The once plentiful passenger pigeon

A stuffed passenger pigeon

My family kept a few National Geographic World Explorer books for kids in the house while I was growing up. The nature ones were my favourite. I poured over them regularly, fascinated by illustrations comparing the eye sight of eagles to humans and the relative speed animals move. On one page, there was an illustration of a covered-wagon-era family on the prairies looking up at a sky darkened by a flock of passenger pigeons passing by. Even then, it amazed me that a bird once so plentiful is now gone like the dodo and thylacine (Tasmanian tiger).

Once passenger pigeons might have been the most numerous birds on the planet, numbering in the billions. Their spiral to extinction was shockingly fast. The passenger pigeon's crime was eating grains and other crops, so people went out and systematically killed them, some were eaten, some fed to pigs and most left to rot. At the start of the 1880's these pigeons were still nesting in the millions. Twenty years later, the last wild passenger pigeon was shot March 24, 1900 in Ohio. Only a few were left in captivity.

The last passenger pigeon died 100 years ago this year. The pigeon, Martha, lived her life in captivity at the Cincinnati Zoo. When she died, Martha was sent to the Smithsonian, stuffed and put on display. There are many passenger pigeons in museum collections, which means genetic material from these birds is available. In the near future, genetically re-engineering recently extinct animals like the passenger pigeon could be possible, but should we?

My husband, who is a curator at the local museum, offered to show me one of the three passenger pigeons held in the museum's collection. Unlike the flocks containing millions of birds once found in the more eastern areas of North America, passenger pigeons were only rare visitors to BC. For anyone who is curious, the Project Passenger Pigeon site contains a lot of information about passenger pigeons including ranges by province and state.

The photo is of the best preserved specimen at the museum held up by my husband as I wouldn't dare hold such an irreplaceable specimen. The iridescent rust body reminds me of the colour of the robin's I see in my backyard, but the body shape is pure pigeon. I kept domestic pigeons around the same time I was pouring over the World Explorer books. I loved the sound my birds made. I wonder if passenger pigeons had the same soft coo?

As a tangent: here is someone collecting the old World Explorer books for the apocalypse.

Turdus migratorius (backyard Robins)

This is not a Robin

How the ordinary can be missed...

Winter is over and I’m delighted to report it’s warm enough to sleep with my window open! My island in the Pacific is temperate, not tropical, so spring warmth is always welcome. With the window open, the first and last thing I hear each day is singing birds.

This time of year, Robins are the ones singing. Around here we don’t have the dainty European ones (they were introduced about in the early 1900s, but didn’t take like the starlings and house sparrows did). Instead, we have a member of the thrush family, Turdus migratorius or American Robin. I’ve read that robins go through puberty once a year (article here) as hours of daylight increase. By this time of year they’re looking for a mate and the search is a noisy affair.

While I was out walking around a local bog, Red-wing Blackbirds were conducting a similar noisy quest as the robins. I heard the blackbirds long before seeing them, a feat my walking companion took to mean I’m an expert on birds. I’m not. Red-wing Blackbirds and Robins are the a few bird calls I recognize (Bald Eagles are another - majestic bird, ridiculous sound). We couldn’t see the birds at first, so we stopped and gazed out into the bog. Eventually, between the dried cattails, we saw flashes of fire-engine* red as the males jockeyed to catch the eye of the females.

I do purposely go out to see birds, but unintentionally I do it poorly. I have a bird identification book and binoculars, which I almost always forget to take with me. I have a check list for local birds that I’m filling out, yet I can’t tell the difference between different species of gulls. I’m a member of the local natural history society and they regularly do outings to watch birds, yet I’ve never gone with them.

There is always at least one Robin in my backyard, so I don’t have to go out of my way to see them. Unlike the Resplendent Quetzal, a bird I traipsed through the jungle in Costa Rica to see, the ubiquitous Robin is easy to ignore. They lack the iridescent green body and brilliant red breast of the quetzal. Even the flashy red wings of blackbirds eclipse a robin’s colouring. A Robin’s breast is the same shade of the liquid that seeps out of a bucket of nails left in the rain. Additionally, most Robin’s wings droop just a bit, giving them a goofy look which is augmented by the bird’s tendency to endlessly pursue their reflection in a window.

