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Canberra-based naturalist, conservationist, educator since 1980. I’m passionate about the natural world (especially the southern hemisphere), and trying to understand it and to share such understandings. To that aim I’ve written several books (most recently 'Birds in Their Habitats' and 'Australian Bird Names; origins and meanings'), run tours all over Australia, and for the last decade to South America, done a lot of ABC radio work, chaired a government environmental advisory committee and taught many adult education classes – and of course presented this blog, since 2012. I am the recipient of the Australian Natural History Medallion, the Australian Plants Award and most recently a Medal of the Order of Australia for ‘services to conservation and the environment’. I live happily in suburban Duffy with my partner Louise surrounded by a dense native garden and lots of birds.

Thursday 8 February 2024

Eye Spy; what colour is a bird's eye?

The simple answer is just about any colour at all; the tricky follow-up though is a two-parter, 'why and how?'. However if you're looking for answers to these questions, you probably don't need to read any further here - mostly we still don't know. For much of what follows I've drawn on a recent (2022) review paper by a team from Louisiana State University led by PhD student Eamon Corbett. This is a free 'preprint' article; the final published article (2023) is available here, but it is not free. 

I think it is very telling that, as of October 2023, only 2% of bird species and 21% of families have been studied in any way to determine what causes their eyes to be the colour they are. As with feathers, the colour of eyes (by which I mean irises) is determined by pigments, or physical structures, or a combination of both, though any given colour may be achieved in different ways in different species. I'll explain what we do know about the basis of various iris colours as we go, but mostly this is going to be  a celebration of the rainbow of colours in which bird eyes may parade. (Not quite a rainbow actually - there are very few green bird eyes that I know of, and I have no examples of them.)

In this post I have primarily used Australian examples, though have included some others where appropriate. Firstly the great majority of eyes are black or dark brown - in fact the proportion of 'dark' eyes is higher in birds than in other vertebrate groups that have been studied.

Barn Owl Tyto alba, Alice Springs Desert Park. It seems that melanins are responsible
for most of the dark eyes, but not all are in the same layers of the eye - not detail
we need to go into here! (Corbett goes into all of this in much more depth,
via the first link in the first paragraph of this post.)
Double-eyed Fig Parrot Cyclopsitta diophthalma, Cairns.
This may be an example of a phenomenon we'll delve into a little more below,
of a dark eye forming a highlight by contrasting with a bright background; the
reverse can also be found in other species.
Yellow-tailed Black-Cockatoo Zanda funerea in our backyard in Canberra.
Perhaps that's enough of black eyes; dark brown ones are probably formed similarly, by layers of melanins. 
Swift Parrot Lathamus discolor, Canberra. A rare bird, seriously endangered by habitat loss,
whose eyes we probably pay much less attention to than to the spectacular plumage.
Brown Quail Synoicus ypsilophorus, Bundjalung NP, north coast NSW.
This quail's eyes are probably reddish-brown, and this colour is fairly common too. (You'll probably be able to see these better by clicking on the photos.)
Brown Tinamous Crypturellus obsoletus, near Sao Paulo Brazil.
(I am determined not to get sidetracked today with interesting stories about
the birds or the locations!)
Noisy Friarbird Philemon corniculatus, Pilliga Forest, northern NSW.
This is a familiar big migratory honeyeater from eastern Australia.
Rainbow Bee-eater Merops ornatus, Longreach Waterhole, Northern Territory.
I'm no artist, or any other sort of expert on colour, but a fairly unusual extension of brown in eyes is hazel, usually described as 'brown-green'; I'm not sure about these, but I'm going to tentatively call them hazel. Always happy to be challenged of course, though I hope we can agree that they are very attractive eyes indeed!
Kori Bustard Ardeotis kori, Ngorogoro Crater Tanzania.
White-fronted Chat Epthianura albifrons, Canberra.
Australian chats are now known to be a group of honeyeaters.
It's of course impossible to determine when 'red-brown' becomes 'red' (not least because it's in the eye of the beholder as much as that of the bird), but here are some pretty impressive red eyes. It used to be assumed that the carotenoid pigments that cause feathers to be red, orange or yellow have the same effect in eyes, but as often is the case, while there are certainly carotenoids in some or even many eyes, it's not that simple. In addition to another class of pigments, pteridines and purines, blood vessels near the surface do the trick in quite a few birds and cholesterol crystals have an effect in some others. I've mentioned already how few species have been studied in detail, but inevitably most of those that have been are from the Northern Hemisphere. However here are a couple of red-eyed Australian birds whose eye secrets have been revealed to make the point that we just can't make assumptions here.
Black Swan Cygnus atratus Canberra. Its red eyes are due to an interaction of
melanins, cholesterol crystals and blood vessels.
Zebra Finches Taeniopygia castanotis, Great Sandy Desert, Western Australia.
Their red eyes are down to pteridine pigments.

