The question of whether bats are drawn to light is often met with a simple “yes” or “no,” but the scientific answer is far more complex, depending heavily on the bat species and the specific characteristics of the light source. Most bats are creatures of the deep night, relying on darkness for safety and successful hunting, so the widespread use of artificial light at night (ALAN) has introduced a significant new variable into their environment. The resulting behaviors range from attraction to light-heavy areas to complete avoidance, creating a mosaic of responses that impact the ecology of these nocturnal insect-eaters. Understanding this relationship requires separating the bat’s direct response to light from its indirect response to the insect prey concentrated around the light.
The Complex Answer: Attraction, Repulsion, and Species Differences
Bat responses to artificial illumination are highly diverse, categorized by their flight speed and foraging style. This reaction is known as phototaxis: directional movement toward (positive) or away from (negative) a light stimulus. Most bat species, particularly those that are slow-flying and forage close to vegetation, show a negative phototaxis and actively avoid lit areas, likely as a strategy to evade predators like owls, which hunt more effectively in bright conditions. Species like the brown long-eared bat and various Myotis species are highly light-averse, viewing lit areas as barriers. Conversely, a smaller group of fast-flying bats, such as the common pipistrelle and noctule bats, may appear to exhibit positive phototaxis near streetlights. This apparent attraction is usually not a response to the light itself but an exploitation of the dense insect populations gathered there. However, some studies suggest migratory species may be drawn toward certain green light wavelengths independent of prey, indicating a true positive phototaxis in some contexts.
The Indirect Link: Why Some Bats Hunt Near Lights
The primary reason certain fast-flying bats congregate around lights is the reliable, high-density food source created by the illumination. Many nocturnal insects, which form the bulk of an insectivorous bat’s diet, are strongly attracted to light, a behavior known as positive phototaxis. High-intensity lights, particularly those emitting UV and blue light, function as magnets for these insects, creating localized feeding hotspots. This concentration of prey allows light-tolerant species to forage efficiently, often capturing larger insects than they might find in dark areas. The common pipistrelle, for example, can be seen swooping repeatedly around streetlights, taking advantage of the high prey availability. For these opportunistic hunters, the benefit of an easy, abundant meal outweighs the potential risk of increased predation in a lit environment. The bats are drawn not by the visual stimulus but by the echo-location signature of the swarming insects, treating the light source as a concentrated, predictable buffet.
Disruption of Navigation and Roosting by Artificial Light
For the majority of species that avoid light, Artificial Light At Night (ALAN) represents a significant threat to their daily survival and movement patterns. Light near a roost entrance can disrupt the bats’ circadian rhythm and delay their nightly emergence, shortening the available time for foraging. This delay causes a mismatch between their foraging onset and the peak availability of their insect prey, which occurs shortly after sunset.
Artificial illumination also creates barriers that fragment the landscape, preventing light-averse bats from using established commuting corridors between their roosts and foraging grounds. Slow-flying species, which often use linear features like hedgerows or river corridors for navigation, will deviate from these routes to avoid light spill, forcing them to take longer, less efficient paths. Blue and UV-rich light is particularly harmful, as it interferes strongly with bat navigation and increases the risk of predation. The avoidance zone for some light-averse bats can extend 8 to 50 meters from the light source, effectively rendering significant areas of habitat unusable.
Choosing Bat-Friendly Lighting Solutions
Mitigating the negative effects of artificial lighting requires reducing light intensity and altering the light spectrum. Lighting fixtures should be designed to minimize light spill, using hoods or baffles to direct light only downward where it is needed. Continuous lighting can be avoided by installing motion sensors or using variable lighting regimes that only activate or brighten lights when human presence is detected.
The color temperature of the light is another significant factor, as bats are least sensitive to light in the longer-wavelength parts of the spectrum. Warm-colored light, specifically amber or red, with a color temperature of 2700 Kelvin or lower, is preferred because it contains minimal blue and UV emissions. Light sources that feature a peak wavelength of 600 nanometers or higher are considered the least disruptive to bats and are less attractive to the insects they prey upon. This careful selection of color and control allows for essential human lighting while reducing the ecological impact on nocturnal wildlife.