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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Birds down under

I've been an avid birdwatcher for many years, and have been fortunate enough to travel to some amazingly cool places in search of avifauna.  Besides exploring my own country, I've been to Canada (several times), Belize (twice), Ecuador (twice), Iceland (twice), England (twice), Scotland, Sweden, Finland, Denmark, Russia, Spain, Portugal, and Malaysia.

One place I've never been, though, is Australia, which is a shame because it's got some incredible animals.  And despite a pretty well-deserved reputation for having far more than their fair share of wildlife that's actively trying to kill you, most tourists come back from trips to Australia alive and with all their limbs still attached in the right places.

The main reason for Australia's unique ecosystems is that it's been isolated for a very long time.  During the breakup of Pangaea, the northern part (Laurasia, made up of what is now Europe, North America, and most of Asia) separated from the southern part (Gondwanaland, made up of what is now Africa, South America, Antarctica, Australia, and India), something on the order of 180 million years ago.  The other pieces gradually pulled apart as rifting occured, but Australia remained attached to Antarctica until around thirty million years ago.  At that point, the whole thing had a fairly temperate climate, but when the Tasman Gateway opened up during the Oligocene Period, it allowed the formation of the Antarctic Circumpolar Current, isolating and cooling Antarctica and resulting in the extinction of nearly all of its native species.  Australia, now separate, began to drift northward, gradually warming as it went, and carrying with it a completely unique suite of animals and plants.

The reason all this comes up is a sharp-eyed Australian loyal reader of Skeptophilia, who sent me a link to a news story about a recent discovery by a dedicated amateur fossil hunter and birdwatcher, Melissa Lowery, who was looking for fossils on the Bass Coast of Victoria and stumbled upon something extraordinary -- some 125 million year old bird footprints.

Lowery's bird footprints [Image by photographer Rob French, Museums Victoria]

At that point, the separation of Australia and Antarctica was some 65 million years in the future, the sauropod dinosaurs were still the dominant animal group, and Victoria itself was somewhere near the South Pole.  Lowery's find led to a full-scale scientific investigation of the area, and uncovered a great many more bird tracks, including some with ten-centimeter-long toes.  Also in the area were the footprints of dozens of kinds of non-avian dinosaurs.

"Most of the bird tracks and body fossils dating back to the Early Cretaceous are from the Northern Hemisphere, particularly from Asia," said Anthony Martin, of Emory University, who led the study.  "Our discovery shows that there were many birds, and a variety of them, near the South Pole about 125 million years ago."

Of course, being a birdwatcher, I'm intensely curious as to what these birds looked like, but there's only so much you can tell from a footprint, or even fossilized bones.  It's simultaneously intriguing and frustrating to think about the fact that these animals -- and all the other animals and plants that lived alongside them -- had every bit of the diversity, all the curious and wonderful and beautiful adaptations and behaviors, that our modern wildlife does.

Imagine what it would be like to transport yourself back to Australia in the early Cretaceous, and witness all of that with your own eyes and ears.  (With, of course, a guarantee of coming back alive and with all your limbs still attached in the right places.  Back then, Australia was a rougher place than it is now.)

So thanks to the reader who sent me the link -- it's renewed my desire to visit Australia.  If I can't see the amazing birds they had 125 million years ago, at least I can have a look through my binoculars at some of the ones they have today.

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Birds of a feather

The word species has got to be the mushiest term in all of science.

It's one of those situations where you think you know what something means until you start pushing on it.  When humans started to put a serious effort into categorizing other life forms -- Aristotle is usually credited with being the first to do this in a systematic way -- it seemed obvious enough.  Members of a species are similar morphologically.  Put more simply, you can tell a cat from a dog because they look different.

The problem is, this starts to cause problems just about immediately.  What about organisms that look very different, but we still consider to be the same species?  Dogs, in fact, are a good example.  Imagine you're an alien scientist arriving on Earth, and you're looking at a St. Bernard and a chihuahua.  If a human said, "These are the same species," my guess is you'd do whatever passes for laughter on your home world, then get back in your spaceship and fly away after writing "No intelligent life" on the map of the Solar System.

Dogs, of course, aren't the only ones; there are lots of examples in nature of different-looking organisms that are considered conspecific.  So in the 1800s, the definition was revised to, "a group of organisms that are capable of mating and producing offspring."  This worked until people started to think about mules, which are the offspring of a horse and a donkey (usually considered separate species).  Then, it was pointed out that although alive and well, (most) mules are infertile, so a word was added to take care of that problem: "a group of organisms that are capable of mating and producing fertile offspring."

