Birds have complex relationships with light that influence their behavior and physiology. Light is crucial for vision, regulating circadian rhythms, orientation during migration, and more. However, artificial light at night from urbanization impacts many species of birds in both positive and negative ways. Understanding how birds interact with light is key to supporting healthy bird populations. This article explores the latest scientific research to answer common questions about how birds respond to light, including street lamps, building lights, vehicle headlights, and other sources of illumination.
Do birds need light to see?
Yes, birds rely on light to see their environment and perform critical behaviors like finding food and avoiding predators. Birds have excellent vision and can detect ultraviolet, violet, blue, green, yellow, and red wavelengths. Some species may also see limited infrared light. However, most birds cannot see well in low light and are essentially blind at night without moonlight or artificial lighting. Birds have high densities of photoreceptors and oil droplets in their retinas to support color vision in daylight. But as light dims, color vision and visual acuity drop rapidly. So most birds are heavily dependent on sufficient light for their vision.
Do birds avoid light at night?
Many species of birds demonstrate behaviors to avoid or minimize light exposure at night. Migrating birds rely on cues from properly timed seasonal light cycles. But artificial light at night can disrupt their navigation by dusk and dawn light. Birds migrating over cities or near bright lights often collide with buildings or become disoriented. Resident birds also alter their behavior in artificially lit areas. For example, European blackbirds avoid light while foraging and reduce activity when exposed to light pollution. Many seabirds only return to their cliffside nests under cover of darkness to avoid predators. Artificial lights can disrupt returning to the nest to feed chicks. Some birds also roost in darker areas during the day to avoid disturbance. So birds take active steps to reduce light exposure at night whenever possible.
Why do birds sometimes flock to lights?
Under some conditions, birds are attracted to lights at night. Migrating birds can become disoriented by artificial light pollution, especially on overcast nights when natural navigational cues are obscured. Birds may then circle the light sources repeatedly. Insectivorous birds, including nighthawks, swifts, and nightjars often feed in large numbers around streetlights. This is likely because the lights attract swarms of insects. Some birds, like seabirds, may also be drawn to illuminated boats and rigs while foraging offshore. Attraction to lights tends to be greater on darker nights with less moonlight. Frogs and bats eating the insects swarms may also congregate, in turn attracting predatory birds. So lights essentially create a temporary ecosystem unlike natural conditions.
Do cavity nesting birds prefer dark nest sites?
Natural cavity nesting sites like tree hollows and tunnels are inherently dark. Cavity nesters like chickadees, titmice, bluebirds, and woodpeckers therefore likely have no innate preference for dark or light nests. Nest site selection depends more on cavity depth, diameter opening, height above ground, and insulation properties. Unlike open cup nesters which often seek concealed sites, cavity nesters may be more flexible since the cavities themselves provide cover. However, persistent artificial light shining into nest cavities could potentially disturb some species. For example, Eastern bluebirds avoided nest boxes illuminated with white LED lights at night. So most cavity nesters may not preferentially select darker nests, but could abandon or avoid sites with high light exposure.
Do nocturnal birds like light at night?
Nocturnal birds like owls are highly adapted to function in low light. Owls have huge eyes to maximize light capture and extremely light-sensitive retinas with a high density of rods but few cones. This gives excellent night vision but poorer daytime color vision and visual acuity. Owls also have light-colored facial disks to reflect sound into their ears for hunting by sound alone in darkness. Many owls avoid light when possible because their specialized eyes are overwhelmed by bright light. But they take advantage of artificial lights to extend hunting through the night. Lights attract more prey to an area, improving hunting opportunities. One study found breeding tawny owls selected nest sites with more artificial light, likely to improve foraging. So darkness is still preferred, but nocturnal birds exploit artificial light opportunistically.
Do diurnal birds avoid activity at night?
Most diurnal birds are active during the daytime and inactive at night when they cannot see well. Low light suppresses activity in diurnal species. Small passarine birds like sparrows and finches have extremely poor night vision. They become largely inactive after dusk and do not leave night roosts until dawn light signals morning. Exceptions are brids that migrate at night like many shorebirds and sparrows. But migrating birds rely on celestial cues for navigation and are not actively foraging. Normal activity patterns are disrupted around bright lights allowing some diurnal species to remain active at night. Otherwise most diurnal birds avoid activity in low light to avoid predation and wasted energy foraging unsuccessfully.
Do birds sleep at night?
