Saturday, January 22, 2022

New (Extinct) Maniraptors of 2021

By my count, 42 new species of extinct maniraptors were named in 2021, which is about par for the course these days. Let's take a quick look at these new taxa and other nomenclatural proposals.

New alvarezsaurs have been described at a fairly steady trickle in recent years, and 2021 gave us Khulsanurus, known from a partial skeleton from the Late Cretaceous Barun Goyot Formation of Mongolia. It was discovered at the same locality as Parvicursor, but can be distinguished based on vertebral anatomy.

No new oviraptorosaur species were described last year; in fact, they may have lost a member. A new specimen of Elmisaurus preserves overlapping material nearly identical to Nomingia, lending credence to the hypothesis that the two are synonymous (in which case the name Elmisaurus would take priority).

Non-ornithothoracean Paravians
2021 was a good year for dromaeosaurids, with the velociraptorines Shri (formerly known by the nickname "Ichabodcraniosaurus") and Kuru (formerly known by the nickname "Airakoraptor") both described based on partial skeletons from the Barun Goyot Formation. Less completely known are Kansaignathus (based on a partial jaw and possibly teeth from the Late Cretaceous Yalovach Formation of Tajikistan), Vectiraptor (based on partial vertebrae from the Early Cretaceous Wessex Formation of the United Kingdom), and Ypupiara (an unenlagiine based on jaw fragments from the Late Cretaceous Serra da Galga Formation* of Brazil). Tragically, the type specimen of Ypupiara was among the fossils lost in the 2018 fire that destroyed the main building of the National Museum of Brazil.

*Ypupiara was described as being from the Marília Formation. However, it was recently proposed that the classic Marília Formation should be split into the separate Marília and Serra da Galga Formations.

Holotype of Shri, from Turner et al. (2021).

Troodontids, on the other hand, got Papiliovenator from the Late Cretaceous Wulansuhai Formation of China, known from a partial skeleton including a nearly complete skull. Its name translates (I think quite adorably) into "butterfly hunter", not based on any inference about its ecology, but on the "butterfly-like" shape of its dorsal vertebrae. There was also Tamarro from the Late Cretaceous Talarn Formation of Spain, known from a partial metatarsal. Meanwhile, a detailed reassessment of small theropods from the Dinosaur Park Formation concluded that Latenivenatrix cannot be reliably distinguished from Stenonychosaurus, and that the two should be (re-)synonymized.

Skull of Papiliovenator, from Pei et al. (2022). (The paper was released as an advance online publication in 2021.)

Last year also saw the description of a new Jeholornis-like avialan, Neimengornis, from the Early Cretaceous Jiufotang Formation of China. It appears to be known from an essentially complete skeleton. However, there have already been suggestions that the type specimen is likely a chimera.

As is often the case, the Jiufotang Formation also contributed some new enantiornitheans in 2021, two to be precise. One of these was Brevirostruavis, which is notable for preserving an elongated hyoid, suggesting that it may have used its tongue for handling food. Although the original description compares this condition to that seen in hummingbirds and woodpeckers, the hyoid elongation seen in Brevirostruavis does not look to my eye quite as extreme as that of those crown birds. My guess would be that Brevirostruavis might have used its tongue to help pick up food and manipulate it within its mouth, but did not use a feeding strategy that involved protruding the tongue very far beyond its snout.

The other new Jiufotang enantiornithean was Yuanchuavis, which preserves a set of eight tail feathers forming a fan-like array. This contrasts with the typical condition seen in most other enantiornitheans (which tended to either lack large tail feathers or have only a single pair of them), but resembles that of Chiappeavis. Unlike Chiappeavis, the central pair of tail feathers in Yuanchuavis was particularly elongate.

Personally, I think one of the most scientifically important new maniraptors to be named last year was Yuornis from the Late Cretaceous Qiupa Formation of China. It is based on a partial skeleton with an essentially complete skull preserved in three dimensions, a rarity for avialan fossils. It is also one of the few enantiornitheans known to have had entirely toothless jaws.

Holotype of Yuornis, from Xu et al. (2021).

A fourth enantiornithean to come out of 2021 was Fortipesavis, known from a foot preserved in Late Cretaceous Burmese amber that had been previously described in 2019.

Non-neornithean Euornitheans
Two non-neornithean euornitheans were named in 2021 based primarily on cranial material, these being Brevidentavis and Meemannavis from the Early Cretaceous Xiagou Formation of China. The type specimen of Brevidentavis was previously reported as the skull of Gansus, but a more likely candidate for an actual Gansus skull was described in the new paper. The lower jaw of Brevidentavis exhibits unusually short, blunt teeth set in a groove (instead of in sockets), whereas Meemannavis has a toothless lower jaw.

Holotype of Brevidentavis, from O'Connor et al. (in press).

One Mesozoic avialan named last year that was not discovered in Asia was Kaririavis from the Early Cretaceous Crato Formation of Brazil. Based on a partial foot, it was unusual for a Cretaceous euornithean in being particularly small (around the size of a sparrow) and in that its one preserved toe claw was large and strongly curved.