It only got worse from here.  An awkward difficulty with the above definition is, what about asexual species?  They kind of don't fit in no matter how you look at it.  Oh, well, maybe they get their own version of the definition.  But what about ring species?  This is a group of populations, often arranged in a ring around a geographical barrier (thus the name) where all of them can interbreed except for the ones at the "ends" of the ring  It's been observed multiple times, including a group of salamanders in California, the Greenish Warbler of central Asia, and a ring of gull species -- the latter of which goes all the way around the world.

So do these represent one species, or many?  Within the ring, some of them are interfertile, and others aren't.  And splitting the ring doesn't help; then you're separating populations that are interfertile.  In fact, like asexual species, ring species seem to be unclassifiable with the canonical definition.

It all comes, my evolutionary biology professor said to us, from the desperation humans have to pigeonhole everything.  "The only reason we came up with the concept of a species in the first place," he said, "is because humans have no near relatives."

Of course, none of this sits well with the creationists, because a central tenet of their beliefs is that each kind of life form was created by God as-is and nothing's changed since.  Which is all well and good until you ask, "What do you mean by 'kind of life form'?"  They respond that God created "discrete forms with genetic boundaries to interbreeding," which they call baramins (a neologism coined from the Hebrew words for "created" and "kind").  So the ring species of gulls isn't a problem because gulls are a "kind."  In fact, you can define "kind" in this context as "a classification of life forms that conveniently makes all of the internal contradictions go away.  Now stop asking questions."  

In any case, there really is no good, consistent definition of species that covers all the exceptions.  Even now that we have genetic analysis -- which is currently the touchstone for classification -- it only further reinforces the fact that evolution generates a continuum of forms, and you're asking for trouble if you try to subdivide them.  Only in cases like ourselves, where there are no living near relatives, does it seem clear-cut.

Take the study out of the University of Colorado that appeared in Nature Communications this week.  It's about a trio of species of birds, so being a rather fanatical birder, it immediately caught my eye.  The species involved (and I use that term guardedly, for reasons that will become obvious) are the Common Redpoll, (Acanthis flammea) the Hoary Redpoll (Acanthis hornemanni), and the Lesser Redpoll (Acanthis cabaret), all types of finch with a characteristic red splotch on the forehead.  

Common Redpoll (Acanthis flammea) [Image licensed under the Creative Commons Cephas, Carduelis flammea CT6, CC BY-SA 3.0]

The Lesser Redpoll is only found in Europe, but the other two occur in North America.  They have pretty obvious color differences; the Lesser Redpoll is brownish, the Hoary Redpoll is almost white, and the Common Redpoll is somewhere in the middle, with reddish flanks.  The size differs, as well, with the Lesser at the small end and the Hoary at the large end.

Lesser Redpoll (Acanthis cabaret) [Image licensed under the Creative Commons Carduelis_cabaret.jpg: Lawrie Phipps derivative work: MPF (talk), Carduelis cabaret1, CC BY 2.0]

However, the differences aren't huge.  We get Common Redpolls at our bird feeders in winter fairly regularly, but Hoary Redpolls are a rare sighting in our area.  Every winter I scan the flocks of redpolls looking for whiter individuals, but I still have never seen one.  However, I may be able to cross that one off the list of "species I haven't seen" -- because the current study has shown that despite the differences in appearance, all three are a single species.

Hoary Redpoll (Acanthis hornemanni) [Image licensed under the Creative Commons Ron Knight from Seaford, East Sussex, United Kingdom, Arctic Redpoll (Acanthis hornemanni) (13667519855), CC BY 2.0]

The color and size differences, the researchers found, are due to a "supergene complex" -- a single cluster of genes that work together to produce a specific phenotype.  What's striking is that despite the differences in that gene complex between the three different groups of redpolls, they are otherwise about as genetically identical as it's possible to get.  And... they're all potentially interfertile.

"Often times we assume that a lot of traits can act independently, meaning that different traits can be inherited separately from one another, but this particular result shows that sometimes these traits are actually tightly linked together," said Erik Funk, lead author on the paper, in an interview in Science Daily.  "At least for these birds, they're inheriting a whole group of traits together as one."

