How coastal lights reshape the night flights of migrating birds
New research reveals that even faint coastal lighting alters speed and height in birds arriving from the sea.
Illuminated nights, altered flights
A new study from the Croatian Adriatic coast has revealed that even relatively low levels of artificial light at night can significantly alter the behaviour of migrating birds. Using paired ornithological radars placed at a brightly lit coastal town and a nearby near-natural site, researchers monitored two full spring migrations and found clear evidence that nocturnal migrants fly more slowly and at lower altitudes when passing over illuminated areas.
Although the overall number of birds in the air did not differ dramatically between the dark and light-polluted sites, the way birds moved through those airspaces did. Sea-crossing migrants approaching land showed the strongest behavioural shifts, suggesting that birds emerging from long, dark overwater flights may be especially vulnerable to artificial light as they reach the coast.
Subtle light, strong effects
Despite the modest brightness of the illuminated site – far dimmer than large urban centres – migrating birds consistently reduced their airspeed when flying over it. Radar modelling showed that sea-crossing migrants travelled almost 1 m/s more slowly above the lit area, while coastal migrants also reduced speed, though to a lesser degree.
The study found that light pollution alone explained a significant proportion of the behavioural variation recorded, demonstrating that even low-intensity lighting used in small towns or coastal developments can disrupt natural migration patterns.
Drops in flight height raise collision concerns
Birds also flew markedly lower over illuminated areas. The radars recorded average declines of more than 50 metres in flight altitude for sea-crossing migrants, with slightly smaller but still substantial reductions for birds following the coastline.
These changes were magnified on cloudy nights, when sky brightness is amplified by reflected light. Under full cloud cover, flight heights at the lit site dropped by an additional 12–16 percent. Because lower altitudes increase the risk of collisions with buildings, masts and other structures, the researchers highlight this as a key concern for conservation.
Sea-crossers more vulnerable than coastal flyers
Migration along the Croatian coast follows two broad directions: birds travelling parallel to the coast and those arriving directly from Italy after crossing the Adriatic Sea. The study confirmed that the latter group showed the strongest response to artificial light, reducing both height and airspeed more when approaching the illuminated shoreline.
This likely reflects an increased reliance on distant visual cues after long sea crossings. The contrast between the dark sea surface and the glowing coastline may draw birds towards lit areas even when those areas do not correspond to optimal habitat or safe migration routes.
Cloud cover compounds confusion
Cloudy and foggy nights have long been known to worsen the effects of artificial light on migrating birds. This study found the same: migration intensity declined as cloud cover increased, while the behavioural effects of light pollution became more pronounced.
Under heavy cloud, birds flew lower and slowed further over the lit site, suggesting greater disorientation when celestial cues are obscured and artificial lighting dominates the nightscape.
Implications for a rapidly developing coastline
Croatia is considered progressive in regulating light pollution, yet coastal tourism continues to expand and illumination levels are rising. The authors warn that even well-managed lighting can disrupt migration in sensitive areas, particularly in regions where birds funnel after crossing large water bodies.
The findings also underscore the importance of dynamic mitigation measures. Short-term “lights out” approaches on nights predicted to have intense migration could reduce collision risk, but such measures require improved access to real-time radar data across Europe.
Until then, the study suggests that new infrastructure along key flyways should be assessed for potential impacts on night migrants – and that reducing unnecessary nighttime lighting remains one of the simplest and most effective steps to protect birds on the move.
December 2025
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