Perhaps one of the biggest overhauls in dinosaur systematics in 2021 was the reinterpretation of geranoidids, eogruids, and ergilornithids as stem-ostriches instead of gruiforms. Although a close relationship between ergilornithids and ostriches had been suggested in the 1950s and contemplated more recently in light of further data on fossil paleognaths, anatomical information from newly described eogruid and ergilornithid specimens seems to strongly bolster this hypothesis. In their revision of these birds' affinities, the authors additionally resurrected the genus Proergilornis, which had been previously synonymized with Ergilornis, and suggested that it was less closely related to extant ostriches than ergilornithids proper.

A new fossil paleognath was also described last year, the kiwi Apteryx littoralis, based on a tarsometatarsus from the Pleistocene of New Zealand. Kiwi fossils are rarely found, and this species is the first one known from the early Pleistocene.

A couple of extinct total-group anseriforms were named in 2021. One of these was the small presbyornithid Bumbalavis from the Eocene Naran-Bulak Formation of Mongolia. (Meanwhile, the type specimen of "Presbyornis" mongoliensis from the same locality was reinterpreted as a stem-mirandornithean similar to Juncitarsus.) The other was the stiff-tailed duck Manuherikia primadividua from the Miocene Bannockburn Formation of New Zealand, the fourth species to be named in the genus Manuherikia. It appears to be stratigraphically separated from the older M. lacustrina.

Two small, early total-group galliforms were described from the Naran-Bulak Formation last year as well, these being Bumbanipodius and Bumbanortyx.

New fossil members of Strisores don't get described every day, or every year for that matter. (The last one I recall was Cypseloramphus from 2016.) That's why I was excited to see the publication of Archaeodromus from the Eocene London Clay Formation of the United Kingdom, known from a partial skeleton. This new taxon is a member of Archaeotrogonidae (formerly thought to be a group of stem-trogons, as their name suggests, later reevaluated as strisoreans), and not only provides new anatomical information on this group, but also suggests that they may be stem-nightjars. The absence of clear examples of Eocene stem-nightjars had been a conspicuous gap in the known fossil record of Strisores, so the common but enigmatic archaeotrogonids filling that space seems like a tantalizing possibility. It would also imply that my earlier hypothesis that the putative archaeotrogonid Hassiavis was a stem-owlet-nightjar is probably wrong, but that's the way science goes sometimes.

Shoulder and forelimb bones of Archaeodromus, from Mayr (2021).

The Naran-Bulak Formation gave us two more new fossil birds in 2021, the possible gruiforms Bumbanipes and Bumbaniralla. Bumbanipes appears to have been a specialized swimming form with morphological similarities to the limpkin (Aramus guarauna) and finfoots, whereas Bumbaniralla resembles messelornithids. Additional new entrants in the extinct gruiform department were the crane-like Palaeogeranos, based on a coracoid from the Oligocene of France, and the recently extinct rail Gallirallus astolfoi, based on a tarsometatarsus from the island of Rapa Iti in French Polynesia. Recent revisions in the generic assignment of recent rails have restricted the genus Gallirallus to the weka (G. australis) and sometimes the New Caledonian rail (G. lafresnayanus), in which case it wouldn't surprise me if G. astolfoi was transferred to a different genus in the future, perhaps Hypotaenidia. In fact, reassignment to Hypotaenidia was proposed last year for another recently extinct rail, the Chatham rail ("Cabalus" modestus), in one study on rail phylogeny.

No new extinct charadriiforms were named in 2021, but a study on the original illustrations used to describe the Moorea sandpiper (Prosobonia ellisi) supported synonymizing it with the Tahiti sandpiper (P. leucoptera).

There has still been no formal proposal to name the clade uniting Phaethontimorphae and Aequornithes, but last year the name Feraequornithes was coined for the group including most aequornitheans other than loons. As it happens, all of the extinct members of the "waterbird clade" that were named in 2021 belong to Feraequornithes.

The excellent fossil record of total-group penguins continued to provide, with new taxa in the form of the giant Kairuku waewaeroa, based on a partial skeleton from the Oligocene Glen Massey Formation of New Zealand, and the smaller Marambiornopsis, based on a tarsometatarsus from the Eocene Submeseta Formation of Antarctica. Perhaps more surprising was the description of a new fossil petrel based on a well-preserved skeleton, Procellaria altirostris from the Pliocene Tangahoe Formation of New Zealand.

Holotype of Procellaria altirostris, from Tennyson and Tomotani (2021).

Among pelecanimorphs, Eopelecanus was described as the oldest known stem-pelican based on a tibiotarsus from the Eocene Birket Qarun Formation of Egypt, whereas a revision of plotopterid specimens from the Paleogene of Washington State resulted in "Tonsala" buchanani being reassigned to Klallamornis.

