As many of you know, I will be traveling to the Hawaiian Islands to complete my Young Birder Odyssey big year. I thought this trip would also be a good way to revive my evolutionary biology series that I've skimmed the surface with in my posts on redpolls and feathers, now I'm going to discuss how islands affect the evolution of the animals living there, and I will primarily be using various examples from the Hawaiian Islands to help provide examples of what I discuss, including giant waterfowl, long-legged owls, and of course the native honeycreepers that rival Darwin's finches as an example of adaptive radiation from a common ancestor (sorry if you were expecting me to write a post on them). Because Hawaii unfortunately lacks the dwarf elephants, monitor lizards, tortoises, ratites, lemurs, azdharchids, tiny iguanodonts, and some of the other animals featured in Trey the Explainer's Biology on Islands video, which I have watched numerous times in preparation for this post, so I will borrow examples from Madagascar, New Zealand, Indonesia, the Galapagos, Mediterranean, West Indies, California's Channel Islands, and others to supplement the Hawaiian examples when helpful.
One sixth of all land area on earth is geographically separated from everything else. Unlike continents, islands are small and secluded. This isolation means only a few selective organisms can exist on them, if they can get there in the first place. There are three main ways animals get to islands: by flying or swimming there, by crossing natural land bridges that are now underwater, or as castaways of storms. Many birds were able to fly from the mainland to islands, often blown off course or intentionally, and establish themselves there. Seals are long-distance travelers that in many cases can also swim to islands if they need to. Another interesting case is of the Komodo Dragon (Varanus komodoensis), which is theorized to have originated in Australia and moved north to escape the receding forest habitat as deserts took over following a landbridge to New Guinea and Indonesia. However, the islands inhabited by dragons today were never connected to Australasia by the landbridge, so scientists theorize the dragons colonized them by swimming (they are surprisingly good swimmers). Inclement weather can blow flying animals off course on migration, leaving them stranded on islands as well. This is how the ancestors of the native bird species and subspecies got there, as well as those of the native arthropods and Hawaiian subspecies Hoary Bat. In short, most of the native wildlife in Hawaii got there by accident.
A land-bridge is an area of a continent that is exposed when sea levels are lower, allowing animals to travel between the future islands and the mainland. This is how life has been able to cross between the Americas and Eurasia during the many intermittent periods in history from the Cretaceous to the Pleistocene when the Bering Land bridge connecting Alaska and Russia was open due to lower sea levels. Another interesting land bridge relevant to island biogeography is one that existed in northwestern Europe called Doggerland. During the last ice age, sea levels were lower, and the British Isles were connected to the rest of Europe by a grassy plain called Doggerland that will one day rest at the bottom of the North Sea. These plains supported a variety of European megafauna including mammoths, bison, horses, lions, Megaloceros, rhinos, reindeer, and nomadic humans. Over time, as the climate warmed, these humans and animals were forced to migrate to higher elevations in Britain and the Netherlands as sea levels rose due to melting ice sheets and a tsunami off the coast of what is now Norway.
The rafting theory states that animals sometimes trapped mats of vegetation that blow out to sea during storms, and when they reach the nearest island, they are able to colonize the new land. This is how all of Madagascar's native land mammals got there as well as the ancestors of the iguanas and tortoises in the Galapagos.
Foster's rule, also known as the island rule or the island effect, is a biological rule stating that members of a species get smaller or bigger depending on the resources available in the environment. The rule was first stated by J. Bristol Foster in 1964, in which he compared 116 island species to their mainland varieties. He proposed that certain island creatures evolved into larger versions of themselves while others became smaller. He proposed the simple explanation that smaller creatures get larger when predation pressure is relaxed because of the absence of some of the predators of the mainland, and larger creatures become smaller when food resources are limited because of land area constraints.
The more famous residents of definitely the giants. In the absence of predators or competition for resources, animals living on islands have grown enormous, such as the moas and Haast's Eagles of New Zealand or the Galapagos Tortoises. Despite the presence of kiwis in New Zealand, the closest relatives of the giant moas are a clade of South American paleognaths called tinamous, which are still capable of flight. Despite their size, moas had one predator before the arrival of humans: Haast's Eagle, which had a 3 meter wingspan. Like the moas, they became extinct shortly after humans arrived. The Galapagos Tortoises are the most famous of the living giant tortoises, but another species of giant tortoise, the Aldabra Tortoise, lives on an island northwest of Madagascar with which it shares it's name. Earlier, I mentioned that Komodo Dragons evolved in Australia and moved northward, there was an even bigger monitor lizard related to the dragons to inhabit Australia known as Megalania or Varanus priscus, which became extinct due to climate change along with its preferred prey of giant kangaroos and wombats. Even Hawaii had its own giant flightless birds (a common theme on islands in the Indo-Pacific). In addition to living and extinct species of Hawaiian Goose or Nene, the Southeast islands were home to four species of flightless geese known as Moa-nalos and the Giant Hawaiian Goose (Branta rhuax).
