Birds enjoy sharper vision than humans. Birds can see certain light frequencies--including ultraviolet--that humans cannot see.
In fact, many songbirds have feathers that reflect ultraviolet light. This light is used to communicate species, gender, and perhaps even social standing. Birds can see this ultraviolet light under normal, daylight conditions. Humans require the assistance of a black light.
Why do birds see better than humans?
1) Both birds and humans have photoreceptive 'cones' in the retina located at the back of the eye. These cones allow us to see color light. The human eye contains 10,000 cones per square millimeter. Songbirds, for example, have up to 12 times this amount or 120,000 cones per square millimeter.
2) In humans, these photoreceptive cones consist of three types. Each cone is sensitive to red, green, or blue light. This is called trichromatic color vision. Birds have an extra cone for tetrachromatic color vision. This extra cone expands the visible light spectrum, allowing birds to see ultraviolet frequencies.
3) During low-light conditions, both humans and birds rely on photoreceptive ‘cell rods’ in the retina. The human eye has 200,000 cell rods per square millimeter. Some birds, such as owls, have up to 1,000,000 cell rods per square millimeter.
4) Bird eyes, on average, account for 15% of the mass of the bird’s entire head. Human eyes, by contrast, account for less than 2% of the head.
5) Bird retinas, in contrast to humans, contain no blood vessels. This prevents light scattering and thus provides birds with greater visual acuity than humans.
In sum, your favorite songbird will notice WindowAlert decals and/or UV Liquid the MOMENT you place them on your window.