Sleep is essential for birds, but sleeping patterns vary across species. Many diurnal birds follow a typical pattern of sleeping at night. Ducks, hawk, hummingbirds, songbirds, and most other daytime species sleep after dusk by finding sheltered roosts and tucking bills under feathers. Falling body temperatures and slowed metabolisms signal sleep. Sleeping at night reduces predation. Nocturnal birds like owls sleep during the day, usually in cavities or dense cover. Other birds have polyphasic sleep cycles with night sleep and short naps. Some birds migrate at night and likely sleep while gliding. Seabirds like albatrosses can sleep on the wing! Light pollution disturbs sleep, but most birds innately sleep at night matching their natural active cycles.
Do artificial lights affect hormones in birds?
There is evidence that artificial light at night interferes with hormone cycles in birds. In particular, it can suppress melatonin levels. Melatonin is a key hormone produced at night in response to darkness. It has many important functions for birds. Melatonin induces drowsiness, lowers body temperature, and regulates sleep. It also controls reproductive physiology and seasonal breeding. Light pollution has been shown to lower melatonin and delay egg laying in birds. Bright lights may also reduce prolactin, a hormone critical for molting. Disruption to normal fluctuations of melatonin, prolactin and other hormones from artificial light can negatively impact birds’ health and fitness.
Do city birds adapt to light at night?
Urban exploiter bird species like pigeons, house sparrows, and European starlings are very common in cities. These species likely adapt to artificial night light over successive generations. Urban populations sleep less at night and start dawn chorusing earlier than rural birds. Alterations to sleep and hormone cycles may support adaptation. Migrating birds may also habituate to some extent, avoiding brightly lit buildings over time. But plenty of evidence shows artificial light still harms urban-dwelling birds via distraction of migrating birds and reduced reproductive success. Full adaptation seems unlikely as light pollution is evolutionarily novel and many effects are detrimental. More research is needed on how urban bird populations try to cope with extremely unnatural light regimes.
Do commercial bird stores use lights at night?
Pet stores and commercial bird breeders often use artificial lighting to manipulate hormone cycles in birds. Supplemental daylight from light bulbs during the winter can prevent seasonal regression of the reproductive system. More hours of “daylight” maintain fertility and induce a spring breeding condition. Lights also extend the perceived photoperiod so birds come into breeding condition earlier. Lights are then turned off at night to simulate short winter nights and further stimulate the hypothalamic-pituitary-gonadal axis. So stores artificially control light to maximize eggs and chicks year-round for sales. However, this constant light manipulation is likely stressful and detrimental to bird welfare over the long-term.
Do bird feeders attract birds at night?
Bird feeders are not likely to attract birds at night when most species are inactive. Nocturnal birds interested in feeders like owls hunt by sound, not sight. However, feeders illuminated by spotlights or security lights can indirectly attract birds. Insects swarming the lights may draw in nocturnal insectivores like nightjars and nighthawks. Night-migrating species traveling high overhead descend when disoriented by lights and may stop at lit feeders. Training a handheld flashlight briefly on a feeder can also draw in birds roosting nearby. But overall, feeders are not a major light source attracting birds. Their bigger value is providing food restoring energy reserves for migrating birds stopping over in the daytime.
Do birds damage lights?
Birds very rarely cause damage to light fixtures. The most likely issue is large perching birds like hawks, crows, and vultures fouling the area directly under lamps with droppings. Birds do not intentionally damage lights, though they sometimes die from hitting illuminated windows and structures. Rare cases of shorting out lights involve electrocution when a bird simultaneously contacts live wires and grounded structures. Nests on electrical equipment can also cause short circuits. Overall, birds around lights are more at risk of harm than the reverse. Preventive measures like shielding wires, placing lights high up, and using bird-friendly designs can reduce conflicts. Proper maintenance and cleaning underneath lights also helps. But bird damage is generally not a substantial problem.
Do birds migrate using moonlight?
The moon provides only limited illumination compared to sunlight. But migrating birds have amazing low-light vision and use moonlight to help navigate at night. On clear nights around the full moon, many diurnal birds migrate after twilight ends. Moonlight provides visual cues combined with stars, geomagnetic sensing, and landscape contours to guide navigation. Even on moonless nights, birds may sense light from the night sky. In experiments, migratory birds use the patterns of polarized skylight as a compass for orientation. So moonlight and other minimal light cues support amazingly sophisticated night navigation abilities in many species. Artificial lights can override and disrupt these natural nighttime cues.
Conclusion
Birds have complex and often conflicting relationships with light at night. Most species did not evolve to cope with artificial light and suffer disruption from urbanization. It interferes with sleep cycles, hormone regulation, and navigation for migrants. But some birds opportunistically exploit lights to extend feeding into the night. Overall, darkness is still the natural condition birds thrive under best. Understanding these species-specific responses to different light conditions allows supporting healthy bird populations. This requires reducing unnecessary artificial lights and using proper shielding when light is needed. With care, we can minimize harm and maximize the benefits birds receive from natural patterns of light and dark.