Islands can also decrease the size of the animals living there due to limited resources and space. One of the most nocticable examples are the Channel Island Fox and Pygmy Mammoth, smaller relatives of the Gray Fox and Columbian Mammoth that live on the mainland in California. I've included other examples, such as Giant Anteater-sized ground sloths and pygmy chameleons
Convergent evolution is when two unrelated animals evolve similar appearances in response to similar environments. One example of convergence that I find most fascinating is one of two Island species imitating each other. In the absence of rodents on New Zealand, a group of ratites shrunk to fill the role of nocturnal opportunists. Kiwis traded flight for a longer bill and an enhanced sense of smell to hunt for insects and worms on the floor of the temperate rainforest, at the cost of good eyesight. What I find most interesting, possibly even more interesting, is that Hawaii has its own version of a kiwi! The Kaua'i Mole Duck (Talpanas lippa) was a flightless species of Duck related to modern-day stifftail ducks (Ruddy, Andean, Lake, Maccoa, Blue-billed, and White-headed, genus Oxyura) that like the kiwis and Kakapo, also gave up flight to hunt for smaller animals on the forest floor at night. Talpanas is unfortunately extinct, but if they were still around, finding one would have been on my list of birding priorities once I got there. Some other cases of convergent evolution in Hawaii include stilt-owls (genus Grallistrix) which evolved long legs similar to resemble those of phorusrhacids, Secretarybirds, and giant flightless Cuban owls of the genus Ornimegalonyx (unlike these owls, Grallistrix kept the ability to fly); and the Hawaiian Honeyeaters, which resemble the honeyeaters of Australasia so closely, they were considered to be part of the same family before elevated to full family status (Mohoidae)
New Zealand isn't just home to flightless birds and monstrous raptors, the islands also act as a time capsule from the age of the dinosaurs. Dense forests of tree ferns and podocarps similar to those in the Lord of the Rings series are found on both islands, and are where many episodes of Walking With Dinosaurs were also filmed. These ancient forests are home to two ancient creatures from the Mesozoic: the Giant Weta and the Tuatara, both coincidentally appearing in Spirits of the Ice Forest (episode 5). The Tuatara looks like a lizard, but is from an unrelated order called Rhyncocephala. Competition from lizards elsewhere has driven Tuataras into extinction, and now survives only on a few small islands off New Zealand’s coast.
Adaptive radiation is the diversification of a clade from one common ancestor to fill a variety of niches and exploit the abundance of food sources in their new environment. This is the reason I chose to use Hawaii as my example location in this blog post. Most people typically think of the tanagers of the Galápagos when adaptive radiation comes to mind, but I’ve chosen the Hawaiian honeycreepers not just out of personal bias, but also because their bill shapes reflect a greater divergence than that in the Galapagos and they’re more colorful (reds, greens, and yellows are more appealing than different shades of gray and brown, sorry Darwin). About 4 million years ago, the ancestors of the drepanidid finches, most likely a flock of rosefinches from the Asian mainland based on genetic analysis, was blown or flew naturally to Hawaii. Some of these finches had genes that gave them large grosbeak-like bills, others had those for long, thin bills, and others had genes for short, straight bills. Over time, finches would mate with birds that had bills which would best enable them to feed themselves and provide food for their young until they could only mate with birds of similar bill shape. This is called speciation. In Hawaii, most of the native finches can be divided into five categories: Generalists like the ‘alauahios and ‘Anianiau; nectarivores like the I’iwi, mamos, ‘apapanes, ‘Ākohekohe, and ‘Ula‘aihāwane; frugivores like the Rhodacanthis grosbeaks, koa-finches, palilas, ‘Ō‘ū, Telespiza finches, and Lanai Hookbill; gleaning insectivores like common ‘amakihis, ‘ākepas, ‘Akeke‘e, and Greater ‘Amakihi, and bark-picking insectivores like the ‘Akialoas, nukupu’us, ‘Ākiapōlā‘au, Kiwikiu, ‘Alawī, ‘Akikiki, and Po‘ouli. Some, such as the ‘akialoas, ‘amakihis, and ‘Ula‘aihāwane blur the lines between niches, exploiting multiple roles based on avalibility of food, and the Laysan and Nihoa finches have even been known to eat the eggs of seabirds when seeds and insects are scarce
Living on an island can not only alter the physical appearance of a species, but also their behavior. Island species
However, being less responsive to predators can also work against a species, and that is what I will talk about in part 2...
