Birds of a feather

The diversity you find among birds is really remarkable.

There are differences in bill shape, from the weird angled beaks of flamingos, to the longer-on-the-bottom fish skewers of skimmers, to the upswept needle of the avocet, to the absurd (and aptly-named) spoonbills and shoebills, to the pelicans -- about whom my dad taught me a limerick when I was little:

A wonderful bird is the pelican.
His bill can hold more than his bellican.
He can stash in his beak
All his food for the week,
But I really don't see how the hellican.

Yeah, it's kind of obvious where I got my sense of humor from.

Of course, it doesn't end there. The impossibly long toes of the South American jacanas (called "lilytrotters" because they can walk on the floating leaves of waterlilies).  The phenomenal wingspan of the albatross.  The insane plumage of the birds-of-paradise.

And the colors.  Man, the colors!  Even in my decidedly non-tropical home we have some pretty amazing birds.  The first time I saw an Indigo Bunting, I was certain that one of my sons had put a blue plastic bird on the bird feeder just to rattle my chain.  There couldn't be a real bird that was that fluorescent shade of cobalt.

Then... it moved.

But nothing prepared me for the colors I saw on my visits to Ecuador, especially amongst the birds of the tanager family.  There are hundreds of species of tanagers in that tiny little country, and because they often travel in mixed foraging flocks, you can sometimes see twenty or thirty different species in the same tree.  These include the Green-headed Tanager:

[Image licensed under the Creative Commons Lars Falkdalen Lindahl (User:Njaelkies Lea), Green-headed Tanager Ubatuba, CC BY-SA 3.0]

The Black-capped Tanager:

[Image licensed under the Creative Commons Joseph C Boone, Black-capped Tanager JCB, CC BY-SA 4.0]

And the Flame-faced Tanager:

[Image licensed under the Creative Commons Eleanor Briccetti, Flame-faced Tanager (4851596008), CC BY-SA 2.0]

Being a biologist, of course the question of how these birds evolved such extravagant colors is bound to come up, and my assumption was always that it was sexual selection -- the females choosing the most brightly-colored males as mates (in this group, as with many bird species, the males are usually vividly decked out and the females are drab-colored). If over time, the showiest males are the most likely to get lucky, then you get sexual dimorphism -- the evolution of different outward appearances between males and females.  (This isn't always so, by the way.  Most species of sparrows, for example, have little sexual dimorphism, and even experienced birders can't tell a male from a female sparrow by looking.)  More puzzling still is the general trend that tropical birds are more brilliantly-colored than bird species in higher latitudes -- a trend that is yet to be convincingly explained.

The reason this comes up today is two papers that came out last week.  The first, that appeared in Science Advances, looks at one of the most amazing things about their evolutionary history -- they were the only branch of the dinosaur clade that survived the cataclysmic mass extinction at the end of the Cretaceous Period.  What allowed birds to make it through the bottleneck that killed all of their near relatives -- and not only survive, but thrive and rediversify?

The evidence is that the extinction event selected for two things; small body size, and a shift toward young being altricial -- born relatively helpless and undeveloped, and therefore requiring more parental care.  Some lineages of birds would eventually increase in body size again, but they never again would reach the colossal proportions that their cousins did during the Jurassic and Cretaceous Periods.

"We have typically not looked at the change in DNA composition and model across the tree of life as a change that something interesting has happened at a particular point of time and place," said Stephen Smith, of the University of Michigan, who co-authored the study.  "This study illustrates that we have probably been missing something...  We found that adult body size and patterns of pre-hatching development are two important features of bird biology we can link to the genetic changes we’re detecting.  One of the most significant challenges in evolutionary biology and ornithology is teasing out the relationships between major bird groups — it’s difficult to determine the structure of the tree of life for living birds."

The study not only elucidated relationships between extant groups of birds, it allowed the researchers to pinpoint when groups diverged from each other, and therefore what innovations were likely to be connected with events occurring on the Earth at the time.

The second study, which appeared in Nature Ecology & Evolution, looked at the question I began with -- the impossibly bright colors that are characteristic of so many bird species.  Colors in birds arise two ways -- pigments (chemicals which absorb some frequencies of light and reflect others) and structural color (due to feathers creating a combination of refraction and interference; this is also known as iridescence).  Most pigmented color in birds is relatively drab -- blacks, grays, and various shades of brown -- the flashing blues, greens, and purples you see in groups like tanagers, hummingbirds, and sunbirds are almost entirely due to iridescence.

The researchers went through images of as many of the 9,409 species of birds currently in existence, along with the current best iteration of the family tree of birds, to try and figure out where along the way iridescence evolved, and how it spread so widely among this class of animals.  

