Owls; the ultimate night riders

I've been meaning to do a post on owls for some time now, but always felt that I didn't have enough decent photos to make it worth while. At this stage I'm probably not going to be able to improve on that any time soon, so rather than go on ignoring these wonderful birds I'll go with what I've got, rely on the owls to carry it, and hope you'll be tolerant!

Australian Boobook Ninox boobook, Nowra, NSW south coast. This roosting bird shows
the basic owl characteristics including very large eyes which can function at incredibly
low light levels, powerful bill, legs and claws, and feathered legs. All owls are carnivorous. 
The body and wing feathers are very soft and fringed to allow almost totally silent flight.
The fourth toe is very flexible, and can either face back (for a two forward/two back setup),
or forward (for three-forward/one back). You'll see both options in the photos which follow.
(There has been a lot of good work done recently on the complex of boobook species which
are found from eastern Asia, through the numerous islands north of Australia to
Australia itself and New Zealand.
Until recently it was called Southern Boobook N. novaeseelandiae, but the English name was
changed when previously included populations from the Lesser Sundas (including Timor),
Tasmania and New Zealand were all recognised as different species.)

One of the key things about owls is that they are, of course, mostly nocturnal (there are a few diurnal ones, but we can generalise to start with). This means that you either have to photograph them at night, while not hurting or scaring them with flashes, or find them in the daytime, when they're almost certainly going to be tucked away in foliage, away from the daytime birds which will harry them mercilessly if they can. Other people manage perfectly well of course, part of which might be due to better equipment, but mostly just better skills! A spinoff of this is that, while I can usually offer you photos of aspects of bird behaviour, pretty well all my owl photos show them roosting or staring down from a branch, and in a photo those two things look pretty similar I must admit!

The owls are a very successful group of hunting birds, with some 250 species found throughout the vegetated continents and they have occupied every habitat from the Arctic to the tropical rainforests. They are also an old group, with the oldest known fossil owl dating back 60 million years from a site in Wyoming. Owls comprise their own Order, with no obvious close relatives, though perhaps the daytime birds of prey (hawks etc) might be the closest. Within the Order there are two Families, the barn owls and the 'typical' or hawk owls, but the latter group totally dominates, with some 230 species to the barn owls' combined 20 or so. In Australia however the 10 breeding species are split evenly between the two Families.

Eastern Barn Owl Tyto javanica, Alice Springs. This is a 'rescue' owl at the excellent
Alice Springs Desert Park, free-flying but unable to be released. While it has the general
owl characteristics described above (here two toes forward and two back)
there are some obvious differences. The iris is dark, while in the majority of typical owls it is
pale, often yellow (the observation holds for all Australian species).
There is an obvious heart-shaped facial disc, fringed with stiff feathers, 
which acts as an external 'ear' to direct sound to the ear openings, hidden by the feathers.
(Other owls also have such a disc, but in most cases it is not as well developed as the barn owls' are.)
Not so obvious here is its lighter build and longer legs, or the difference in calls.
While typical owls in general have a call which is a variation on hooting,
barn owls have a rasping shriek or hiss.
(As with the boobook group discussed earlier, there have been major recent advances in
understanding barn owls relationships too. Until recently there was just one
Barn Owl Tyto alba recognised throughout the world, but now there are three.
'Ours' is found from Australia to southern Asia, while another is in Europe and
much of Africa, and the third in the Americas.)

Another very important owl characteristic isn't really shown in these photos either, and can only be properly seen from front-on.

American Barn Owl Tyto furcata Santa Cruz, Galápagos. This bird was roosting
in a rock crevice, trees being scarce commodities in most of the Galápagos.
Unlike nearly all other birds, its eyes are in the front of its face, enabling it
to have binocular vision; in most birds the eyes are on the sides of its
head, so the only way it can have the sort of 3D vision than an owl (and we)
regard as normal, is to turn the head from side to side.
This Sulphur-crested Cockatoo exemplifies the contrast.