What? No bird-related stuff? I know this is mostly a birding, bird conservation, ornithology blog, but occasionally I like to discuss other science topics as they relate to birds. Since Jurassic World: Fallen Kingdom will be released a month from today, I've decided to write a blog post on one of its most blatant inaccuracies: featherless dinosaurs. Yup, I'm going to mantle the big question of which dinosaurs had feathers. This will be the first in a series on the connection between birds and non-avian dinosaurs: this one on feathers, several on the evolution of avialans throughout the Jurassic, Cretaceous (including on how they survived the mass extinction), Paleogene, and Neogene periods; one on lesser-known bird families that are now extinct, one on my thoughts on Fallen Kingdom if I ever see it (this most likely will happen), and possibly more. I'm not sure what I will call this mini-series, but I'm pretty sure I might go with "Paleobirding." This wasn't an easy decision to make, as I felt I've had to write this because unlike with most fictional creatures depicted in movies, dinosaurs are real and the general public often accepts inaccurate movie versions as fact without looking at the evidence. I'm writing this to stop those misconceptions from spreading and give an accurate view of how dinosaurs lived.
Before starting, I have one simple request: PLEASE keep discussions in the comments sections civil, I don't want to have to constantly deal with fanboys who don't like to see their childhood movie monsters ruined by science. If the comments get out of hand, I will have to shut this down. Science does not care about your opinions, so just admire the beauty of it. If you don't want to learn about the possibility of all dinosaurs having feathers, then don't read this blog post, or much of this series on dinosaurs in general. For those who don't think feathered dinosaurs are scary, I know many people who vehemently disagree, especially those who have seen cassowaries
Let's start with the big question: what are feathers?
In short, "feathers" are filament-like projections that first evolved from scales. C.M. Kosemen thinks feathers first evolved as sensory organs like whiskers and then diversified. It's unknown when feathers first evolved, but they may have been present in the earliest dinosauriformes. To appease the diehard Jurassic Park fanboys lurking on the internet, I will mention that feathers are actually a highly derived type of scale. The scutes on crocodylians, feathers in dinosaurs, and pycnofibres (the hairlike filaments in pterosaurs) all evolved from the same skin covering. It was originally thought that feathers and scales were made of two different forms of keratin, however, it is now known that the keratin that forms feathers is present in crocodilian embryos. The evolution of feathers is depicted in five stages, based on an analysis of feather evolution in a 1999 paper by Richard Prum. Each of these stages in feather evolution has been found on dinosaur fossils except for stage 3, which is known from cretaceous amber.
Feathers can be broken down into seven different structures:
Numerous speculative theories have been proposed on the purpose of feathers: The first one is that down and semiplume feathers (the feathers present in ratites) were used to regulate temperature (feathers are more efficient at trapping and shedding heat than hair is, according to a study on Red Kangaroos and Emus); which would most benefit dinosaurs living polar and alpine regions (see figure 2) or in deserts (Madagascar, Negmet). Wing and tail feathers were then likely used for courtship displays similar to ratites, pheasants, and birds of paradise, although most evidence of sexual dimorphism in non-avian dinosaurs is not conclusive (see the clip from Dinosaur Revolution featuring a pair of courting Gigantoraptor I attached as a speculative example). Pennaceous feathers would have been used on avialans and small dromaeosaurs like Microraptor and Sinornithosaurus to glide from branches, but this was not powered flight as they could not flap their wings. Powered flight would later evolve in enatiornith birds with the appearance of a breastbone or keel for flight muscles to attach to and give birds more lift when flying.