As for the red eyes that follow, we can only assume that some of them are due to some combination of the above effects, and some are not! The swan's eyes are emphasised by being on a black background, and here are a few more (though by no means all I could have used) using this strategy. The whole body may be black, like the swan's...

White-winged Choughs Corcorax melanorhamphos, Canberra. I can be pretty
confident that blood vessels play a role here, as choughs can make their eyes
bulge and almost glow when they're threatening or being threatened.
Eurasian Coot Fulica atra and chicks, Canberra.

Metallic Starling Aplonis metallica, Cairns, north Queensland.
Pacific Koel Eudynamys orientalis, south coast NSW.

Spangled Drongo Dicrurus bracteatus, Canberra. This tropical and subtropical bird only
rarely comes as far south as Canberra, but this one hung around just around the
corner from us for a few days back in 2012.
...others have just a black head or even just a mask.
Black-fronted Dotterel Charadrius melanops, Cobar, western NSW.
Eastern Spinebill Acanthorhynchus tenuirostris, Blue Mountains, NSW.
Hooded Dotterel Charadrius cucullatus, Dhilba Guuranda–Innes NP, South Australia.
Chestnut Teals Anas castanea, south coast NSW. In this case only the males have the advantage
of the black contrast, though the females have the red eye too.
For many other species, like the female teals, the red eye alone is enough it seems, though in all these cases it is framed in a paler plain background.
Red-collared Lorikeet Trichoglossus rubritorquis, Darwin.
Olive-backed Oriole Oriolus sagittatus, Nowra, NSW.
Grey-cowled Wood Rail Aramides cajaneus, northern Peru.
These eyes really almost do glow in the dim rainforest understorey.
Fire-eyed Diucon Pyrope pyrope, Chilean Patagonia.
This tyrant flycatcher's eyes really are its outstanding feature.
Crested Duck Lophonetta specularioides, Chilean Patagonia.
And red grades to orange. This is probably often due to the influence of yellow carotenoids on some of the features that produce red, but remember the earlier comments on how much we don't yet know.

Crested Pigeon Ochyphaps lophota, Wagga Wagga NSW.
Dusky-legged Guan Penelope obscura, Peruibe, southern Brazil.
Hildebrandt's Starling Lamprotornis hildebrandti, Ngorogoro Crater, Tanzania.
Many birds have striking yellow eyes; in those few that have been studied contributing factors include pigments such as melanins, carotenoids, pteridines and purines, collagen fibres and oil droplets, in varying combinations. Take your pick regarding the following, all of which are in different Orders.
Australasian Shoveller Spatula rhynchotis, Canberra.
Pearl-spotted Owlet Glaucidium perlatum, Tangarire NP, Tanzania.
Grey Currawong Strepera versicolor, Gluepot Reserve, South Australia.
(This is the confusingly black mallee subspecies.)
Dolphin Gull Leucophaeus scoresbii, Ushaia, Argentinian Tierra del Fuego.
Collared Sparrowhawk Accipiter cirrocephalus (with House Sparrow lunch)
in our Canberra back yard.
Australasian Grebe Tachybaptus novaehollandiae, Canberra.
Before we get to white eyes, there are some I can only describe as 'pale', almost white but tinged with yellow or grey.
Yellow-billed Spoonbills Platalea flavipes, Canberra.
Osprey Pandion haliaetus, Hervey Bay, Queensland.
Bar-shouldered Dove Geopelia humeralis, Darwin.
Male Magpie-lark Grallina cyanoleuca, Canberra.
Which brings us to those birds, and there are quite a few, with striking white irises. These eyes may actually have various pigments, but in combination with colourless light-reflecting structures or chemicals these may produce white irises. And no, I'm afraid I don't really understand that either! Here are some, nonetheless, which I find particularly impressive.
Blue and Yellow Macaw Ara ararauna, Pantanal, south-west Brazil.
Grey-crowned Babbler Pomatostomus temporalis, Alice Springs, central Australia.
Hardhead (also known, much more helpfully, as White-eyed Duck) Aythya australis, Canberra.
New Holland Honeyeater Phylidonyris novaehollandiae, south-west Western Australia.
Pacific Gull Larus pacificus, Binalong Bay, Tasmania.
Radjah Shelduck Radjah radjah, Darwin. I'm not sure why this 'white on white'
works so well, at least for me, but I find it compelling.
Blue bird irises are not common, but when they are encountered they are most striking. As usual the mechanism has not been looked at very much, but it is assumed that the principle (as with most blue feathers) is the Tyndall Scattering Effect, where minute particles in a medium reflect light of a wavelength we see as blue. No pigments are involved, and indeed blue pigments are almost unknown in birds.
Chestnut-headed Oropendola Psarocolius wagleri, Costa Rica central ranges.
Galápagos Cormorant Nannopterum harrisi, Isla Fernandina.
This is the world's only flightless cormorant.
Little Black Cormorant Phalacrocorax sulcirostris, Canberra.
Spiny-cheeked Honeyeater Acanthagenys rufogularis, Cocoparra NP, NSW.
It is not such a big step from this to mauve, which is even rarer in birds' eyes.
Male Satin Bowerbird Ptilonorhynchus violaceus, National Botanic Gardens, Canberra.
Some birds also have, dare I say it, somewhat weird iris patterns; you'll need to click on these two pictures to enlarge to be able to see the effect..
I don't often feature this species, Australia's worst exotic bird pest, here, but Common Mynas
Acridotheres tristis, here in Sydney, have a most unusual iris comprising a ring of
white spots around the otherwise dark iris.
Golden-collared Toucanet Selenidera reinwardtii, Wild Sumaco Lodge, north-eastern Ecuador.
Toucanets in this genus have already striking green-yellow irises but with added large dark
patches on either side of the pupil.
In some species the irises change colour with age (as plumage commonly does) or are determined by sex.
Channel-billed Cuckoo Scythrops novaehollandiae, above and below, the world's largest cuckoo.
The adult above at Karumba, tropical Queensland, has startlingly red eyes (and eye ring).
This juvenile at Wollongong, south of Sydney, has brown irises which later turn red.
Likewise Silver Gulls Chroicocephalus novaehollandiae (the abundant and ubiquitous
Australian gull) has pure white irises as an adult - most of which have both feet by the way -
but they are dark in immature birds (below).

