Most mammals (including the great apes) and many species of birds have dark brown eyes.
We forget that humans are unusual in having a wide variety of eye colors: brown, blue, grey or hazel/green.
And in fact elsewhere in the animal kingdom, we find the same eye colors recurring. This Ugandan Grey-necked Crowned Crane, Balearica regulorum, has pale grayish-blue eyes.
This Common Cormorant in Central London has eyes greener than those of any human outside Scifi movies:
And this dragonfly goes one step further:
But we also find eye colors that in humans would need contact lenses:
Look at this White-eyed Buzzard in India:
Or the lemon yellow eyes of the Indian Jungle Owlet:
or the tawny eyes of the lion:
and the tangerine eyes: of the Namibian Southern White-faced Scops Owl, Ptilopsis granti
Maddest of all, the cherry-red eyes of the loon (aka Great Northern Diver):
The dragonfly aside, the mechanism for all these eye colors lies in the outer layer of the iris, called the stroma. Dark brown eyes result from the presence of melanin in the stroma. Blue eyes happen when the stroma contains no pigment and is translucent. This layer scatters the white light, and it scatters the shorter blue wavelengths the most, giving rise to the perception of blue eyes. If the layer has a little more collagen, the blueness is dampened and the eyes look grayish. Green eyes result from the presence of a little melanin only, mixing with the reflected blue to create green.
There are actually two types of melanin, and the full range of colors found in my photos above depends mainly on which type of melanin is found. Here is a great chart, if you’d like to know more.
Melanin Content and Eye Color
| Eye color | Melanin Presence on Front Layer of Iris | Melanin Presence on Back Layer of Iris | Dominant Pigment Type |
| Brown | Heavy | Normal | Eumelanin |
| Blue | Light | Normal | Eumelanin |
| Gray | Even less than blue | Normal | Eumelanin |
| Green | More than blue eyes, less than brown | Normal | Pheomelanin |
| Hazel | More than green, less than brown | Normal | Pheomelanin and Eumelanin |
| Amber | Heavy | Normal | Pheomelanin |
| Red or Violet (in humans) | None or extremely little | None or extremely little | n/a |
PS Humans are also unusual in having the colored iris surrounded by a large white area, the sclera. This makes it easy to notice the direction of other people’s gaze, a useful trait when cooperating with others.
Eyes on the Wild 
Thanks for the informative (and pretty) post. Your theory about human eyes makes sense. Another possibility is use for averting one’s gaze, which can deliver a number of different messages depending on context. A truly useful adaptation.
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As usual, great story – providing me with a whole new outlook at my fellow creatures. Amazing, the things they didn’t teach you in medical school!
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I’ve read that the red eyes of the loons are part of the mating plumage and that they go grey in the winter months when they lose their breeding look. All the birds on our lake seem to have on the black and white and red eyed look in the summer, breeding pairs or no… I’ll have to take a closer look next season, especially as the fall comes on.
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How interesting. If that is true, I wonder what the biology and mechanics of that change are. Keep me posted.
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