I have sometimes wondered if owls are often described as 'wise' because we share this facial characteristic with them; ie they look a bit like us so must be wise! I wouldn't put any arrogance past us...

A couple more observations on barn owls and their differences from the 'main' owl Family, before a bit more on what makes owls so special, then going on to focus on the hooting majority. I'm very confident that the pale undersides of the Western Barn Owl (Tyto alba), its hair-raising screeches, silent flight and habit of roosting in church towers in Europe are the basis of many ghost stories. Members of the barn owl Family usually roost by day in hollows (presumably buildings were used by the Western Barn Owl when hollows were no longer available), whereas other owls make themselves as inconspicuous as possible within or behind foliage. Barn owls often don't defend a territory at all, and when they do it is usually by silent display. 'Typical' owls do so fiercely, and proclaim ownership with long hooting sessions.

Owls in general have other adaptations too that are of interest. For instance an owl can spread the bones of the lower mandible outwards, to extend the gape and enable large prey items to be swallowed whole. Like most carnivorous birds they regurgitate pellets of indigestible bones, feathers, fur and scales (which are invaluable in determining their diet).

Pellet from a Powerful Owl Ninox strenua which became a celebrity when it lived
in the Australian National Botanic Gardens in Canberra for a few weeks in autumn 2007.
It left when it had exhausted the supply of Ringtail Possums and Sugar Gliders;
this pellet is clearly composed of fur and bones.

Owls have never evolved echolocation to help them hunt at night, so have remarkable vision, with very large eyes relative to their skull size. This leaves little room for muscles to move the eyes, so they have developed extremely flexible necks, with twice as many neck vertebrae as we do, and special adaptations to prevent blood flow from being constricted when they swivel their head through about 270 degrees. In addition to the overall size of the eyes, they have a very large maximum pupil size, to enhance light gathering when there isn't much of it. They also have a preponderance of rod cells over cone cells in the retina; rod cells are much more light sensitive, while cone cells are good at colour vision. Hence an owl can see well at low light levels, at the expense of distinguishing colour.

The very large eyes of this Tropical Screech Owl Megascops choliba are evident.
We were actually sitting patiently waiting for an Ocelot (that never appeared for the
two nights we were there) at a bait station in the Pantanal of south-western Brazil,
but this was a nice compensation. (The lighting allegedly doesn't bother the Ocelot,
but we saw no evidence of that! Luckily it didn't seem to faze the owl either.)
The screech owls form a group of some 23 species found throughout much of the
Americas; this species is found widely in South America.

Some of the most impressive owl adaptations however are in their hearing, and many cases they are known to swoop and catch prey that they can't see at all. In an experimental situation Western Barn Owls could catch mice in a totally darkened room by homing in on the rustle of their feet on the floor. Even in light, other owls can grab unseen mice from grass clumps or under snow. They hear roughly the same range of frequencies that we do, but hear ten times as well in the frequencies that correspond to rustling noises in grass or leaves. Their ear openings, like their eyes, are unusually large and are covered loosely by special feathers which don't interfere much with sound. The facial disc, which we mentioned earlier as being especially well-developed in barn owls, is another important factor in gathering and focusing sound. 

This Short-eared Owl Asio flammeus is not a barn owl, but has a facial disc which is
equally well developed. This beauty was perched by the roadside near Lake Titicaca in the
high, cold mountains in the far south-east of Peru late one afternoon. That would have been
enough for us, but our very experienced local guides had never seen one in this part of the world.
The nearest it comes to here normally is on the north coast of Peru, and that rarely.
This medium-large owl hunts mostly by night, but also is regularly active in the daytime
and evening, concentrating on small mammals. It is found right across the Northern
Hemisphere and the southern half of South America, making it not only the most
widely-distributed owl in the world, but is arguably the most widespread of any land bird

 

As an another example of that last comment, here is a Short-eared Owl of the Gálapagos
sub-species, roosting in a chasm on Genovesa, in the Pacific 1000km from South America.
Note that because this one isn't gripping a perch, it's spread its toes widely.