And what they found was that 415 distantly-related branches of the tree have iridescent feathers, and the common ancestor of all modern birds -- something like eighty million years ago -- was very likely iridescent.

"I was very excited to learn that the ancestral state of all birds is iridescence," said Chad Eliason, of the Field Museum in Chicago, who was the paper's lead author.  "We've found fossil evidence of iridescent birds and other feathered dinosaurs before, by examining fossil feathers and the preserved pigment-producing structures in those feathers.  So we know that iridescent feathers existed back in the Cretaceous -- those fossils help support the idea from our model that the ancestor of all modern birds was iridescent too."

There are still a lot of questions left unanswered, however.  "We still don't know why iridescence evolved in the first place," Eliason said.  "Iridescent feathers can be used by birds to attract mates, but iridescence is related to other aspects of birds' lives too.  For instance, tree swallows change color when the humidity changes, so iridescence could be related to the environment, or it might be related to another physical property of feathers, like water resistance.  But knowing more about how there came to be so many iridescent birds in the tropics might help us understand why iridescence evolved."

Which is extremely cool.  Something to think about next time you see one of those brilliant little flying jewels flit by.  The stunning colors we appreciate every day on our bird feeders and in the wild have a very long history -- going back to a trait that evolved something like eighty million years ago.

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Rose-colored glass

My wife, Carol Bloomgarden, is an amazing artist, and participates in art shows all over the northeastern United States.  (Her work is called micrography -- it's drawings made from patterns of tiny handwritten text.  You can, and should, check it out at her website.)  Because showing framed art work requires moving lots of stuff around -- not only the work itself, but the canopies, frames, and stands on which to display it -- I frequently accompany her to her shows.

My usefulness is best summed up in a line from a t-shirt a student of mine used to wear: "I May Not Be Very Smart, But I Can Lift Heavy Objects."

In any case, in between setup and breakdown, I usually have lots of time to wander around the show and see what the other artists are selling.  Last year, one of the booths belonged to a very talented jeweler who made jewelry out of (amongst other things) fragments of Roman glass.

Carol hinted at me that she loved this jeweler's work, so for her birthday I got her a necklace and matching set of earrings made from chunks of turquoise-colored glass dating to about 300 C.E.

The Romans were outstanding glassmakers, and a lot of their work survives (unfortunately, much of it in fragmentary form).  And one curious thing about a lot of Roman glass is that it has a patina -- an iridescent sheen on the surface, sometimes refracting light and creating a metallic or rainbow appearance.  There is nothing in the existing writing from that era indicating that those effects were created deliberately; it seemed to be some sort of byproduct of the aging of the piece.

Fourth century C.E. Roman glass from a glassworks in Syria, showing the gold patina over pale green glass [Image is in the Public Domain courtesy of its creator, Marie-Lan Nguyen]

Researchers in materials science at Tufts University became curious about how these coatings were produced, and did microscopic analysis of the surfaces of pieces of Roman glass.  They came to a surprising conclusion; the gold, silver, or rainbow-colored coatings were (1) naturally produced after the pieces were buried, and (2) were photonic crystals -- regular, periodic microlayers of precisely-arranged molecules, of the same sort used in semiconductors and solar cells, which have the effect of generating light interference and an opalescent or iridescent appearance.

It turns out that the interaction between the glass surface, rainwater, and the minerals in the soil results in a very slow, orderly deposition of thin films on the artifact's surface, and in two thousand or so years, you have something truly spectacular.  "It's really remarkable that you have glass that is sitting in the mud for two millennia and you end up with something that is a textbook example of a nanophotonic component," said Fiorenzo Omenetto, who co-authored the study.  "While the age of the glass may be part of its charm, in this case if we could significantly accelerate the process in the laboratory we might find a way to grow optic materials rather than manufacture them."

"This is likely a process of corrosion and reconstruction," said Giulia Guidetti, also a co-author.  "The surrounding clay and rain determined the diffusion of minerals and a cyclical corrosion of the silica in the glass.  At the same time, assembly of 100 nanometer-thick layers combining the silica and minerals also occurred in cycles.  The result is an incredibly ordered arrangement of hundreds of layers of crystalline material... [so] the crystals grown on the surface of the glass are also a reflection of the changes in conditions that occurred in the ground as the city evolved -- a record of its environmental history."

So here we have another example of the kind of fascinating crossover you see in the very best science -- in this case, between materials science and archaeology.  With possible applications to engineering.  

I know I'll think about this study every time Carol wears her Roman glass jewelry.  

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Pink, pink, gold

When I was in Ecuador in 2019, I was blown away by its natural beauty.  The cloud forests of the mid-altitude Andes are, far and away, the most beautiful place I've ever been, and I've been lucky enough to see a lot of beautiful places.  Combine that with the lovely climate and the friendliness of the people, and it puts the highlands of Ecuador on the very short list of places I'd happily move to permanently.

