Eye in the Sky

Char­lie Port­lock gives us the facts on why ul­tra­vi­o­let light mat­ters to we hunters

Air Gunner - - Con­tents -

Do you know a su­per­hu­man? If you’ve ever met some­body who’s had rad­i­cal eye surgery then it’s pos­si­ble that you do. Re­cent med­i­cal de­vel­op­ments have meant that pa­tients with se­ri­ous cataracts or eye can­cer can have their corneas (the lens in the eye) re­placed with ar­ti­fi­cial ones. With around 3% of peo­ple this can lead to an abil­ity to see colours in the ul­tra­vi­o­let spec­trum, a fre­quency of light nor­mally un­de­tectable to hu­mans, but in­te­gral to the lives of bees, rap­tors and yes, pi­geons.

Hu­mans can see some of the ul­tra­vi­o­let spec­trum (UV) un­der a black light bulb – the kind you’ve prob­a­bly seen in clubs or light shows, that shows up dust and the white of your cloth­ing. How­ever, we don’t see the full colour spec­trum of ul­tra­vi­o­let, only the white and shad­ows. There’s ac­tu­ally an­other di­men­sion of vis­ual per­cep­tion be­yond our own. Try to imag­ine that our world is not ac­tu­ally the colour­ful place that we think it is; com­pared to pi­geons and rap­tors, what we see is the monochrome of an old western on an ana­logue TV, rather than the full-colour, high­def­i­ni­tion im­age of the mod­ern dig­i­tal dis­play. Com­pared to pi­geons, we’re al­most colour­blind.

Hu­mans have three kinds of op­ti­cal cone cells in their eyes (red, green and blue) and we can see around 7 mil­lion colours. Pi­geons are tetra­chro­mats and have four of these cone cells – one of them be­ing sen­si­tive to

UV – as well as a kind of coloured oil in each that hu­mans lack. This set-up en­ables them to see colours in much higher def­i­ni­tion, along with the en­tire UV spec­trum of light. There’s lit­er­ally an­other world, an­other fron­tier of colour, that’s invisible to us.

SU­PER­BIRDS

Pi­geons also have al­most twice our abil­ity to de­tect and process move­ment. They are tasty birds, so they need to be able to avoid ae­rial preda­tors like pere­grine fal­cons – the fastest bird on earth – as well as cor­rectly plot their land­ing within dense fo­liage whilst fly­ing at high speeds. If you’ve ever rid­den a bicycle on an ag­gres­sive down­hill, you’ll know how fast 25mph can seem when you’re in a small ve­hi­cle. If a rel­a­tively small pi­geon ex­pe­ri­enced ve­loc­ity in the same way that we do, it wouldn’t be able to process vis­ual in­for­ma­tion at the nec­es­sary speed to en­sure a safe land­ing.

At 60 cy­cles per sec­ond we can­not de­tect a flash­ing light like a strobe or a pro­jec­tor, see­ing it as a con­tin­u­ous beam. This com­pares to 90-110 cy­cles per sec­ond for pi­geons who would see a mod­ern dig­i­tal film as a se­ries of slides. With this kind of acu­ity we can ex­pect pi­geons to be about twice as sen­si­tive to move­ment as we are. They, like many birds, may also ex­pe­ri­ence the world in a kind of slow mo­tion, un­less in flight, in which case things would prob­a­bly ap­pear ‘nor­mal’. If you’d evolved to be in your el­e­ment on the wing at 70mph, then feed­ing on the ground or roost­ing would seem rather se­date by com­par­i­son, mak­ing any un­to­ward move­ment from ap­proach­ing preda­tors – in­clud­ing air­gun­ners – that much more ob­vi­ous.

Many birds have feath­ers that use ‘struc­tural’ colours – the ones we can see eas­ily – for dis­play pur­poses, but since the 1980s re­searchers have been aware that the feath­ers of many birds re­flect UV light, of­ten mak­ing their plumage ap­pear com­pletely dif­fer­ent and sig­nif­i­cantly more com­plex to other birds. To be­gin to ap­pre­ci­ate pat­terns of avian plumage, sci­en­tists use a spec­tropho­tome­ter which ap­prox­i­mates how birds might see. They’ve have made some in­ter­est­ing dis­cov­er­ies; for ex­am­ple, the hum­ble blue tit is dif­fi­cult to sex with the hu­man eye, but the birds them­selves have no trou­ble be­cause the crown of the male specif­i­cally re­flects UV light. This kind of new knowl­edge has many im­pli­ca­tions for those study­ing bird be­hav­iour, in­clud­ing hunters.

WHITER THAN WHITE?

Op­ti­cal bright­en­ers are used in all com­mer­cial de­ter­gents, and some soaps and sham­poos, and are used to cre­ate the daz­zling white ef­fects that we’re so used to see­ing on ad­verts. Van­der­berg Air Force base in the US in­structs their men not to use laun­dry with op­ti­cal bright­en­ers be­cause it makes them more vis­i­ble at night un­der ar­ti­fi­cial light, as well as through in­frared op­tics. It’s stan­dard prac­tice in the US Forces to avoid de­ter­gents con­tain­ing these in­gre­di­ents for rea­sons of safety against snipers, but you prob­a­bly don’t want to wash your hunt­ing clothes with this stuff – best stick to wa­ter and el­bow grease.

