Window Warnings

Bird Vision

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. Wild birds 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 quadchromatic 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 wild bird will notice WindowAlert decals the MOMENT you place them on your window!

Best Placement


1) Bird strikes often occur when birds perceive a LINE OF FLIGHT through one window of your home and out another. This also occurs on buildings with corners made of glass. In these situations, position your decals to block a bird’s perceived line of flight.

2) Bird strikes may occur when birds see OPEN SKY reflected in your window. This may occur on certain windows or areas of a window based on the time of day or orientation to the sun. Use your decals to break up this reflection of open sky.

3) Bird strikes are often a problem near BIRD FEEDERS. Bird feeders should be positioned closer than 3 feet or farther than 30 feet from your windows. Feeding birds are often frightened and when they are, they take wild, evasive flight. If your feeder is close to the window, birds fly away from the house. If they do happen to strike a window, they may not have reached full speed and may survive. Believe it or not, the safest feeders are window feeders which attach to the window itself. You may find such feeders on this website.

4) Bird strikes are a problem with highly-reflective windows, windows near bodies of water, and windows near indoor mirrors. These situations may be difficult to remedy.


PROBLEM: Some highly territorial birds vigorously defend their territories and will even attack their own reflections in windows. They do this because they perceive their reflection to be an intruder.

When a bird repeatedly attacks a window, it puts its own health and energy at risk. This activity can compromise the success of its nest and its brood.

SOLUTION: CardinalAlert is a window decal designed to be placed along the bottom edge of problem windows, eliminating reflections form birds that mount their attack from a windowsill perch.


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