But back to owls' superpowers. One way owls use their hypersensitive hearing is to turn the head until the sound reaching each ear is at exactly the same volume - which of course occurs when the object/prey is exactly in front of the owl. But there's a second, even more amazing, mechanism in play too. If the source of a sound is off to the right, it not only sounds marginally louder on that side but arrives at the right ear fractionally sooner than it gets to the left. By turning the head again until the sound reaches both ears simultaneously, again the owl knows that the prey is now right in front. But, even without moving its head (which could alert the prey) it can locate the prey precisely within a 60 degree arc. And to achieve all this, the owl can distinguish a time difference between one ear and the other of just 30 millionths of a second! My brain threatens to shut down when I try to process that information.

In Australia all owl species nest in tree hollows, except the generally uncommon Eastern Grass Owl Tyto longimembris (which nests on the  ground). Elsewhere however there are owls which breed on cliffs or in old bird nests, and hardly any of them bother even to line the nest site. 

 

One of my favourite owls, the little Burrowing Owl Athene cunicularia, actually nests underground, mostly in burrows provided by rodents or badgers - or at least that's what most of the information says. On looking into it however I've realised that this mostly refers to North America and ignores the fact that probably more Burrowing Owls live in South America. (North Amercan bias, surely not?😀) On going through my South American field guides, I discover that in fact most South American Burrowing Owls dig their own, which makes sense as burrowing mammals such as ground squirrels and badgers are absent there; skunk burrows might be the best bet for South America owls, but they're not abundant.

Young Burrowing Owls at the mouth of their burrow, Sipan archaeological site,
north-western Peru.

Adult Burrowing Owl in the same arid area. One of my favourite Burrowing Owl
stories is their habit of collecting dried mammal dung and leaving it near the
burrow to attract edible dung beetles! Tool use, surely?

As you'll have already noticed, most owl plumage is subtly (and beautifully) mottled and camouflaged, presumably to reduce the degree of daytime harassment by mobbing birds. Chicks however begin life with very fine down feathers for the first week or so, which is replaced by a thicker down called a mesoptile through which full feathers eventually grow.

Crested Owls Lophostrix cristata, Tortuguero, east coast of Costa Rica. The chick is still
wearing its mesoptile down. This handsome owl can be 40cm tall (female owls are bigger
than the males) and is found in tropical rainforests in South and Central America. Quite a few
owls have these 'ears', which have nothing to do with hearing, and everything to do with display.
This distinctive owl is the only member of its genus.

Here is a slightly better portrait of a pair of this lovely owl at
Sacha Lodge in the Ecuadorian Amazon

I thinks that's about enough of facts and figures. For the rest I'm just going to introduce a small range of owls from four continents, and see what stories might arise. I'll start with a few Australian species, noting that my photo coverage of them is woefully inadequate.

Australian Boobook at a cave entrance, Undara Lava Tubes NP, north Queensland.
Owls lurk around the tubes waiting for bats to emerge at dusk. This subspecies,
which is found across the western two thirds of Australia, is paler and redder
than the south-eastern subspecies in the first photo of this post. I can't
explain which this one's eyes look dark rather than the expected yellow.

Here is the celebrity Powerful Owl Ninox strenua in the National Botanic Gardens in
Canberra in 2007 that I mentioned earlier. The largest of the boobook group of owls
(and the largest Australian owl) it stands an imposing 60cm high, and is found in tall
forests along the south-east coast and hinterlands of Australia. However it is reasonably
comfortable in suburbia too and in Sydney is known to prey on big fruit bats which form
colonies there, and regularly on stray cats. Otherwise its regularly takes prey as big as
Ring-tailed Possums and Greater Gliders, Pied Currawongs, cockatoos and even lyrebirds.
I first heard its deep slow resonant 'whoo whoo' decades ago bouncing off the cliffs at night
in Gariwerd (Grampians) National Park in western Victoria, and have never forgotten it.