Soon, the Robins will sort out who to mate with and the songs will fade. Fragments of delicate blue egg shells will be discarded as the next generation of ordinary Robins are hatched. From my desk, I can watch a Robin bounce over the ground, stop and tug an earthworm out of the ground, a comical procedure. As the bird flies away with a worm in its beak, I’m always left wondering how does the Robin know the worm is there?

* actually, around here, most of the fire-engines are yellow.

Circling Vultures

They didn't photograph as well as I hoped

Since it was sunny on the long weekend, my significant other took some time to wash the car. I came out to check out what he was doing. In the still damp and shiny clean back window, I saw the reflection of a mass of black birds circling the house, an scene reminiscent of a horror movie. A nice thermal must have formed above the house as I’m not about to expire any time soon.

Looking up, I saw that each individual had a bald head. Identifying them was easy - they were turkey vultures, common to my area and a bird I’ve been able to identify since I was a kid. There were too many birds to accurately count, my best guess was there was more than thirty.

More vultures joined the group as they shifted their spiral slowly away until the mass was out of sight. This time of year they migrate south in groups. I had no idea they did so in such a large group. Turkey vultures migrate down to southern California or even as far as South America.

There was a time, when I was growing up, I was fascinated by the idea of birding. I’ve never been a serious birder as the hours don’t agree with me and I think going to extremes over a life list of birds can get a little silly, but I still like to identify the birds I see. I scraped together allowance money and bought the ‘Birds of North America’ in the early 80’s. It is still my only bird field guide. Every time I don’t bring my bird book out on a hike with me I see an interesting bird and regret not having my book.

My bird book doesn’t have a lot to say about turkey vultures. It describes them as “ a common carrion eater, scavenging in fields and along road sides.” Carrion eaters have many benefits. According to the The Turkey Vulture Society, these birds prevent spread of disease by cleaning up dead things - no food is wasted in nature. Since they might have to stick their head inside a carcass to get a tasty morsel, being bald means they don’t have to mess up their feathers for a meal.

Turkey vultures don’t make the haunting cry of a hawk, or even the peep of a chick. They have to resort to hisses and grunts as they lack the vocal cords other birds have. I’ve never hears a turkey vulture make a sound. 

As a tangent: I'm always amazed that there is a society for everything.

Aliens for dinner

Not the alien I mean

Sounds like I invited some extraterrestrials over to share a meal but, what I really mean is eating invasive species as a form of revenge against the damage they inflict on our native species.

As people move around, we tend to take critters (and plants - which I’m not going to discuss) with us. Rats and cats have been introduced across the globe, both of which have been known to decimate bird populations - especially on islands where birds have lost their ability to fly. Pigs were deliberately left on tropical islands by passing sailors to provide future food.

Sometimes animals are intentionally introduced as a means to fix a problem. For example, Australian sugar cane crops were being decimated by cane beetles, so in 1935 just over 100 cane toads were introduced to control the cane beetles. The cane toads adapted well to their new environment, now there are over 200 million - however they didn’t control the cane beetles. Instead cane toads caused all sorts of other problems as they are toxic to the animals that try to eat them.

Want to make a buck? 
How about introduce a critter that produces a luxury product, like beaver fur. In 1946, 50 beavers from Canada were introduced to the southern tip of South America for just this reason. It turned out great for the beavers as there were no predators to worry about. The beavers went on to do what beavers do - gnaw down trees and build dams. Unfortunately, the forests in that region can’t handle beaver damage like North American forests can, so the damage is extensive. Active programs are still underway to remove the beavers.

Not all introduced species create these kind of problems, however there is always a risk that a local species will be displaced by the newly arrived animals. The result is a loss to our global biodiversity as our world-wide ecosystem is becoming more and more homogenized.

