Alps ski resorts are a place of utmost joy for humans but a danger zone where birds must risk their lives. That is because many die after failing to see lift cables and crashing into them mid-flight. Scientists have found a way to save birds: install black-and-white warning markers instead of red ones so they are visible to color-blind birds.
A team led by Marjorie Liénard of the Department of Life Sciences at the University of Liège in Belgium said in the Journal of Experimental Biology on the 23rd (local time) that "the black grouse (scientific name Lyrurus tetrix), which accounts for most ski resort collision accidents, has very poor vision and cannot see the red color of warning markers."
◇ High-contrast black-and-white warning markers are effective
At ski resorts in the French Alps, nearly 600 incidents of birds dying after colliding with ski lift cables have occurred over the past 60 years. Black grouse accounted for 70% of them. Resorts installed warning markers 3.5–15 cm wide so birds could distinguish the cables. And because the black grouse's comb is red, they also made the markers red, thinking birds would see the red markers better as they recognize their peers. Even so, collisions involving black grouse continued.
The Belgian team began the study believing the cable-hung warning markers did not consider the black grouse's eyesight. First, they determined the field of view by checking how far the bird could see when turning its head. They imaged the retina from every angle—front, back, and sides of the head. Surprisingly, the black grouse had a 353-degree field of view, tops among birds. Only the 7 degrees directly behind, which it could not see even when turning its head, were a blind spot.
But its ability to detect obstacles while flying was inferior to humans'. Calculations showed the smallest object recognizable at a distance of 32 m was about 3 cm. That is 4–5 times larger than what humans can distinguish at the same distance. In other words, black grouse need warning markers larger than what humans see.
Simon Potier, the paper's first author, said, "In bright daylight, a sign measuring 9 cm by 9 cm composed of 3-cm squares can be recognized at a distance where collision risk can be reduced." He also explained that to give birds time to react as they fly toward the cable, the spacing between signs should be kept within 16 m.
A bigger issue was color vision, the ability to detect colors. The team extracted photoreceptor proteins from the eyes of black grouse that died in collisions to see which colors they were sensitive to. Black grouse can see yellow, green, blue, violet, and parts of ultraviolet, but they struggle to distinguish red, which has a longer wavelength.
The researchers said warning markers hung on cables should avoid red and be made in colors with strong contrast, such as white and black or violet and yellow. That is because dark-colored cables against a green forest background or white markers against snow are hard to see.
Then why can't black grouse with red combs see red? The team explained that black grouse detect different light from the red comb. Liénard said, "The comb not only reflects red but also reflects ultraviolet that humans cannot see."
◇ Polka-dot patterns that prevent collisions
Many birds also die in cities from collisions. Birds die after flying into transparent soundproof barriers or glass building facades. In the United States, it is known that 1 billion birds a year are lost to collisions. According to a survey by the National Institute of Ecology (NIE), 8 million wild birds a year in Korea die after hitting transparent windows.
In the past, people tried to prevent collisions by sticking several large stickers of raptors such as eagles or hawks on transparent windows, but the effect was limited. Experts say that is because they did not consider birds' behavior and vision. Birds perceive a stationary sticker not as a predator but simply as an obstacle to avoid. Unless eagle or hawk stickers are densely placed, there remain many places where small birds can collide.
Based on the National Institute of Ecology (NIE) findings, the Ministry of Climate, Energy and Environment announced the "guidelines for reducing wild bird collisions with transparent windows" in 2019. The most effective prevention method is applying the so-called "5×10 rule." Because birds fly fast, they tend to try to pass through narrow spaces. However, they avoid spaces narrower than 10 cm horizontally and 5 cm vertically, judging they cannot pass through.
In that case, if small dots are placed at 5×10 rule intervals, the entire transparent window can become an impassable area for birds. In fact, experiments at home and abroad have shown that windows with dot-pattern stickers saw collisions decrease by 90%. The Ministry of Climate, Energy and Environment and local governments are currently attaching dot-pattern stickers to transparent soundproof barriers.
References
Journal of Experimental Biology (2026), DOI: https://doi.org/10.1242/jeb.250727
Global Ecology and Conservation (2019), DOI: https://doi.org/10.1016/j.gecco.2019.e00795
Ministry of Climate, Energy and Environment (2019), https://mcee.go.kr/wonju/web/board/read.do?menuId=1032&boardId=969600&boardMasterId=232&condition.hideCate=1