Many birds die or are severely injured trying to fly through glass or glass-like structures that offer an unobstructed or reflective view of vegetation or open sky. This is because birds “see” much differently than us mammals do.
There are two types of light receptors in animal eyes; rods and cones. Rods are better for night vision because they are sensitive to small quantities of light and are used extensively by nocturnal animals. Cones detect specific colours (or wavelengths) of light, so they are more important to colour-orientated diurnal animals (animals active during the daylight).
Many mammals, including dogs—have only two cone types, which is like having only two colour channels on a television. Humans and primates are a bit better as we have three cones for red, green, and blue wavelengths. However, birds have us beat! Birds have four single-cone types: red, green, blue, and ultraviolet (UV). What’s more, birds’ cone cells contain a coloured oil droplet, which may allow them to distinguish even more colours! Even within the range of wavelengths that are visible to humans, birds can detect colour differences that humans are not capable of registering--even the ability in some species to “see” magnetic fields to assist with annual migration direction!
Birds seeking places to perch, eat or hide may be attracted to indoor houseplants, green walls and other indoor vegetation that are visible through windows because they can see them much better than us and the reflection of light plays havoc with their internal visual processing.
As birds fly quickly (even year-round residents, Black-capped Chickadees can fly at speeds of up to 24 km/hr), and also have very light and hollow bones (for flight), the force of impact of hitting a window (even at relatively slow speeds) can be lethal at worst, and extremely damaging at best. Injuries from a window collision can be obvious like a broken wing, but internal damage might not be visible. Birds that hit windows or suffer any type of traumatic injury should be assessed by an authorized wildlife custodian before release.
An estimated 16 to 42 million birds are killed every year in Canada by colliding with glass, about 44% of those happen at residences.
Mirrored glass is especially dangerous, but regular glass is also highly reflective depending on light conditions. Windows are an obvious threat, but clear balcony railings, glass windscreens, bus shelters, glass walkways and link-ways, all pose the same risk.