What brought me there were the birds.  It's a tiny country, but is home to 1,656 species of birds -- about one-sixth of the ten-thousand-odd species found worldwide.  Most strikingly, it has 132 different species of hummingbirds.  Where I live, in upstate New York, we have only one -- the Ruby-throated Hummingbird (Archilochus colubris) -- but there, they have an incredible diversity within that one group.  Because each species is dependent on particular flowers for their food source, some of them have extremely restricted ranges, often narrow bands of terrain at exactly the right climate and altitude to support the growth of that specific plant.  You go a few hundred meters up or downhill, and you've moved out of the range where that species lives -- and into the range of an entirely different one.

The most striking thing about the hummingbirds is their iridescence.  My favorite one, and in the top five coolest birds I've ever seen, is the Violet-tailed Sylph (Aglaiocercus coelestis):

[Image licensed under the Creative Commons Andy Morffew from Itchen Abbas, Hampshire, UK, Violet-tailed Sylph (33882323008), CC BY 2.0]

What's most fascinating about birds like this one is that the feathers' stunning colors aren't only due to pigments.  A pigment is a chemical that appears colored to our eyes because its molecular structure allows it to absorb some frequencies of light and reflect others; the chlorophyll in plants, for example, looks green because it preferentially absorbs light in the red and blue-violet regions of the spectrum, and reflects the green light back to our eyes.  Hummingbirds have some true pigments, but a lot of their most striking colors are produced by interference -- on close analysis, you find that the fibers of the feathers are actually transparent, but when light strikes them they act a bit like a prism, breaking up white light into its constituent colors.  Because of the spacing of the fibers, some of those wavelengths interfere destructively (the wavelengths cancel each other out) and some interfere constructively (they superpose and are reinforced).  The spacing of the fibers determines what color the feathers appear to be.  This is why if you look at the electric blue/purple tail of the Violet-tailed Sylph from the side, it looks jet black -- your eyes are at the wrong angle to see the refracted and reflected light.  Look at it face-on, and suddenly the iridescent colors shine out.

So the overall color of the bird comes from an interplay between whatever true pigments it has in its feathers, and the kind of interference you get from the spacing of the transparent fibers.  This is why when you recombine these features through hybridization, you can get interesting and unexpected results -- as some scientists from Chicago's Field Museum found out recently.

Working in Peru's Cordillera Azul National Park, on the eastern slopes of the Andes, ornithologist John Bates discovered what he'd thought was a new species in the genus Heliodoxa, one with a glittering gold throat.  He was in for a shock, though, when the team found out through genetic analysis that it was a hybrid of two different Heliodoxa species -- H. branickii and H. gularis -- both of which have bright pink throats.

"It's a little like cooking: if you mix salt and water, you kind of know what you're gonna get, but mixing two complex recipes together might give more unpredictable results," said Chad Eliason, who co-authored the study.  "This hybrid is a mix of two complex recipes for a feather from its two parent species...  There's more than one way to make magenta with iridescence.  The parent species each have their own way of making magenta, which is, I think, why you can have this nonlinear or surprising outcome when you mix together those two recipes for producing a feather color."

The gold-throated bird apparently isn't a one-off, as more in-depth study found that it didn't have an even split of genes from H. branickii and H. gularis.  It seems like one of its ancestors was a true half-and-half hybrid, but that hybrid bird then "back-crossed" to H. branickii at least once, leaving it with more H. branickii genes.  All of which once again calls into question our standard model of species being little cubbyholes with impermeable walls.  The textbook definition of species -- "a morphologically-distinct population which can interbreed and produce fertile offspring" -- is unquestionably the most flimsy definition in all of biology, and admits of hundreds of exceptions (either morphologically-identical individuals which cannot interbreed, or morphologically-distinct ones that hybridize easily, like the Heliodoxa hummingbirds just discovered in Peru).

In any case, the discovery of this hybrid is fascinating.  You have to wonder how many more of them there are out there.  The fact that its discovery ties together the physics of light, genetics, and evolution is kind of amazing.  Just further emphasizes that if you're interested in science, you will never, ever be bored.

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Look at that shine!

There's this bird called the cassowary, have you heard of it?

I think a better name for it would be the "Giant Blue-headed Australian Death Turkey."  They're ungainly-looking things, but (1) they're big, and (2) they're fast.  An adult GBADT can be two meters tall and weight 55 kilograms.  Not only that, but if they feel threatened, they don't run or fly away as any normal species of bird would do.  No, this is Australia.  What they do is run toward people, jump up, and kick them with razor-sharp talons, attempting -- sometimes successfully -- to disembowel them.