Rufous Owl Ninox rufa, Atherton Tableland, north Queensland. Another big owl, not
much smaller than the Powerful Owl. This one is solely tropical, found scattered across
northern Australia and New Guinea. It has a similar diet to the Powerful. A well-known
pair roosts in the Darwin Botanic Gardens, but I've never had the pleasure.

Barking Owl Ninox connivens near Townsville, north Queensland. I really did hesitate
before posting this awful photo (a scan of an old slide) but in this case I decided that it
was (just) better than nothing. Bigger than a Boobook, this is a woodland owl
found across virtually the whole continent, especially in woodlands and open
forests. Its call really be mistaken for a barking dog, especially when a pair
is duetting.

Tawny-bellied Screech Owl Megascops watsonii, Amazonia Lodge,
near Cusco in southern Peru. Like the Tropical Screech Owl we met earlier
(and all others in the genus) this one has prominent ear tufts. Note how it's
just opened its left eye a slit, to keep track of me.

A familiar group of owls to anyone who's birded in South and Central America is the pygmy owl genus Glaucidium. They are mostly active in the daytime and guides regularly use their call to attract other small birds which mob them. However while the Americas are their stronghold, there are also members of the genus in North America, Asia, Europe and Africa. Here are four species of pygmy owl, three from the Americas but also an African one.

Austral Pygmy Owl G. nana, from the icy Patagonian southern cone of South America,
here alongside Perito Merino Glacier in Argentina. The largest female weighs barely
100g, but they have been recorded taking birds as large as 160g, though insects
form a large part of the diet.

Ferruginous Pygmy Owl G. brasilianum, Chapada dos Guimarães, south-western Brazil.
(Despite its scientific name, it is found widely in South and Central America).
They mostly feed on insects but like the other pygmy owls they are feisty hunters
who are undeterred by the size of a potential lunch.

The same bird as above, with the back of its head facing us. These 'false eyes' are
characteristic of several pygmy owls, presumably to make the mobs of small birds
think they're always under observation, though it doesn't seem to deter them.
Below is closer view of a pair of these pretty little owls, this time from the Pantanal
in south-western Brazil.

Pacific Pygmy Owl N. peruanum, Olmos area, central northern Peru.
It is found along the dry Pacific coast from Ecuador to the northern tip of Chile, and is
the only pygmy owl found in the lowlands west of the Andes. Until fairly recently this pygmy
owl was lumped in with the Ferruginous Pygmy Owl, but their calls are very different.

Pearl-spotted Owlet G. perlatum, Tangarire NP, Tanzania. This is one of four
African pygmy owls, except that they're called owlets there. It's a fairly familiar
little owl, found very widely across sub-Saharan Africa; even I've seen it in four
different countries. Like its neotropical relatives it is often active by day.

The scops owls form a big genus (some 60 species, the largest owl genus) of Old World owls, mostly Asian though a few are found in Africa and Europe as well. Until fairly recently the American screech owls were also included, though that is now agreed to have been erroneous, especially with regard to their calls. Like screech owls they are medium small owls with prominent ear tufts. They are mostly quite nocturnal.

African Scops Owl Otus senegalensis, Tarangire NP, Tanzania. Like many
owls, scops owls have lovely camouflage. This one mostly eats invertebrates.
(Sorry about the stick!)

Madagascan (or Rainforest) Scops Owl Otus rutilus, Ankarana NP, Madagascar.
It is widespread on the island, but not a lot seems to be known about it.

This lurking behaviour is typical of owls which are totally nocturnal (or nearly so). As I've mentioned before they are fiercely attacked by large numbers of daytime birds of many species, which respond to the calls of others to join in the mob.

Buffy Fish Owl Ketupa ketupu, Kinabatangan River, Sabah, Malaysian Borneo.
Both scientific names are based on a Javanese name for this species. It is one
of half a dozen large owl species in Asia and Africa which are primarily fish-eaters.
This one may swoop from a perch to take fish from the surface,
or stand by the water, or even wade into it.

