Seeing in the ultraviolet

Even in the visible spectrum, birds can discriminate more subtle color distinctions than we can, thanks to their at-least-five functional cone photoreceptor types (we only have three). But it’s in the ultraviolet (UV) part of the spectrum where they literally can see what we can’t.

Somewhat disappointingly, birds don’t generally have secret UV patterns the way that, for example, some flowers do (Andersson 1996). Instead, they seem to use UV to augment signals we can already see: bluebirds turn out to reflect UV, as do the spots on some thrushes, and so on. But the UV can still contain information invisible to our eyes. In the Alpine Swift and the European Starling, better-fed chicks reflect more UV from their skin; their parents can use this information to give more food to scrawny chicks in good times, or to cut their losses and favor the healthiest chicks in lean times (Bize et al. 2006).

Starling adult and fledglings – who may be too old and feathered to reflect much UV from their skin now, but are definitely still hungry.
Photo by Tina S. White.

Kestrels use UV to locate their prey. Voles make paths, “runways,” through the grass where they live, and mark those runways with urine and feces. Unfortunately for the voles, those marks reflect UV light, so their marked runways are basically big, UV signs saying HERE BE VOLES. Kestrels look for those UV marks and preferentially hunt near them, staking out the voles that marked the runways (Viitala et al. 1995). It might be time for the voles to consider a change…

Kestrel with a captured vole.
Photo by Pat Gaines

Seeing in the dark

King penguins dive deep to find prey, and in so doing, they encounter a problem: light attenuates very quickly in water. Foraging in the daytime at 200-300 m below the water’s surface is equivalent, light-level-wise, to foraging at the surface at night. King Penguins have large eyes (one full centimeter longer from the pupil to the back of the eye than our eyes) and can open their pupils quite wide, both of which help with vision in the dark, but they aren’t enough. To be able to see as well as possible at low light levels, the sensitivity of the retina—the thing at the back of the eye that detects the image after the light comes in through the pupil—has to be maximized; and for that to happen, it has to be “dark-adapted,” which takes about 30 minutes. (This is probably why your bedroom looks really dark when you go to sleep, but if you wake up in the middle of the night, it seems less dark: your retina has become more dark-adapted in the meantime.) But King Penguins usually only dive for 15 minutes. And even if they did dive for longer, it doesn’t seem like a great idea to be bumping into things in deep water for half an hour while you wait for your eyes to adapt…

The King Penguins solve this problem by having an extremely dynamic pupil: not only can it open wide, it can also close down very, very small. By changing size, our pupils can adjust the brightness of an image on the retina by about 16-fold; their pupils can adjust the brightness by 300-fold. When they are on the surface in bright daylight, they close their pupils down very small, so that the retina sees very low light levels and can become dark-adapted even in sunlight; then when they dive, they open the pupil wide, and the dark-adapted retina gives them maximal light sensitivity in the dark depths (Martin 2012).

King Penguin; notice the pinhole pupil.
Photo by Su Yin Khoo

Then, on the other end of the visual-innovation spectrum, there are the lazy cormorants. Cormorants hunt fish in waters that are often murky and full of sediment, with very low visibility. Rather than evolving highly specialized eyes to deal with this, cormorants simply hunt in the murk, disturbing fish by swimming nearby or probing with their bill, and then striking out with that spring-loaded S-shaped neck at the resulting “escaping blur.” They then bring their capture to the surface to examine it in the sunlight—their visual field gives them a good view of anything held in their bill—before eating it (Martin 2012).

This blur looks edible.
Photo by thriol.

We tend to think of natural selection as resulting in specialized, refined competencies: the cheetah’s run, the Peregrine Falcon’s stoop, the mantis shrimp’s color vision. The cormorants’ hunting strategy is a reminder that sometimes, “not very good” is good enough.

Seeing with sound

Even by owl standards, the Tawny Owl’s world is dark. Tawny Owls hunt in the black hours between dusk and dawn, underneath closed woodland canopies that block out most moon- and starlight. Their sensitivity to light is about 2.5 times greater than ours, but even that isn’t enough to allow them to see the details of the forest floor on most nights. Instead, to catch their tiny, rustling prey, they use their ears. Owls have large external ear structures, “discs” of feathers around their ear openings, which allow them to not only hear very soft sounds but to localize them: i.e., to pinpoint exactly where the sound came from. (We do this constantly, and you don’t notice until it stops working. For a brief time I lost most of the hearing in one ear, which threw my sound localization out of whack, and every sound seemed to be coming from a direction that made no sense: I would lie in bed at night listening to a gecko that I knew was in a cage to my right, apparently crashing around above and behind me.)

The Tawny Owls can use their sound localization to strike at and catch prey below them even when they can’t see them. However, while they can figure out what direction a sound is coming from, it’s harder to figure out how close it is, and it’s hard to judge a pounce if you don’t know how far down the ground is. So Tawny Owls stay on the same territory all their lives, getting to know exactly how high is each perch, so that when they hear the rustle of dinner, they can pounce (Martin 2012).

Tawny Owl. Photo by Sergey Yeliseev

Oilbirds, who fly, mate, nest, and raise chicks inside pitch-dark caves, take this even further. They imitate that other flying, pitch-dark-cave-dwelling animal, the bat: they echolocate.

Oilbird inside a cave. You’re welcome.

Here’s a real picture.
Photo by Quinten Questal

Oilbirds don’t echolocate as well as bats do. They use a relatively low-frequency sound—low enough that we can hear their clicks—which means that they get fairly poor resolution with their echolocation “images”: the larger the wavelength of the sound you use, the courser the resolution. When Konishi & Knudsen (1979) suspended plastic discs of varying diameters to see how small a disc had to be to be invisible to the Oilbird’s clicks, the Oilbirds successfully avoided all discs 40 cm in diameter, avoided some of the 20 cm discs, and crashed into the 10 and 5 cm discs as if there was nothing there.

Things smaller than 20 cm aren’t important enough to avoid anyway.
Photo by Mike’s Birds

Like the cormorant’s vision in the murky water, the Oilbird’s echolocation isn’t great—but it’s good enough. Being able to see down to about 20 cm resolution will keep you from crashing into cave walls and at least the thicker stalactites. They won’t be able to find their mates and their chicks with echolocation, but they can use calls for that.

And really, who am I to criticize their echolocation? I’d crash into an elephant in pitch darkness, not to mention a tiny disc. Good job, Oilbirds!

That’s right, show some respect! And turn off the bright sun please…
Photo by barloventomagico

References

Andersson S. 1996. Bright ultraviolet colouration in the Asian Whistling-thrushes (Myiophonous spp.) Proceedings: Biological Sciences 263:843-848.

Bize P et al. 2006. A UV signal of offspring condition mediates context-dependent parental favouritism. Proceedings of the Royal Society B 273: 2063-2068.

Konishi M, Knudsen E. 1979. The oilbird: hearing and echolocation. Science 204:425-427.

Martin G. 2012. Through birds’ eyes: insights into avian sensory ecology. Journal of Ornithology 153(S1):23-48.

Viitala J et al. 1995. Attraction of kestrels to vole scent marks visible in ultraviolet light. Nature 373:425-427.