The sexes of Black-necked Storks Ephippiorhynchus asiaticus, here at Karumba
on the Gulf of Carpentaria in Queensland, differ in eye colour.
The female on the left has staring yellow eyes, while the male's are brown.

The story is the same with Galahs Eolophus roseicapilla, except that the female has
red eyes (here on the right) and the male's are again brown.

Earlier, while discussing red irises, I showed some examples of contrast with the surrounding feather colour, especially black plumage. Here are some more examples of eye colours accentuated by their surrounds, with black beings a 'popular' background. And I realise that I've feature a couple more red ones here - oh well, they're worth admiring!

Blue Dacnis Dacnis cayana, Peruibe, south coast Brazil.
Magpie Tanager Cissopis leverianus, Trilha dos Tucanos near Sao Paulo, Brazil.
Masked Flowerpiercer Diglossa cyanea, Yanacocha near Quito, Eucador.
Plush-crested Jay Cyanocorax chrysops, Pantanal, south-west Brazil.
(And apologies for the unfortunate background!)
Torresian Crow Corvus orru, central Australia.

And this one is in direct and, I find, pleasing contrast with the crow's white-on-black pattern.

White Terns Gygis alba, Lord Howe Island.
Here the black eye is showcased by the startlingly white plumage.
Finally - which may come as some relief to you - an entirely different kind of contrast which I'm pretty sure isn't just random. There are two species of oxpeckers, a genus of starlings, which make a living in sub-Saharan Africa picking ticks from the skin of large grazing mammals.The Yellow-billed Oxpecker Buphagus africanus is found right across the continent, while the Red-billed B. erythrorynchus is found only in the east. In each case the eye colour forms a strong yellow-red contrast with the bill, which I find fascinating.
Two oxpecker species on a giraffe in Serengeti NP in Tanzania.
The Red-billed has glaring yellow eyes, and the Yellow-billed (which actually has a red
tip to its bill) has bright red irises.
I had intended to finish with another way of drawing attention to eyes, that is with bare coloured skin surrounding them, but I think this is long enough already, and I'll leave that for another day. 

As for why all these different iris colours and patterns have evolved, I think that such evidence as there is (again thanks to Corbett's exhaustive review) points to the primary function as being in signalling to others of the same species, including offering information as to sex, fitness, ages etc. I'm content to leave it at that until there is further information available.

I hope you've been as interested in this topic as I've found myself being. If not at least I hope you've enjoyed gazing into a lot of very attractive eyes! Either way, thanks for reading this far.

NEXT POSTING THURSDAY 29 FEBRUARY
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