In my part of the world, we have lots of introduced species (tropical areas often have more - Hawaii and Florida are perhaps the hardest hit with alien invaders). There are green crabs, manila clams, carp, house sparrows and grey squirrels to name a few. Another that has become ubiquitous in North America is the European Starling. These noisy birds like open country - like orchards and grain fields. They often flock together in massive flocks where they scour the area for fruit and insects to eat. They indiscriminately eat crops intended for human consumption which has put them on the hit list of many farmers. They also out compete local birds, for instance swallow species like the purple martin, for nest sites.

Why would anyone introduce starlings? 
In the late 1800’s, Eugene Schieffelin decided to introduce every bird mentioned in Shakespeare's works into North America. As part of this odd plan, 60 starlings were released in 1890 into New York’s central park. Now, there is an estimated population of 200 million.

So what can be done? 
One option is to eat the invaders. I was at an event recently put on by the Penticton Museum and Archives for the opening of the traveling ‘Aliens Among Us’ exhibit created by the Royal BC Museum. The exhibit highlights alien species in BC. At the opening, breaded and fried starling breast was offered - the meat was dark and gamey, reminiscent of goose, and was quite good. For Okanogan fruit growers, eating starlings must be a delicious form of revenge.

It would take a lot of effort to harvest enough starling breast to make a full meal. I’ll just keep the idea in the back of my mind in case there is a zombie apocalypse and starlings are all I can catch.

As a tangent - people are not considered ‘aliens’ in this context because people tend to move themselves around (i.e., natural dispersal) - although governments might label people as aliens for various reasons. By this same logic, extraterrestrial aliens would only be considered aliens if they hitched a ride to earth on a space shuttle instead of their own spaceship.

Examples of iridescence from the fall fair

I caught these two beauties showing off their iridescent plumage at the fall fair. It always amazes me that such a range of colours can be produced from an optical trick.

The talking crow

A picture of a crow - but not the talking crow

“Wanna ball,” demanded a small metallic voice. I stopped peddling my bike to put a foot on the ground and look around. I was alone on the road, not even a car to be seen. It was near the end of summer in the early 80's. My precious blue bike had been bought for me at a garage sale for twenty-five cents – the kind I had to peddle backwards to brake. All summer long, I rode my bike to the corner store, the swimming hole and anywhere else I wanted to go. On this particular dusty hot day, the kind signaling the approaching summer's end, I was biking along Headquarters Road, a rural road lined with fields of yellowed grass circled in barbed wire fences.

On a wire nearby, perched a crow looking me in the eye. He fluffed up his glossy black feathers and tilted his head. “Wanna ball,” he stated, as though voicing a common crow need. I stared at him, which he took for encouragement to continue. “Wanna ball, wanna ball, wanna ball ...”

It was getting late; as curious as the crow was, I needed to get home. So I balanced my bike and started to peddle away. The crow took flight, landing on the fence a short distance ahead of me, continuing to make the same demand. As I picked up speed, he just flew beside me in silence. The crow followed me all the way home (I guess he thought I was hiding a ball somewhere). Once we reached my house, I told my mom about the bird. She didn't believe me, and I don't blame her based on my overactive imagination, until the crow demanded a ball from her. 

Thanks to G.Hanke for the crow picture (I didn't have a camera back then)

Winter colour - or gratuitous ducks

I went out for a walk yesterday to a near by pond. The day was grey - the iridescence in these feathers provided the only natural colour around. All these ducks were hanging out hoping for food. Their wait paid off as many walkers (us included) brought duck snacks.

A wood duck (male)

A bufflehead (male)

A ringneck duck

A pair of American wigeons

A usual duck pond suspect - a male mallard

A duck-alanch (they thought we had food)

nature at the back door

The monster-from-a-horror-flick stage 

of a lady bug's life

In my office, I've arranged it so that from my desk I look out into the backyard. My husband and I have removed most of the lawn and whats left contains more clover than grass – which the neighbour's bees love when it's blooming. Raised garden beds now dominate the space. In the spring, I'll plant them with veggies. Right now, they contain remnants of last summer's garden – kale, parsnips and turnips ready for harvest. Plus, one bed is currently home to our crazy chickens (crazy because as soon as one catches sight of me, they all come running). We also have berry bushes and herbs waiting for spring.

