When the lights stay on, diurnal birds forget how to sleep

Pranad Patil has been leading wildlife tours since 2008, and he has grown used to animals confounding expectation. But one encounter still stays with him. On the outskirts of Ahmedabad, long after sunset, he spotted a  — a small, typically diurnal hawk  — circling an illuminated streetlight pole, picking off insects drawn to the glow. "It certainly looked stressed," said Patil. "The eyes were really wide open. The bird looked sort of in a trance. It definitely was not a normal thing."

Patil, a naturalist based in Mumbai, has since catalogued a handful of such sightings. A drongo and a fantail, both in his building complex in Andheri, hunting for insects at 4 am around tube lights. A red-vented bulbul doing the same in Pench.

In each case, he says, the behaviour was clearly triggered by the presence of artificial light — remove the light, and the bird would almost certainly have been roosting. "If the tube light was not there, they would definitely not have been active at that time," he said.

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What the studies say

These are, in one sense, unremarkable anecdotes. Birds behave strangely around streetlights; most city-dwellers have noticed something of the kind. But they sit within a rapidly accumulating body of evidence that artificial light at night. ALAN, as researchers abbreviate it, is reshaping how birds move through time, not just through space.

A recent paper published in Ecology Letters, which analysed 36 light pollution studies spanning 30 bird species, found consistent physiological and behavioural disruptions across the board: extended activity periods, increased nocturnal foraging, and significant sleep disruption, with downstream effects on metabolism, hormonal cycles, and immune function. Brighter light intensities were associated with more severe shifts.

The same pattern has now been documented in one of India's most recognisable raptors. In February 2024, citizen scientist Polycarp Joseph recorded three brahminy kites foraging at the Vaddy fishing harbour in Kerala's Kollam district — between 11 pm and 12:30 am, nearly six hours past what would be their natural period of activity.

The harbour is illuminated like a sports stadium at night, with LED arrays in the auction yard and across the fishing boats. A paper published in the Journal of Ornithology documents this as the first recorded instance of nocturnal foraging in brahminy kites in India.

The Vaddy harbour during the day is a scene of intense competition, with hundreds of black kites, house crows, little egrets, and feral dogs all competing for discards and freshly landed fish. Under artificial light, after midnight, most of that competition dissolves.

Why the problem is bigger than we imagine

For Patil, the competitive logic makes sense, but what concerns him more is what happens to the birds that take this gamble repeatedly. "We can understand our bodies and counter sleep loss," he said. "We can take medication or sleep in a dark room in the daytime. The birds don't have that sense. If a bird does this once, maybe it keeps on doing it regularly. And if the light is there permanently, it affects the sleeping cycle, which then affects the hormonal cycle, which then affects breeding cycles and migration patterns."

This cascading logic — disrupted sleep becoming disrupted hormones becoming disrupted reproduction — is precisely what the Ecology Letters study begins to map. Sleep disruption emerged as the most consistent physiological impact in the literature, and the study's authors note that its downstream effects on metabolic rate and immune function are likely significant even when they do not manifest as changes in lifespan or body size, the broader life-history traits the researchers were tracking.

Patil is unsparing on the question of whether the conservation community has caught up to this. Light pollution and noise pollution, he argues, remain poorly understood relative to the more visible threats of habitat loss and chemical contamination. "These things are equally bad," he said. "For wildlife, there are no measures to counter it. They cannot go into a dark room in the daytime and catch up on their sleep."