Think I'm joking?  This is an actual (i.e. un-Photoshopped) photograph of a guy trying to avoid being killed by a furious cassowary.

The reason this comes up is that cassowaries have another strange feature besides being, essentially, emus with daggers strapped to their feet.  Their black feathers have the quality of iridescence -- something you might not notice if it was leaping at you -- but from a safe distance, their feathers have an oily rainbow sheen.

This is more than just simple pigmentation.  The structures in the feathers containing the black pigment are called melanosomes, and they come in a variety of shapes and sizes in different species.  The brightly-colored throat patches ("gorgets") in hummingbirds are the color they are because of melanosomes.

But if the pigment they contain is black, how do hummingbirds display their amazing array of colors, and how do cassowaries gain their sheen?

The reason is a phenomenon called optical interference, and has to do with the multiple clear layers of keratin that separate the layers of melanosomes.  Light passing through those clear layers is refracted, and crosses light waves refracted by other layers -- and because of this, some wavelengths of light undergo destructive interference (they cancel each other out) and others constructive interference (they reinforce each other).  In our local Ruby-throated Hummingbirds, the keratin layers are spaced so the wavelengths that reinforce are ones that our eyes see as being in the red region of the spectrum; other colors get cancelled out.  Thus, the ruby throat of the Ruby-throat.

But change the spacing of the layers, and you change what colors reinforce.  So you can get the Violet-tailed Sylph of Ecuador...

[Image licensed under the Creative Commons Joseph C Boone, Violet-tailed Sylph 2 JCB, CC BY-SA 4.0]

... the aptly-named Magnificent Hummingbird of Mexico, Central America, and southern Arizona...

[Image licensed under the Creative Commons Don Faulkner, Magnificent Hummingbird (7047734993), CC BY-SA 2.0]

... and over two hundred others, each with its own different spacing of the keratin layers in the feathers, and thus, each with its own array of spectacular, iridescent colors.

What's fascinating about this evolutionarily is that cassowaries and hummingbirds have been separate lineages for a long time.  Their last common ancestor is estimated at eighty million years ago, so predating the extinction of the non-avian dinosaurs by a good fourteen million years.  And some birds don't have this kind of iridescence -- their feather colors come from ordinary pigments, not a lot different than different colors of paint.  So how did two widely-separated groups of birds end up landing on the same solution for being colorful?

It's a very striking example of convergent evolution, where different organisms end up becoming superficially similar (usually only on one or two traits) because of similar selective pressures.  And apparently the innovation came about a long time ago in both lineages, as I found out in a paper this week in Science Advances that details information about some fossil feathers from relatives of the cassowary that were around 52 million years ago, during the Eocene Epoch.

In "Cassowary Gloss and a Novel Form of Structural Color in Birds," by Chad Eliason and Julia Clarke of the University of Texas - Austin, we read about an incredibly detailed analysis of feather fossils from the Green River Formation in Wyoming.  Using an electron microscope, the authors were able to measure the spacing of the melanosome layers and keratin layers, and determined that the species the feathers came from -- the lithornithid Calxavis (or Calciavis) grandei -- was black, with a deep iridescence on the wings.

The idea that we could actually find out what color an extinct species was using its fossilized feathers is amazing.  When I look at "artist's reconstructions" of prehistoric animals, I have to remind myself constantly that all the colors are just guesses based on analogies (sometimes incorrect ones) to modern species.  But now we actually have a pretty good idea of what a bird looked like who last flew around fifty-some-odd million years ago, which is kind of mind-boggling.

You have to wonder what other characteristics Calxavis shared with its modern cousins.  Unfortunately, we still know next to nothing about the behavior of long-extinct animals, so more than likely we'll never have anything more than guesses about how it acted when it was alive.

Who knows, maybe it even rushed at prehistoric predators and tried to rip them apart with its talons.  I mean, the Giant Blue-headed Australian Death Turkey's bad attitude has to come from somewhere.

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This week's Skeptophilia book-of-the-week is one that should be a must-read for everyone -- not only for the New Yorkers suggested by the title.  Unusual, though, in that this one isn't our usual non-fiction selection.  New York 2140, by Kim Stanley Robinson, is novel that takes a chilling look at what New York City might look like 120 years from now if climate change is left unchecked.

Its predictions are not alarmism.  Robinson made them using the latest climate models, which (if anything) have proven to be conservative.  She then fits into that setting -- a city where the streets are Venice-like canals, where the subways are underground rivers, where low-lying areas have disappeared completely under the rising tides of the Atlantic Ocean -- a society that is trying its best to cope.

New York 2140 isn't just a gripping read, it's a frighteningly clear-eyed vision of where we're heading.  Read it, and find out why The Guardian called it "a towering novel about a genuinely grave threat to civilisation."

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]