Birders tend to hate it when confronted with "lumpers," as they call researchers who merge species together, therefore reducing the number of potential birds to chase after.  They much prefer "splitters," who take previously single species and subdivide them, like another "winter finch," the Red Crossbill (Loxia curvirostra), which according to some taxonomists isn't a single species but several -- possibly as many as seven.  In any case, my point here is that this kind of thing happens all the time.  Like I said at the beginning, we think we have a clear idea of what's meant by a species until we start examining it.

But to me, this only increases my fascination with the natural world.  It's a beautiful, subtle, and complex interlocking web of organisms, and maybe the most surprising thing of all is that we do think it should be simple and easily classifiable.  As usual, our scheme for understanding the world turns out to be woefully inadequate -- and once again, science has come to the rescue by turning a lens on a small and unassuming bird as a way of pointing out how much more we have to learn.

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As I've mentioned before, I love a good mystery, which is why I'm drawn to periods of history where the records are skimpy and our certainty about what actually happened is tentative at best.  Of course, the most obvious example of this is our prehistory; prior to the spread of written language, something like five thousand years ago, most of what we have to go by is fossils and the remnants of human settlements.

Still, we can make some fascinating inferences about our distant ancestors.  In Lost Civilizations of the Stone Age, by Richard Rudgely, we find out about some of the more controversial ones -- that there are still traces in modern languages of the original language spoken by the earliest humans (Rudgely calls it "proto-Nostratic"), that the advent of farming and domestication of livestock actually had the effect of shortening our average healthy life span, and that the Stone Age civilizations were far more advanced than our image of "Cave Men" suggests, and had a sophisticated ability to make art, understand science, and treat illness.

None of this relies on any wild imaginings of the sort that are the specialty of Erich von Däniken, Zecharia Sitchin, and Giorgio Tsoukalos; and Rudgely is up front with what is speculative at this point, and what is still flat-out unknown.  His writing is based in archaeological hard evidence, and his conclusions about Paleolithic society are downright fascinating.

If you're curious about what it was like in our distant past, check out Lost Civilizations of the Stone Age!

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

Bird trees

I'm a birdwatcher, which in my specific case kind of crosses the line into a mild mental illness.  I've traveled all over the world to see new birds, which is kind of cool, but it's also led me to do stuff like standing calf-deep in the snow, in far-below-freezing temperatures and gale-force winds, to see a rare species of duck that happened to end up for some reason in nearby Cayuga Lake in midwinter.  From the standpoint of seeing a bird species I'd never seen before, it was a great day.

It was a somewhat less-than-great day with respect to frostbite, hypothermia, and people in passing (heated) cars rolling down their windows to yell "What the hell is wrong with you?"

The reason this all comes up because of how excited I am about a recent release of new information by the Bird 10,000 Genomes Project, which has as its fairly lofty goal the sequencing of the genomes for all ten-thousand-plus species of birds currently living on the Earth.

The cool thing about genetic information to determine relationships is that it's much more accurate than relying on such obvious characteristics as external appearance or behavior.  Through genetic analysis, the B10K Genomes Project, as it's affectionately known, has found the following surprises:

• Flamingos are fairly closely related to grebes, a family of small diving water birds, and both as a group are more closely related to pigeons than to any other species of aquatic bird.
• Likewise, the bizarre flightless dodos, now extinct but once common on two remote islands in the Indian Ocean, are most closely related to pigeons.
• The three main groups of birds that regularly prey on mammals -- hawks and eagles, owls, and falcons -- aren't closely related at all, and their similarities seem to have developed through convergent evolution.
• Despite superficial similarities in appearance and behavior, vultures in North and South America are only very distantly related to vultures in Africa and Europe.
• Hummingbirds, swifts, and nightjars (such as the more-often-seen-than-heard whippoorwill) are all on the same branch of the bird family tree.  A different branch includes such disparate groups as loons, pelicans, albatrosses... and penguins.
• Emus, ostriches, and kiwis -- flightless species that are on the same basic branch, a group called ratites -- all descend from a common ancestor that could fly, and apparently evolved flightlessness independently.

Here's a circular representation of what we know so far, with illustrations of a few selected species:

Clockwise from the top: golden eagle, thick-billed murre, ruddy turnstone, white-bellied storm petrel, western bronze ground dove, squirrel cuckoo, Anna’s hummingbird, marbled wood quail, little spotted kiwi, redwing blackbird, akiapolaau, black sunbird, wall creeper, Cape rockjumper, chestnut wattle-eye, chowchilla, Amazonian royal flycatcher, rosy-faced lovebird and keel-billed toucan.  [Image courtesy of S. Feng et al./Nature, 2020; Illustrations by Jon Fjeldså]

So the study is pretty cool, expanding greatly what we know about the feathered dinosaurs we see flitting about every day.  As the B10K Genome Project site puts it:

The B10K project will allow the completion of a genomic level tree of life of the entire living avian class, decode the link between genetic variation and phenotypic variation, uncover the correlation of genetic evolutionary and biogeographical and biodiversity patterns across a wide-range of species, evaluate the impact of various ecological factors and human influence on species evolution, and unveil the demographic history of an entire class of organisms...  We envision this project will have significant scientific and public impact that will change our understanding of avian biology and evolution, which in turn will affect our understanding of other organisms and open doors to new areas of research.