Now, let's talk about the direct evidence of feathers in each of the dinosaur groups, but before we dig in, it's worth mentioning that the traditional phylogeny of dinosaurs lists two branches: Saurischia (sauropodomorphs, herrerasaurids, and theropods) meaning "lizard-hipped" and Ornithischia (ornithopods) or "bird-hipped." However, in 2017, Matthew Baron, David Norman and Paul Barrett proposed that theropods were more closely related to ornithischans than to sauropods and herrerasaurs, leading to the formation of a new clade called Ornithoscelida.
Lastly, despite what many clickbait articles and videos on dinosaurs would say, pterosaurs are NOT dinosaurs. I will devote a full section of Paleobirding to the differences between birds and pterosaurs.
Part of the rationale of Baron et al for transferring theropods out of Saurischia and into the new Ornithoscelida is that herrerasaurids and sauropodomorphs noticably lacked any preservation of feathers at all. Quite the opposite was found on sauropods, in fact, since many scale impressions have been found that have been attributed to sauropod species. In other words: no feathers on sauropods
While most of the dinosaur groups (aside from sauropods) will be handled individually, I will address Ornithischia as a whole. The first ornithischian to be found with feathers was Psittacosaurus in 2002 (the genus has been known since 1923, for anyone who was wondering), which preserved bristle-like filaments on the tail. Then in 2009, a heterodontosaurid called Tianyulong which had long filaments on the back, tail, and neck of the animal. Originally, these two species led scientists to think feathers evolved in the (at the time of this discovery) two dinosaur groups independently. Then, in 2014, the discovery of a basal neornithischian found in Siberia was announced. Kulindadromeus, as the dinosaur would be called, suggested that the common ancestor of dinosaurs was feathered (except for sauropods and herrerasaurs, which may not be dinosaurs if you define a dinosaur by the presence of feathers).
When it comes to feather preservation, non-coelorusaurian theropods, which include coelophysids, allosaurs, megalosauroids, and ceratosaurs, are not as well studied. The arms of the carnosaur Concavenator had structures resembling quill knobs, but these may be attachment points for ligaments and not related to feathers at all. Sciurumimus is another interesting species, as the classification is not decided on. If it is a megalosauroid, that would support the hypothesis that this group of dinosaurs had feathers.
Tyrannosaurs were a family of theropods that first appeared in the Jurassic as medium-sized carnivores, and later evolved into the apex predators of Asian and North American formations during the late Cretaceous. Big tyrannosaurs like Tyrannosaurus and Tarbosaurus don't need an introduction, but some of their earlier relatives do. Two tyrannosaurids have been discovered with direct evidence of feathers: Dilong and Yutyrannus, both of which lived in China during the lower Cretaceous. These two species are important because Dilong provided evidence that tyrannosaurs had feathers in the first place, and Yutyrannus confirmed that even large tyrannosaurs likely had feathers. Another thing to note about Yutyrannus is that the Yixian formation, where it and many other feathered dinosaurs lived, would have been very cold during the Early Cretaceous when these animals lived
What about Tyrannosaurus rex? Once the discovery of Yutyrannus was announced in 2012, many people were scared that not even T. rex was safe from getting the feather treatment, leading to the "science ruined dinosaurs" movement. These fears were alleviated when a paper examining several tiny (the largest is 30 square centimeters across) scale impressions on the back of the neck, pelvic region, and tail of the well-preserved BHI 6230 specimen or Wyrex, claimed to have found conflicting patterns between gigantism in dinosaurs and feather integument and concluding that Tyrannosaurus was mostly scaly. When popular media outlets reported about the paper, they sensationalized the study by claiming it marks a return to the T. rex of Jurassic Park and that they were "still lizards" after people had "gotten used to the idea of giant fluffy killer birds." While it is true that rex would have been mostly scaly, these scales would give the animal a leathery appearance because of how small the scales are. Bell et al interpreted this find by suggesting more derived tyrannosaurs likely lost or did not have the filaments of their basal Asian relatives. The distribution of these scales lends support to the position that these animals were mostly scaly or featherless as adults, but does not mean they were featherless at all growth stages. The paper suggests that they might have possessed a feather cape or mohawk on the upper part of their body. These scales might actually be feathers, as the paper notes the scaly feet of modern birds are actually feathers that secondarily evolved back into scales. The authors suggest that this might have been the case with Tyrannosaurus, and as Mark Witton notes, everyone wins the scaly vs feathered debate. This opens up a variety of possibilities: Witton notes that avian skin is more dynamic than reptilian skin, and allows for tons of variations based on the animals life stages and time of year, changing between feathers and scales with the seasons. This could mean T. rex was born with feathers but lost them as it got older, or it could have grown a coat of feathers as an adult in fall and molted this coat in spring. In conclusion, the Bell paper concludes that T. rex would have been largely filamentless in life and would have possessed a leathery or smooth appearance, and does not disprove that it was completely featherless in all stages in life.