Now that there is a variety of landscape, nature has moved in (we don't apply any nasty chemicals in our yard). I've observed the full cycle of a ladybug's life from it's monster-from-a-horror-flick larvae stage to its glossy-red-poka-dotted adult stage. Jumping spiders and bee flies visit, neither of them stay still long enough for me to get a good photo. A hummingbird has taken up residence in the holly behind the fence. A Bewick's wren raised a family in a pile of cedar branches which we almost carted off to the dump (we noticed the birds just in time). I'm looking forward to the day I see a garter snake move in to help with the slugs. Someday, frogs would be nice too.

Producing tasty fruit and veggies is my goal for the backyard, but, it's nice to see creatures finding a home in my yard. Even in winter, lots of creatures are making use of the yard.

You are what you eat – the colour version

A flamingo from a local butterfly garden

Every kid knows that a flamingo is pink because of what it eats. They filter water through their beak to catch brine shrimp and algae. The beta carotene in their food is converted to the pink pigments in their feathers, without this pigment source the bird would be white. Unfortunately, flamingos aren't found on my Pacific island except in captivity. But, we do have critters using the same pigment trick.

Recently, I met up with the local Natural History Society (I'm a member) for a beach seine at night because that was when the best low tide was this time of year. Based on the wind storms recently, we were lucky the wind had dropped off and it wasn't raining. The surf was manageable with the net for people wearing hip-waders and dry-suits – so not me as I don't own either. Two people took the net out into the surf. The first seine was over sand resulting in hardly any fish. So, the net was taken out and hauled in a second time over eelgrass. All sorts of interesting intertidal creatures were pulled up.

Everyone gathered around to check out the fishes, crabs and shrimps. The fish catch included: walleye pollock, English sole, stary flounder, sharpnose sculpin, sailfin sculpin, sandlance, roselip sculpin, tubesnout, high cockscomb, a type surf perch, Pacific spiny lumpsucker (the cutest fish ever) and a penpoint gunnel. Each type was put into a clear ziplock bag along with plenty of water and passed around. By holding the bags up to my headlamp, I got a good look at each critter.

The penpoint gunnel intrigued me because it was neon green – a tropical water colour in our temperate zone. A picture can be found here, the fish looks like an eel, but isn't. This guy hangs around in eelgrass or sea lettuce beds waiting to ambush little crustaceans and mollusks. The bright green colour of the one we found would allow it to blend in almost perfectly (they also come in other colours to match other seaweeds). Like the flamingo, the penpoint gunnel gets it's colour through what it eats. The green comes from the sea lettuce.

Few of the fish and invertebrates were held on to for a local museum's tide pool, the rest were released. As we packed up our gear, another beach seine group arrived. In the darkness, all we could see of them was dark shapes and headlamps – it was like looking at ourselves a couple hours in the past.
  
As a tangent, my trips to the beach seem to coincide with when my rubber boots are muddy. Once again they are clean.

Ready for take off

The barn swallows from last year returned to nest on the light fixture beside our front door. The light is in our carport, so perhaps they should be called carport swallows instead. Last year they raised three chicks. This year five chicks are crammed into the little nest - ever since they hatched we've been afraid one would fall out. Now, they're ready to fledge any day. It's been fun to watch them grow up but, we'll be changing the light fixture to discourage them coming back next year as they make a mess.

Part 3: Blue Feathers

Here is part 3 of my 4 part series on the scattered blues. Check out part 1 here and part 2 here.

Blue feathers have evolved in many species of birds. A blue jay's plumage is an excellent example with blue and white. You can see the black and blue of a Steller's jay in your own backyard. A male mountain bluebird has blue plumage of this type along with the head feathers of the male lazuli bunting; both can be found in central British Columbia. We know that feathers don't contain blue pigment, so the colour must be a result of the feather's structure.