There’s an­other fac­tor. Many mod­ern cloth­ing dyes re­flect UV light, so your photo-print dig­i­tal camo might not be as ef­fi­cient in dis­guis­ing your pres­ence as you think. If you’re won­der­ing if your cloth­ing is as stealthy as ad­ver­tised, then you can test it by buy­ing a small black light and tak­ing a good look in the dark. If your cloth­ing shines, then you can be sure that a pi­geon, or any other day-fly­ing bird, will be able to see it, too. Sports wash or tech­ni­cal de­ter­gents along with UV ‘ killing’ sprays can coun­ter­act this ef­fect, but these so­lu­tions are hardly ideal.

Cloth­ing man­u­fac­tur­ers might not like this in­for­ma­tion, but for pi­geon shoot­ers lurk­ing in shadow or us­ing ar­ti­fi­cial hides, the role of UV in con­ceal­ment should not be un­der­es­ti­mated. I’m not sug­gest­ing that we’ll al­ways stand out like a bea­con when wear­ing these fab­rics, but in cer­tain light­ing con­di­tions, when us­ing cer­tain equip­ment, we might be sig­nif­i­cantly more ob­vi­ous than we re­alise. In short, if we’re shoot­ing pi­geons or any other bird species, then per­haps we should be cam­ou­flag­ing in the fourth di­men­sion.

BIRD FACTS

If you’ve ever won­dered why that buz­zard sits idly in his tree, it’s worth re­mem­ber­ing that an­i­mal urine and fae­ces are also re­flec­tive to UV and that birds of prey can ac­tu­ally plot the ter­ri­to­rial mark­ings of small mam­mals, like voles, and fol­low them right back to their nests as if on a run­way. Rap­tors can see both the dis­tant and the zoomed im­age si­mul­ta­ne­ously, and if they al­ready know where to look from the urine trail, it’s just a ques­tion of wait­ing for their prey to turn up. Nice!

Un­like rap­tors who have the binoc­u­lar sight typ­i­cal of preda­tors, pi­geons have largely monoc­u­lar vi­sion. Each eye sees to the side of the body in a wide arc and these arcs of sight are like two semi-cir­cles that hardly cross. As you’re read­ing this page, the im­age is du­pli­cated in both eyes. Not so for pi­geons; they have rel­a­tiv­ity poor depth per­cep­tion for which they com­pen­sate by bob­bing up and down when mov­ing be­cause this helps them to ori­en­tate with their sur­round­ings.

Berries, nuts, grains and seeds also re­flect light in the UV spec­trum, un­like most green leaves, which makes them easer for pi­geons to spot.

In 2008, the pi­geon passed the ‘mir­ror test’ – be­ing able to recog­nise its re­flec­tion in a mir­ror; it is one of only six species, and the only non-mam­mal, that has this abil­ity. A hu­man child is typ­i­cally around three years old be­fore it has this level of cog­ni­tive func­tion.

THE WIDER VIEW

Mov­ing for­ward, there’s a lot more work to be done around how an­i­mals ex­pe­ri­ence the world on both a phys­i­cal and cog­ni­tive level. As hunters, it’s con­ve­nient for some of us to underestimate our quarry, to see the crea­tures we hunt as ver­min or ‘ lower an­i­mals’. This is surely a mis­take left over from some ar­chaic biblical idea that we’ve been put on this earth to lord it over the ‘ lesser beasts’. The truth is that mod­ern sci­en­tific re­search is show­ing us just how much the an­i­mal and plant worlds have to teach us, and there’s a lot to learn. When we start to in­ves­ti­gate the phys­i­ol­ogy and be­hav­iour of even a com­mon crea­ture like the wood­pi­geon, it’s sur­pris­ing what in­cred­i­ble things it has to teach us.

An abil­ity to step out of our own sen­sory world and into that of our quarry en­ables us to be­come bet­ter hunters, but even more im­por­tantly, it’s a re­minder, that we’re not the most highly evolved crea­tures on the planet. All an­i­mals are per­fectly adapted to live suc­cess­fully within their re­spec­tive habi­tats – apart from hu­mans, it seems – and through a bet­ter un­der­stand­ing of these adap­ta­tions we are able not only to put more food on the ta­ble, but also to ap­proach the nat­u­ral world with a lit­tle more hu­mil­ity. We’re not bet­ter, we’re just dif­fer­ent. Best of luck in the field, Char­lie.

Beards don’t re­flect UV, thank good­ness

We all know not to wear white, but does our camo glow to pi­geons?

Is this what a pi­geon sees?

This is what camo looks like to our eyes

“Un­like rap­tors who have the binoc­u­lar sight typ­i­cal of preda­tors, pi­geons have largely monoc­u­lar vi­sion”

I was lucky with these two

The USAF says some de­ter­gents can negate cam­ou­flage

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