Before you accuse me in the comments of ruining your childhood hero (which I will probably delete), I will add that in the unlikely event of T. rex and Spinosaurus meeting, T. rex would likely win in a fight, either crushing Spino's neck with those bone-crushing jaws that deliver the highest bite force of any known animal or scaring the fish eater back into the water.
Sometimes known as "ostrich dinosaurs" for their resemblance to modern ratites, ornithomimids are a group of omnivorous theropods that lived during the Cretaceous and were prey for many large predators. To add to their resemblance to modern ostriches, we have found two ornithomimids with preservation of feathers: Ornithomimus and a pygostyle on the extremely large and unusual Deinocheirus.
Compsognathids, or "compies" as they are sometimes known, are small theropods that lived from the late Jurassic to the early Cretaceous and were small predators of insects, lizards, and early mammals. One species of compy, Sinosauropteryx, had preserved feathers so well, we even know what color it was! Microscopic pigment cells called melanosomes on the fossil of Sinosauropteryx was analyzed to find it had a reddish brown coloration like a fox or Red Panda, as well as a banded tail like many procyonids (raccoons, coatis, ringtail) or a Ring-tailed Lemur. To add to the resemblance to raccoons, analysis of the fossil found that Sinosauropteryx had a bandit mask over its eyes.
Therizinosaurs were large theropods that were most likely herbivorous, and used their long claws to hold branches closer to their mouths like pandas or sloths rather than to disembowel prey with. We know from many therizinosaur species that they had filamentous intigument covering their entire bodies, even the biggest ones. One of these, Beipiaosaurus, is one of the largest dinosaurs with direct evidence of feathers second only to Yutyrannus.
Oviraptorosaurs (or "chickenparrots" as paleontology fans sometimes call them) are dinosaurs that take the bird-like appearance of the ornithomimids a step further; looking like a chimera of a parrot, a galliform, and a non-avian dinosaur. Many species of oviraptorids have been found with direct evidence of feathers preserved such as wings and pygostyles. This means that all oviraptorids definitely had feathers, even the biggest ones like Gigantoraptor, although it can be argued from Bell 2017 that like Therizinosaurus, Gigantoraptor would have considerably reduced feathers, but not completely lost them.
Alvarezasaurs are small insect-eating theropods that due to their close relationship to other maniraptorans, we know definitely had feathers.
Scansoriopterygids are a testament to the amazing ability non-avian dinosaurs had to evolve and fill every niche imaginable, as many members of this group had a long finger used to probe trees for insects like that of an Aye-aye or the tongue of a woodpecker. All scansoriopterygids had four long feathers on the tail, composed of a central rachis and vanes. However, unlike in modern-style tail feathers, the vanes were not branched into individual filaments but made up of a single ribbon-like sheet. They also had simple feathers covering the body like many dinosaurs.
More famously known as "raptors," dromaeosaurids are birdlike theropods that lived during the Cretaceous period. Numerous species of dromaeosaur have been found with direct evidence of feathers, including complete feather preservation in Microraptor, Zhenyuanlong, and Sinornithosaurus, as well as quill knobs on Velociraptor, Rahonavis, and Dakotaraptor. In short, all dromaeosaurs regardless of how big they are had feathers preserved
What about Velociraptor, the antagonist of the first three films and semi protagonist of Jurassic World? Things are not looking good for this "walking medieval torture machine," as the JPlegacy website called the movie versions of this species, which are closer in size to Utahraptor and Achillobator than an actual Velociraptor, which was the length of a mountain lion and weighed as much as a turkey (in Michael Crichton's defense, he followed Gregory S. Paul's dromaeosaur taxonomy, which lumped Deinonychus, Saurornitholestes, and others into Velociraptor). Basically the fossils of many dromaeosaurs, as well as quill knobs on Velociraptor itself, mean this genus was undeniably feathered. As Brian Switek says, "A Velociraptor without feathers isn't a Velociraptor."
Troodontids are more closely related to birds than their cousins the dromaeosaurs. We know from the remains of Jinfengopteryx that troodontids had filamentous feathers covering their body and pennaceous feathers on the wings and tail
Lastly, we get to modern birds, which all are undisputedly feathered.