In the late 1800s, just after the discovery of Rayleigh scattering, naturalists used this new concept to explain why blue feathers were blue. Since they didn't have the tools to examine the nanostructure (structure in the order of a billionth of a meter) of a feather, naturalists assumed that within the feather there existed transparent cells full of particles that were tiny enough to create Rayleigh scattering. Like the sky, blue light would be more efficiently scattered. These transparent cells would also contain pigments to absorb the longer wavelength colours. As a result, to our eyes these birds would appear blue.

Because Rayleigh scattering is incoherent, it produces the exact same colour irregardless of the observation direction. Since blue feathers in natural light don't change colour depending on what direction the naturalists looked at them, the assumption that their colour was formed through Rayleigh scattering seemed valid. But, in the 1930's, scientists examined a a non-iridescent blue feather under a directional light source. Colour variations were observed as the light source was moved – an iridescent characteristic that called into question the hypothesis of Rayleigh scattering making the feather blue.

By the 1940's, a cool new gadget came on the market – the electron microscope. Now naturalists could directly examine the internal nanostructure of blue feathers. Based on this first look, they interpreted the internal feather structure to contain randomly spaced objects. This meant scattered light would be incoherent leading giving support to the hypothesis of Rayleigh scattering. It took decades of further research to change this hypothesis and in the mean time many textbooks were written explaining that blue feathers were the result of Rayleigh scattering. By the 70's, scientists finally determined that the nanostructures were, in fact, not fully random. Instead they were a quasi-ordered matrix – not quite the perfect order of iridescence but not the full randomness required for Rayleigh scattering. Under natural light from all directions, like sunlight, these feathers appear to be the same colour from all directions. However when a directional light is shone on blue feathers the colour will change depending on the light direction.

Since the colour of a Steller's jay's feather comes from its internal structure on a tiny scale, a damaged feather would lose its blue colour. The dark pigments in the feather, that act to help show off the blue, would make damaged feather would look almost black. So if you are lucky enough to find a Steller's jay feather, take care of it.

Thanks to G. Hanke for the photo of mountain blue birds.

Part 1: Blue Skys and Blue Feathers – The Scattered Blues

A while back I wrote about why the sky was blue and why some feathers are blue (here) – well I didn't quite get it right, so I'm trying again. I've written a more detailed explanation which I'll post in four parts.

When I look around, I see lush greens of temperate rain forest, rich browns of fertile soil, lively yellows in fluttering butterflies, and luscious reds in ripe berries – but, not a lot of blues. If the sky is clear, it's the biggest blue object around, extending from horizon to horizon. Water reflects the blue of the sky, adding another layer of blue. On a lucky day, I'll catch a glimpse of a Steller's Jay showing off it's blue and black plumage, or a shimmering silver-blue dragon fly will dart by. I might even see a rare blue flower. On a gray winter day, the blue eyes of my favorite companion may be the only brilliant blue around. Other natural places have their own blue components, but in general, blues aren't common in nature. In fact, world-wide there just isn't a lot of natural blue pigments, thus the blues we see are often the result of optical properties within an object. These colours created as the result of an object's structure are called, creatively, 'structural colours'. Blue is a very common structural colour, and to understand why we'll need to start with some optics.

Sunlight is called 'white light' because it appears colourless. Within this colourless light lurks the full colour spectrum. Once, people thought white was the fundamental colour of light, and colours formed when something was added into the light. This theory was changed after the careful experimentation and observations of Sir Isaac Newton. Around 1670, Newton shone light through a prism creating a rainbow of hues on the other side. From this result, he concluded that white light contains all colours and that the prism simply separates them. Therefore, colour results from interactions between an object and light.

We now understand that white light is made up of tiny waves (which are simultaneously tiny particles if you want to add complexity). Light waves travel at the same speed but can have different wavelengths, that is, the distance between successive crests. Our brains perceive the different wavelengths as different colours. The longer wavelengths form reds, oranges and yellows, and the shorter wavelengths form greens, blues and violets. If you could watch waves of light pass by, more waves of blue would pass compared to waves of red – this means that the blue light has more energy. Light travels outward from its source, the sun, in a straight line until it collides with something. This collision could release all the hues in the spectrum or just a select few.

