-growing cities. The falcon does not have a rerouting option. The city does. Shielding upward-emitting lights, switching to amber spectra near green corridors, and turning off non-essential building illumination between midnight and dawn during peak migration weeks are interventions that cost very little and give a bird crossing India at night its sky back.