To summarize, heterodontosaurids, basal neornithischians, ceratopsians, and most ceolorusaurian theropods likely had feathers, while ceratosaurs (a group of theropods), sauropods, ornithopods, thyreophorans, and pachycephalosaurs did not. Coelophysids and carnosaurs are uncertain in their feather preservation, but would most likely have had quill knobs. Basal ceratopsians like Psittacosaurus most likely had elongate cylindrical integument analagous to feathers, which larger ceratopsians would have lost. Heterodontosaurids, basal neornithischians, early tyrannosaurs, comsognathids, and early tyrannosaurs had filamentous integument covering their entire bodies. Oviraptorosaurs and scansoriopterygids had complex feathers of multiple stages following the Prum model. Lastly, deinonychosaurs and avialans all had multiple stages of feathers including asymetrical flight feathers.
This post was inspired by a presentation given by Garret Van Gelder at the NYSYBC 2018 Kickoff Meeting, as well as the work of Nick Turinetti and Tom Parker for Saurian, a open-world video game where you play as a dinosaur and try to survive, and Trey the Explainer's "Which Dinosaurs Had Feathers?" and "Did T. rex have feathers?"
All copyrighted images belong to their respected owners. Please notify me if I neglected to credit your work. All copyrighted images in this post are protected under FAIR USE for reasons of Commentary, Education, Criticism, Parody, and Social Satire.
All About Feathers, academy.allaboutbirds.org/features/all-about-feathers/.
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Short answer: Yes! Long answer: Well, it's very complicated so let me explain it in this post...
Oh man, this is it, this is the blog post. I've been talking about and putting off writing a blog post on this subject that people have both been hoping for and others have been dreading my Darwin Day special on Nicholas Mason and Scott Taylor of the Cornell Lab’s Fuller Evolutionary Biology Program's paper on redpoll speciation in Molecular Ecology. This paper basically says that Common, Lesser, and Hoary Redpolls are all the same species based on their genetic evidence. I'm writing this post not because I'm a sadistic lumper that wants to steal lifers, but because I want to increase knowledge about a question that has plagued birders and ornithologists since the Civil war, but now we finally have an answer. For some reason with these three animals in particular, an annoying phenomena has occured and it's people being stubborn with how many species there are based on the scientific evidence, so to make it up to the dissenting party, I'm going to talk about some antitheses to the "one to rule them all" doctrine many have claimed over the question of how many species redpolls are in order to appease that part of my audience. I will be presenting my unbiased scientific opinion, without bringing any of my personal feelings into this discussion whatsoever.
Before I go into depth about redpolls, it's helpful to define the major species concepts that taxonomists use:
The division of redpolls into different species dates back to before the Civil War. In 1861, legendary ornithologist Elliot Coues (one of the founding fathers of the AOU) described eight separate redpoll species based on their visual appearances. Over time the AOU consolidated Coues’ list, but Hoary Redpoll, which has a snow-white breast, was still considered a separate species from Common Redpoll, which has a brown-streaked breast.
According to Kenn Kaufman several studies have documented the genetic similarities between Common and Hoary Redpolls since the 1980s. No study has examined more than 11 regions of the redpoll genome, making ornithologists wonder whether they were missing the true differentiating genetic markers. Until now...
Mason and Taylor looked beyond the plumage into strands of the birds’ DNA in the most extensive look ever at the redpoll genome. Whereas previous genetic analyses of redpolls looked at just 11 regions of the genome (at most), Mason and Taylor examined 235,000 regions, comparing DNA from 77 redpolls, including specimens from museums around the world. They found no DNA variation that distinguishes Hoary Redpolls from Common Redpolls. Furthermore, another redpoll species found in Europe—the Lesser Redpoll—also had extremely similar DNA sequences. This extreme similarity among all the redpolls stands in marked contrast to studies of other groups of birds—such as Black-capped and Carolina Chickadees—which show differences at many regions of the genome. One of the key differing factors among distinct species is assortative mating, or members of a group breeding with each other more than another group. According to Mason, “There are no clear-cut genetic differences, which is what we would expect to see if assortative mating had been occurring for a long time.” Instead, Mason says the world’s three redpoll species seem to be “functioning as members of a single gene pool that wraps around the top of the globe.”But how could it be that Hoary and Common Redpolls look so different given that their genetic makeup is basically the same? For that answer, Mason and Taylor delved into the birds’ RNA. (A quick flashback to high-school biology: If DNA is like the body’s blueprints, RNA is like the construction foreman communicating the instructions to build physical features, like hair or feathers.)