Scattering describes how light is diverted from its original straight path. Light scatters in two ways: coherent and incoherent. When scattering is coherent, spectacular effects such as iridescence can occur. Like a ball bouncing back from a flat wall, the light reflects precisely because the reflecting surface is geometrically regular. Similar colour light waves augment each other, further intensifying the effect. An iridescent feather's colour can change depending on viewing angle, a phenomenon easily observed in a Anna's Hummingbird gorget. Incoherent scattering resembles the result of throwing a rubber ball at a pole – it could bounce away in any direction. In this case, the scattering objects are randomly distributed relatively far apart. Scattering at one object occurs completely independently of the scattering at the other objects. Both coherent and incoherent scattering occur regularly in nature and can provide the mechanism for creating blue colours.

The photo is of a hyacinth macaw I took years ago at the San Diego Zoo.

Sea foam – and why it can be bad for birds

The word “sea-foam” has a feminine mystique to it. I picture it used for frilly prom dresses and girly princess rooms. In the real world, sea foam is never the pretty light aqua colour of paint chips, instead it looks like dirty cappuccino foam – a yellowed off-white often with chunks of stuff in it. As the sea sloshes, salts, chemicals, dead plants, decomposing fish and sea weeds are churned into sea foam. Individual bubbles link together, and when a surface wave passes these bubbles they mass together as they swirl upwards to make foam on the sea surface.

Sea foam on the water's surface changes how wind energy is transferred into the water. This transfer of energy is a type of friction, called the wind stress, and it can play a part in important oceanographic processes such as currents and upwelling. What sea foam does is add another layer between the wind and ocean, so instead of the wind pushing the water, the wind has to push the foam and then the foam pushes the water, which is a much less efficient process. Sea foam can do much worse things.

In the coastal ocean red tides occur, which is a dangerous form of algal bloom. A red tide is better described as a Harmful Algal Bloom or HABs, because HABs aren't all red and not all red tides are HABs. Specific phytoplankton (little water plants) full of toxins are the culprit and are bad news for shell fish and anything that eats them, like us. Recently, migrating birds along the Washington State coast were found dead in large numbers at the same time as an HAB occurred. The birds weren't poisoned; instead they were freezing to death. Why? One or more of the HABs causing phytoplankton were getting churned up into the sea foam. Birds would rummage around in this foam getting themselves covered in it, because normally sea foam isn't a danger to them. But in this case, the phytoplankton-laced foam coated the bird's feathers is such a way that they lost waterproofing and insulation, so that the poor birds froze.

What do blue jays and the sky have in common?

I've only ever caught a glimpse of a blue jay out of the corner of my eye as a flash of blue. I have, however, had plenty of opportunity to watch Steller's jays. Instead of the blue and white of a blue jay, a Steller's jay is black and blue and every bit as pretty. The blue made in these jays' feathers uses the same principle as the blue sky.

In 1810 Goethe explained blue skies as follows: “If the darkness of infinite space is seen through atmospheric vapours illuminated by the day-light the blue colour appears.” So, the colour comes from some mechanism within the atmosphere during the light of day. (I really like the phrase 'darkness of infinite space.')

Fast forward a few years to John William Strutt (1842-1919), also known as Lord Rayleigh, who was the first to describe the actual mechanism that makes the sky appear blue. He also studied the dynamics of seabird flight, so I'm assuming he often looked off into the sky. Anyway, the effect is called Rayleigh Scattering.

Absolutely tiny particles, like water drops and dust, can be so small they reach a point (about a tenth of the wavelength of light) where light will bend around the particle. The bent light is then scattered differently than it would be for larger particles, in that shorter wavelengths (like blue) are more strongly scattered than longer ones (like red) so we end up seeing the blue. So why not violet, as its wavelength is shorter than blue? It turns out our eyes are just more sensitive to the blue.