I'm really looking forward to seeing what else they uncover.  It might not explain my obsession with trying to see every bird there is -- something a friend of mine calls "Pokémon for Adults" -- but it certainly will give me something new to think about when I'm shivering in the snow looking for rare ducks.

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This week's Skeptophilia book-of-the-week is one that has raised a controversy in the scientific world: Ancient Bones: Unearthing the Astonishing New Story of How We Became Human, by Madeleine Böhme, Rüdiger Braun, and Florian Breier.

It tells the story of a stupendous discovery -- twelve-million-year-old hominin fossils, of a new species christened Danuvius guggenmosi.  The astonishing thing about these fossils is where they were found.  Not in Africa, where previous models had confined all early hominins, but in Germany.

The discovery of Danuvius complicated our own ancestry, and raised a deep and difficult-to-answer question; when and how did we become human?  It's clear that the answer isn't as simple as we thought when the first hominin fossils were uncovered in Olduvai Gorge, and it was believed that if you took all of our millennia of migrations all over the globe and ran them backwards, they all converged on the East African Rift Valley.  That neat solution has come into serious question, and the truth seems to be that like most evolutionary lineages, hominins included multiple branches that moved around, interbred for a while, then went their separate ways, either to thrive or to die out.  The real story is considerably more complicated and fascinating than we'd thought at first, and Danuvius has added another layer to that complexity, bringing up as many questions as it answers.

Ancient Bones is a fascinating read for anyone interested in anthropology, paleontology, or evolutionary biology.  It is sure to be the basis of scientific discussion for the foreseeable future, and to spur more searches for our relatives -- including in places where we didn't think they'd gone.

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

Second verse, same as the first

One of many brilliant moments in the movie Jurassic Park is when the main characters realize that the cloned dinosaurs -- which were supposed to be sterile -- had reproduced, and Ian Malcolm has this slow smile cross his face, and says, "Life finds a way."

It's amazing how resilient life can be.  I got a wonderful reminder of this from my friend, writer and blogger Andrew Butters (whose awesome blog Potato Chip Math should have way more followers), who just a couple of days ago sent me a story about bird evolution I'd never heard of before.

Let's start with the "evolution" part first.  Ever heard of iterative evolution?  I hadn't.  It is the tendency -- apparently documented in a number of lineages -- where two separate closely-related lineages develop exactly the same adaptation independently.  If you're thinking, "Wait, isn't that convergent evolution?" you've got the gist, but there's a difference; convergent evolution is when two relatively unrelated species end up evolving similar features because of being under the same kinds of selective pressures.  The two then end up looking similar for one or two obvious traits, but in other respects they are still completely different species.  (If you want some examples with photographs, I dealt with this here only a month ago.)

Iterative evolution, on the other hand, occurs within a clade of already closely-related populations, and distinctive features evolve more than once, generating what may as well be the same species twice.  The suspicion is this occurs because of mutations in regulatory genes, where interfering in embryonic development results in the same change no matter where exactly the mutation occurred.

As implausible as this sounds, it's not at all unlikely if the two mutations in question are both losses of function, where the mutation causes a gene to switch off.  As a rather rough analogy, consider a car's engine.  Two completely separate mutations -- say, undoing the battery cables and removing the fuel pump -- will both result in the car not starting, even though the underlying cause is entirely different.

Which brings us to the Aldabra rail (Dryolimnas cuvieri subsp. aldabranus), a flightless bird native to Aldabra Island in the Seychelles.  It's the only extant species of flightless bird in the Indian Ocean.  Flying is an energy-wasteful activity if it's not conferring some significant benefits to the bird, so in the absence of predators, it's selected against.  That comes to a screeching halt, however, when predators are introduced to the habitat -- which on various islands across the world have included rats, cats, brown tree snakes... and humans.  The result is that flightless species have taken a huge hit worldwide.