The physical differences among redpolls are associated with patterns in their RNA, not their DNA. In other words, the variation we see in plumage and size is probably not a matter of genetic variation, but of genetic expression. It’s kind of like how two humans might have the same gene for brown hair, but one person’s might be lighter than the other’s—that gene is being expressed differently. In the same way, Hoary and Common Redpolls have remarkably similar sets of genes, but those genes are expressed differently, causing the plumage and bill-shape differences we see. Mason has said “We didn’t find distinct characteristics to separate the redpoll types, but rather a continuum, or a progression, of physical traits, and many redpolls were somewhere in the middle.” Interesting indeed, considering that Redpolls with intermediate plumage are common in certain areas, like north-central Alaska, and are often impossible to ID.
Redpolls follow a gradient of streaking on the flanks from the most streaks on flammea to the least on hornemannii. Then you have intermediate birds like the one in the center which can be real nightmares to ID. This could be either a dark HORE, a light CORE, or a hybrid, which if we use the Biological Species Concept to define a species, would support lumping them. Photos, from left to right, by Sharon Watson, newfoundlander61, Guy Lichter, Stuart Oikawa, and Chris Wood via Flickr.
While the evidence supporting Redpolls as a single species is convincing, it was not mentioned in the 2016 AOS Check-list supplement and rejected in the 2017 supplement (the same tragic year that Thayer's Gull was lumped), to the relief of many listers whom were already annoyed that they lost one tick with Thayer's gone and Yellow-rumped Warbler and Willet not being split. Kenn Kaufman can’t imagine there being too much conflict over consolidating the redpolls. “In recent years there have been more ‘splits’ than ‘lumps,’ so if continuing taxonomic work occasionally takes one away, it’s not that big a deal.” Johanna Beam, a young birder studying at St. Olaf university in Minnesota, has actually spoken to Scott Taylor, one of the authors of the paper, and on the topic of the lump, she has said: "but sadly he (Scott Taylor) thinks they're not going to accept it until they get actual breeding data from the arctic, which is some nasty work because of funding, mosquitoes, lodging etc up there."
It's not over yet, Canadian finch expert Ron Pittaway notes that the AOU recently rejected a similar proposal, also based on genetic evidence, to lump the three North American species of rosy-finches, which breed in high mountain areas of western North America,.
To summarize, the consensus on if Common, Lesser, and Hoary Redpolls are all the same species is yes simply based on the Phylogenetic Species Concept I talked about earlier and DNA analysis conducted by Mason and Taylor. I have asked multiple respected people in the birding community for their thoughts and they mostly agree with me. You can choose whether you agree or disagree with me, but for now I offer this quote on the study by Scott Taylor himself: “I think this makes them a more interesting bird, it means they’re part of an exciting, complicated system that can make a single species look different across different parts of its range." Keep in mind that if you do disagree with me, this is opinion purely based on evidence.
I would like to thank Tessa Rhinehart and William Von Herff for giving me and supporting the idea to write about this complex subject, as well as Sam Bressler and Ethan Gyllenhaal for helping me find access to the original Mason and Taylor paper (which was unfortunately behind a paywall); this post couldn't exist without your help. I also would like to acknowledge the amount of work Nicholas Mason and Scott Taylor put into their study, as well as Gustave Axelson and Jesse Greenspan for writing concise summaries of the study on the Cornell Lab and Audubon websites respectively. I lastly want to thank Kenn Kaufman, Ron Pittaway, Lindsey Duval, John Puschock, Ryan Zucker, Ethan M, Johanna Beam, Nathan Martineau, Jerald Reb, Jared Gorrell, Ben Sanders, Alex Sundvall, Joseph Kurtz, Greg Neise, Dominic Garcia-Hall, TIm Swain, Ise Varghese Mac, Sharon Steitler, Aidan Place, Alberto Lobato, Brian C. Johnson, Alvaro Jaramillo, and many other birders for offering their diverse opinions on this subject.
Thanks for reading and have a happy Darwin Day!
All copyrighted images belong to their respected owners. Please notify me if I neglected to credit your work. All copyrighted images in this post are protected under FAIR USE for reasons of Commentary, Education, Criticism, Parody, and Social Satire.
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