Blue feathered birds like blue jays also use Raleigh scattering to get their blueness. Within the structure of their feathers are transparent cells full of tiny particles and pigments. The pigments absorb the longer wavelength colours while the particles scatter efficiently scatter the blue light. To out eyes, these birds look blue.

If you find a blue jay's feather, take care with it because this structure is fragile. If it got crushed the blue colouring would vanish.

Update 26 November 2010 - I didn't quite get this one right, check out here for the start of a better explanation.

Iridescence part II – Hummingbirds

I've already discussed the iridescence of my little fish and how that vivid colouring originates from interference and phase-shifts within a thin layer of material that is as deep as the light's wavelength. Now, I'm going to move on to the iridescence in a male hummingbird's gorget. In a medieval world, a gorget was armor designed to protect one's throat – a term that has been extended to include the metallic-like iridescent feathers on the hummingbird's throat, the purpose of which is likely to attract lady hummingbirds. It takes a combination of optical effects at different scales acting together to create iridescence in a feather. Iridescence is considered a 'structural' colour because it is produced by optical effects instead of pigments.

Hummingbirds are fascinating. Their wings beat so fast I can hardly see them and these tiny creatures are the only group of birds that can fly backwards, sideways or straight up. With a revved up metabolism, their little hearts beat around 1200 times a minute and they only live about three to five years. Which leads me to the question: what would happen if a hummer drank caffeine? Would it be able to beat its wings fast enough to fly out of our time-space continuum?

As part of Newton's investigations in the early eighteenth century regarding the nature of light, he hypothesized that the iridescent colours found in bird's feathers were due to the presence of thin films. Newton was right about the thin films, however, he didn't discover the actual mechanism. It took another 100 years before interference was put forward as the mechanism behind iridescent colours in birds and many more years until this theory was accepted. Plenty of birds display iridescence in either small ways (like a pigeon's breast) to the scintillating colours of a peacock's display. It's feathers that provide the light-reflecting layers that create iridescence.

Feathers are complex in structure and fill a wide variety of roles. They keep a bird warm, prevent it from getting a sun burn, provide a streamlined shape, and aid in waterproofing. More importantly, tail and wing feathers allow a bird to fly (with the exception being the birds that don't fly, where feathers provide buoyancy, insulation and even formal wear for penguins). Feathers supply colour for camouflage and for the males these colours help with attracting mates. Birds have evolved to use many different techniques to produce the colours in their plumage including pigments, structural colours and a combination of the two. Through pigments they can produce yellows, reds, browns, and blacks. Whites and blues are often formed by selectively scattering incoming light in tiny air pockets within the feather barbs, while greens are produced by combining yellow pigment with air-pocket scattering effects. Iridescence is the most intensely striking and vivid structural colour birds produce.

On a hummingbird, the small-scale external structure of the feather is not flat, but raised in a v-shape as a series of ridges and troughs. This microscopic structure preferentially reflects light towards an observer directly facing the bird. At all the other angles almost no light gets reflected, which makes the feathers appear black.

On an even smaller scale, within the hummingbird's feathers there is a stack of three (usually) thin films. Each film has a thickness equal to approximately half the wavelength of the light it intensifies most (visible light wavelengths range from 380 to 740 nanometers, and a nanometer is one billionth of a meter). The films themselves are constructed from eight to ten layers of an irregular mosaic of thin elliptical discs and little pockets of air. The elliptical disks are about 2.5 microns long by one micron wide – about the size of a yeast cell. Different hummingbird feather colours are produced through a combination of the above structural effects. The elliptical disk thickness tends to decrease as the colours pass from red towards blue while at the same time the size of the air pockets increases. For more complexity, iridescent feathers often also contains pigment which absorbs the background light, allowing the structural colours to be even more vivid.

The iridescence-producing feather structure I've described is a complex phenomenon which is actively being researched. There are even on-going attempts to simulate feather iridescence. If this technology pans out, someday I may be able to wear a coat with reds as vivid as an Anna's Hummingbird's gorget.

thanks to G. Hanke for the photo