Artificial light at night disrupts migratory behaviour in sparrows
Captive trials show even dim night lighting can change nocturnal activity and migratory restlessness in migrant sparrows, while House Sparrows are far less affected
A simple question - what happens when nights get brighter?
Artificial light at night (ALAN) is spreading across towns, villages and rural roads, changing the natural light-dark cycle that birds have evolved with.
This study tested that idea using captive trials repeated across three seasons, comparing two migratory sparrows, Gambel’s White-crowned Sparrow and White-throated Sparrow, with a largely non-migratory species, the House Sparrow.
How the experiment worked
Researchers housed birds individually in indoor cages under controlled daylengths that matched the local photoperiod in spring, summer and autumn.
Each bird experienced near-dark nights as a control, and then nights with one of four light levels: 0.15, 0.5, 1.5 and 10 lux - a range chosen to reflect levels birds can encounter from skyglow and rural lighting through to brighter urban illumination.
Video recordings were used to measure locomotor activity across day and night. For the migratory sparrows, the team also tracked classic markers of migratory restlessness (Zugunruhe), including behaviours known as beak-up and beak-up-flight.
Migrants responded strongly - even to dim light
The clearest pattern was that the migratory sparrows (both Zonotrichia species combined) changed their behaviour under ALAN across all seasons.
At night, they became more active under artificial light than they were in near-darkness. During spring and autumn, when they were in a migratory state, this included increases in behaviours associated with migratory restlessness.
House Sparrows (Passer domesticus) were different. Their night-time activity remained minimal, and any behavioural shifts were subtle and mostly limited to the highest light intensity.
Not ‘more light = more migration’ - a threshold effect
One of the most important findings was that migratory restlessness did not simply increase as nights got brighter.
At low and moderate light levels, the migratory sparrows showed stronger expressions of migratory-restlessness behaviours. But at the brightest treatment (10 lux), those characteristic migratory behaviours dropped sharply, even though overall activity could remain high.
In practical terms, dim night lighting may be enough to stimulate migratory drive - but brighter conditions may disrupt the cues that allow proper migration-related behaviour to be expressed.
A summer surprise - effects even outside migration seasons
The study also found that ALAN altered the migratory sparrows’ nocturnal activity even during the summer ‘stationary’ season.
That matters because summer is not a time when you would expect strong nocturnal activity in these birds. The results suggest artificial night lighting can interfere with behavioural rhythms more broadly, not only during active migration windows.
No payback during the day - potential energetic costs
A particularly worrying result was that migratory sparrows did not consistently reduce daytime activity to compensate for the extra movement they showed at night under ALAN.
If a similar pattern occurs in the wild, increased night-time activity without adequate recovery could carry energetic costs - especially for migrants that already face tight fuel budgets and time pressure.
Why House Sparrows may cope better
House Sparrows are strongly associated with human settlements and have lived alongside artificial lighting for many generations.
The contrast seen here suggests resident, human-adapted species may be more tolerant of night lighting, while nocturnal migrants are more behaviourally sensitive - including at light levels that can occur well beyond city centres.
What this means for conservation and lighting policy
Artificial light at night is often treated as an unavoidable side-effect of modern life. This work adds to evidence that it can directly alter behaviour in nocturnal migrants, potentially in ways that matter for migration success.
The encouraging part is that light pollution is one of the more solvable pressures. Better shielding, targeted dimming, warmer spectrum choices and switching off non-essential lighting during peak migration periods could all reduce exposure - especially where migratory routes funnel birds past lit landscapes.
January 2026
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