But the Aldabra rail has survived somehow, and a recent paper has demonstrated that the flight-capable parent species, the white-throated rail, which is found on various islands including Madagascar, has on Aldabra evolved flightlessness at least twice.

[Image licensed under the Creative Commons Charles J Sharp creator QS:P170,Q54800218, White-throated rail (Dryolimnas cuvieri cuvieri), CC BY-SA 4.0]

In "Repeated Evolution of Flightlessness in Dryolimnas Rails (Aves: Rallidae) After Extinction and Recolonization on Aldabra," which appeared in the Zoological Journal of the Linnean Society, by Julian Hume (of the Natural History Museum at Tring) and David Martill (of the University of Portsmouth), we read about incontrovertible evidence that the flightless subspecies evolved again after the first population was wiped out.

The authors write:

The Aldabra rail, Dryolimnas cuvieri subsp. aldabranus, endemic to the Aldabra Atoll, Seychelles, is the last surviving flightless bird in the Indian Ocean.  Aldabra has undergone at least one major, total inundation event during an Upper Pleistocene (Tarantian age) sea-level high-stand, resulting in the loss of all terrestrial fauna.  A flightless Dryolimnas has been identified from two temporally separated Aldabran fossil localities, deposited before and after the inundation event, providing irrefutable evidence that a member of Rallidae colonized the atoll, most likely from Madagascar, and became flightless independently on each occasion.  Fossil evidence presented here is unique for Rallidae and epitomizes the ability of birds from this clade to successfully colonize isolated islands and evolve flightlessness on multiple occasions.

This inevitably brings up the question of whether the two populations -- before and after the Pleistocene inundation -- are the same species.  This is a harder question than it sounds.  First, and most superficially, since the canonical definition of species is "a group of morphologically similar individuals all of which are capable of interbreeding and producing fertile offspring," it's impossible to tell if these are the members of the same species because the pre-inundation birds are all dead.  But it's not even as simple as that, because when you look closely, you find that species is in contention for being the mushiest definition in all of science.  There are scads of nearly-identical populations that can't interbreed, very different looking-ones that can, and ones that show a continuum of types between two extremes, making the question of "where do you draw the line?" a real problem.  (I dealt with this here a long time ago, if you're curious.)

So whether they're the same species or not is probably a meaningless question, a bit like drawing an arbitrary line on the ground and then arguing about why something falls on one side of the line or the other.  But the phenomenon itself is fascinating; that two virtually identical populations could evolve twice in the same place is kind of amazing.  It's almost as if when the first population was destroyed, Mother Nature said, "No, we need some flightless rails here," and the flight-capable parent species recolonized the island and evolved flightlessness a second time.

[Nota bene: no, I don't actually believe Mother Nature guides evolution in some kind of conscious fashion.  So don't yell at me, I was just being flippant.  Evolution is completely non-goal-oriented; it's the law of whatever works at the time.  So for the benefit of the serious and literal-minded readers, I should probably go back and rewrite the preceding paragraph, but I'm not gonna because I still think it's funny.]

Anyhow, this is a pretty cool example of a process that until now I had honestly never heard of.  It does reassure me that life will indeed find a way despite the idiotic and self-destructive things we're currently doing to the Earth's biodiversity.  We might be long gone by then, victims of our own short-sighted greed, but evolution will afterwards continue to do its thing -- producing a new set of endless forms most beautiful and most wonderful, to quote Darwin.

Some of which, beyond all expectation, may look a great deal like the ones that vanished.  And I find that a comforting thought.

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This week's Skeptophilia book recommendation of the week is a fun one: acclaimed science writer Jennifer Ackerman's The Bird Way: A New Look at how Birds Talk, Work, Play, Parent, and Think.

It's been known for some years that a lot of birds are a great deal more intelligent than we'd thought.  Crows and other corvids are capable of reasoning and problem-solving, and actually play, seemingly for no reason other than "it's fun."  Parrots are capable of learning language and simple categorization.  A group of birds called babblers understand reciprocity -- and females are attracted to males who share their food the most ostentatiously.

So "bird brain" should actually be a compliment.

Here, Ackerman looks at the hugely diverse world of birds and gives us fascinating information about all facets of their behavior -- not only the "positive" ones (to put an human-based judgment on it) but "negative" ones like deception, manipulating, and cheating.  The result is one of the best science books I've read in recent years, written in Ackerman's signature sparkling prose.  Birder or not, this is a must-read for anyone with more than a passing interest in biology or animal behavior.

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

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!]