13 August 2024

Just because an animal has UV receptors in its eyes does not mean it can see ultraviolet light. A recent BBC video, below, reveals some surprising things about the use and perception of ultraviolet light in starlings (Sturnus vulgaris) and raptors, especially golden eagles (Aquila chrysaetos). For instance:

• Starlings and golden eagles both have UV receptors in their eyes.
• Female starlings have feathers that reflect UV. The more UV a female reflects the more successful she is at breeding. Male starlings like the glow we humans cannot see.
• UV light scatters more. If you can see UV light, it makes images blurry.
• Raptors have UV receptors in their eyes but they cannot see it because their lenses filter it out. The golden eagle’s vision is sharper because he cannot see UV.
• Scientists used to think kestrels hunted by seeing the UV reflective paths of rodent urine. Nope. Kestrel eyes filter out UV so that theory has been disproved.

Interesting conclusions:

• Because I thought that raptors could see UV, I used to wonder how flashy UV-reflective songbirds managed to evade predators. Answer, the predators cannot see that flashy stuff!
• UV light damages the eye so there is an advantage to not seeing it for most of one’s life.
• Human eyes have UV receptors but we cannot see it because our lens filters out UV. There are exceptions based on age and lack of lenses.
  • Exception#1: Young people up to age 30 can see near UV, the wavelengths closest to our visible color range, per a 2018 Univ of Georgia study.
  • Exception#2: Those without lenses in their eyes can see near UV. This includes those born without lenses and those who had cataract surgery in the early days. Claude Monet had cataract surgery in 1923 with no lens replacement and could see near UV.

Read more about human perception of UV light in this Live Science article: Can Humans See Ultraviolet Light?