Until recently this was included in the genus Bubo, but it turns out that this was also a false grouping. Bubo still exists, as a genus of 10 species of horned owls (in the Americas) and eagle-owls (in Eurasia and Africa). These are seriously big owls, some of them considerably larger than our Powerful Owl! The Eurasian Eagle-Owl Bubo bubo can stand 75cm tall and weigh up to 4.5kg.

McKinder's Eagle-Owl, a subspecies of Cape Eagle-Owl Bubo capensis, central Kenya.
(Be kind, it took me some time to get an angle that showed even part of its face!) It was hidden
in a fig against a cliff face; the species is strongly associated with rocky areas across southern
Africa and in scattered populations in east Africa.

Our guide in this instance was an impressive young local man who's dedicated his life to studying owls - he'd been following this individual for seven years - and working with local communities, educating and advocating against the traditional persecution of owls, a serious problem all over the world.

Great Horned Owl Bubo virginianus, El Cajas NP, 4000 metres above sea level in
southern Ecuador. This huge owl is found throughout North America and much of
northern and eastern South America. We found this one by chance, sitting quietly
as one of our party poked about in a thicket. (Thanks Steve!)

Stygian Owl Asio stygius, Morro del Calzada Reserve, northern Peru.
A pair of these very imposing dark, brooding owls was roosting in the open
in this little forested montane reserve. They are scattered in central and South America
and the Caribbean, with a large range centred on southern Brazil. Despite being
large and widespread it seems that not a lot is known about them.
It is in the same genus as the Short-eared Owl that we met previously.

Spectacled Owl Pulsatrix perspicillata, Pantanal, south-western Brazil.
This large solitary owl is found in lowlands in Central America and throughout the
north-eastern half of South America. It has a wide range of habitats;
this one was in open woodland and grassland, and the only other one
that I've seen was in the Amazon rainforest in Ecuador.
The genus is a small solely Neotropical one.

The final genus in this little owl discovery tour is one familiar to Europeans and North Americans - think of the Tawny Owl and Barred and Great Grey Owls respectively. However, as is often the case, my offerings are South American, and there's a twist...

Mottled Owl Strix virgata, in a hotel garden in San José, Costa Rica.
This pair were tucked away in a massive stand of bamboos that must have
been over ten metres high. Another one that is found widely in Central and
northern South America, often around humans. It's mostly a perch-and-pounce hunter.

Black and White Owl Strix nigrolineata, Caño Negro, northern Costa Rica.
Another medium-sized owl from Central America and the far north-west of
South America. I find this one particularly striking.

And that brings us to a bird that belongs to a short list of birds that I most treasure having seen.

This owl was at San Isidro lodge on the eastern side of the northern Ecuadorian Andes,
at 2000 metres above sea level, and is only known from the vicinity of San Isidro.
It looks very like the Black and White Owl, and also like the closely related
Black-banded Owl (Strix huhula - how I love that name!) from the lowland rainforests
to the east. However the Black and White is only known from the western slopes of the Andes, and
the Black-banded is rarely found higher than 500 metres, and never above 1500.

In plumage and, reportedly, in voice, it seems midway between both those species. Indeed the general approach to it is to tentatively regard it as an isolated sub-species of Black-banded, though it seems to me there is little basis for that. It is widely known in birding circles as the 'mystery owl'. Obviously we need DNA (without the traditional method of obtaining it, of course!). Perhaps feathers from under a roost or eggs that have hatched would do? Of course that's easier said than done, though I'm surprised that no-one has succeeded yet - there are some brilliant bird guides in that part of the world. My bet would be on it being a new species, but it seems we must be patient for a while yet! Meanwhile I love the story and am very fortunate to have seen the bird.

Well it's been a long post, and one that I've been meaning to do for some years. If you've read this far, my thanks for doing so, and I hope it means that you're as fascinated by owls as I am. I hope too that I've been able to add something small to your knowledge and enjoyment of them.

NEXT POSTING THURSDAY 19 SEPTEMBER

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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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