Bird eyes display a remarkable array of colors, far beyond the browns and blues seen in many other animals. This diversity in avian irises is not merely ornamental; it offers a glimpse into complex biological processes and ecological adaptations. Unlike human eyes, which are primarily brown, blue, or green, bird eyes exhibit an astonishing spectrum of hues. Understanding these colors involves exploring their underlying biological mechanisms and roles.
The Spectrum of Avian Eye Hues
Bird eyes showcase a wide range of colors, from dark shades like black and brown to vibrant reds, oranges, and yellows. They can also display striking blues, greens, and even white irises. Passerine birds, such as songbirds, often have black to dark brown eyes, while non-passerine birds like hawks, owls, and ducks frequently exhibit lighter eye colors.
Yellow eyes are notable in species like the Short-eared Owl, Great Egret, Yellow-eyed Junco, and adult male Common Goldeneye. Red eyes are common in birds such as the American Coot, Red-eyed Vireo, adult Cooper’s Hawk, Common Loon, and Eared Grebe.
Blue eyes are found in various aquatic birds, including Great Cormorants, Northern Gannets, and Brown Pelicans. The Double-crested Cormorant’s eyes can turn teal-blue during breeding season, and the Blue-eyed Cockatoo also lives up to its name. Some birds exhibit less common colors, such as the orange eyes of the Curve-billed Thrasher or the multi-colored irises of the Red-bellied Grackle.
Factors Shaping Eye Color
A bird’s eye color is determined by a complex interplay of genetic programming and the presence of various pigments and structural elements within the iris. Genes dictate the production and deposition of these coloring agents, though specific eye color genes are still being identified. For instance, a mutation in the SLC2A11B gene in domestic pigeons affects the production of yellow pteridine pigment, leading to a “pearl” (white) eye color.
The primary pigments contributing to avian eye colors include melanins, carotenoids, pteridines, and purines. Melanin, the most common pigment, is responsible for shades of black, brown, and grey. Carotenoids, which birds obtain through their diet, produce yellow, orange, and red hues. The availability of carotenoids in a bird’s diet can directly influence the vibrancy of these colors.
Pteridines and purines are additional pigments that contribute to bright eye colors, including yellows, reds, and white. These pigments can exist in crystalline forms within the iris, creating reflective properties. Beyond pigments, structural coloration also plays a role, especially in producing blue and iridescent effects. This occurs when light interacts with microscopic structures, such as collagen fibers, scattering certain wavelengths and creating colors like blue, even without blue pigment.