Although not as many new raptors were named last year as there were in 2020, they were still pretty well represented. Arguably the most exciting was Archaehierax, known from a partial skeleton from the Oligocene Namba Formation of Australia. It was nearly as big as the wedge-tailed eagle (Aquila audax, the largest bird of prey in Australia today), and phylogenetic analyses (which have otherwise almost never been done on fossil accipitrimorphs!) suggest that it was a crown accipitrid, but not especially closely related to any living species.

Tarsometatarsus of Archaehierax, from Mather et al. (in press).

The other new fossil accipitrids of 2021 were also quite large species, Buteo dondasi from the Pliocene Chapadmalal Formation of Argentina and Buteogallus irpus from the Pleistocene of the Dominican Republic and Cuba. B. irpus is notably based on specimens formerly assigned to Titanohierax (now restricted to fossils from the Bahamas) and "Amplibuteo" woodwardi (now restricted to fossils from the United States). Its description further suggested sinking members of the genus Amplibuteo into Buteogallus, which had been foreshadowed in previous papers. Another new raptor was the owl Margarobyas abronensis from the Pleistocene of Cuba, a close relative of the poorly-known bare-legged owl (M. lawrencii) that still lives in Cuba today.

Among coraciimorphs, there was Ueekenkcoracias from the Huitrera Formation of Argentina, based on a partial hindlimb. It was described as a stem-member of the group uniting ground rollers and rollers, in which case it would be the first representative of this clade known from South America. However, a later study argued that Ueekenkcoracias more closely resembles the enigmatic Eocene bird Palaeopsittacus, in which case it may not be a telluravian at all. Probably less controversial are the three Pleistocene woodpeckers described from the La Brea tar pits (listed in order of decreasing body size), Breacopus, Melanerpes shawi, and Bitumenpicus.

The London Clay Formation had a good year as far as new fossil birds were concerned, with Tynskya waltonensis revealing new details about messelasturid morphology, though the phylogenetic affinities of these parrot-like birds remain mysterious. Perhaps even more remarkable was the psittacopedid Parapsittacopes, known from a very well-preserved partial skeleton that sheds new light on the anatomy of these stem-passeriforms. It further adds to the already impressive ecological diversity of psittacopedids, exhibiting a slightly widened beak that may have allowed it to feed on fruits and flying insects.

Skull of the holotype of Parapsittacopes, from Mayr (2020). (The print version of the journal retroactively dates the paper to 2020, but really, the paper was first released in 2021.)

Last but not least, a few fossil crown passeriforms were also described in 2021, the possible suboscine Crosnoornis, based on a nearly complete skeleton from the Oligocene of Poland, and the magpie Pica praepica from the Pleistocene of Bulgaria.

Type specimen of Crosnoornis, from Bochenski et al. (2021).

Saturday, January 1, 2022

Review of 2021

It appears that I've set a new record for an all-time low in annual post count on this blog (beating out last year). The main reason for this is pretty clear: 2021 was the year that I was supposed to turn in my PhD thesis. As of the time of writing, the deed has been done, so the main task left before I can earn my degree is to pass my viva (which will presumably be held within the next few months). I still need to do a fair amount of work to prepare my final thesis chapter for publication, not to mention think about what I'll do after I graduate, but the most grueling part of the PhD might just be behind me. Does that mean I'll resume a (more) regular posting schedule here? Well, I'd like to, but I'm making no promises...

However, writing up my thesis did not completely halt my other activities (which was a good thing... I think). On the academic side of things, I attended and presented at a few (virtual) conferences and co-authored a paper on online science outreach. I also received the immense honor of consulting for the educational studio Kurzgesagt on several of their projects, including a poster depicting the tree of life, a poster about the last non-avialan dinosaurs, a video on paleoartistic depictions of extinct animals, and their calendar for 2022 (which features prehistoric life). The research team at Kurzgesagt was an absolute pleasure to work with, and I came away from each project feeling like they made a very dedicated and honest effort to consider all of my feedback.

Kurzgesagt's "Map of Evolution" poster depicting the tree of life. This was the first project that I worked with them on and I'm very pleased with the final result.

My friend Joan Turmelle and I have continued to run our YouTube channel Through Time and Clades. Our biggest accomplishment so far, I think, is that we have completed both of the long-form lecture series that we set out to make: Joan's "Humanity, a Prologue" (covering human origins) and my "Dinosaurs, the Second Chapter" (covering crown bird evolution), with plans to release annual updates incorporating new research from our respective fields. Although I'd be the first to say that my videos are far from ideal in some ways (for example, I know that the multi-hour length of some episodes can be a real deterrent), don't let it be said that I haven't tried to make information on the evolutionary history of Cenozoic birds available in a reasonably accessible and comprehensive manner. We are also working on a companion website that will present the material from our lecture series in what we hope will be a more approachable format for some, though that is in early stages still. Another pleasant surprise for our channel last year was that we received an invitation to participate in Paleo Rewind, an annual collaboration among paleontology-focused YouTube creators to recap the year in paleontological discoveries.

A collage of title slides from my YouTube series "Dinosaurs, the Second Chapter", in which Joan and I discussed the origins, evolution, and diversity of crown-group birds.

Astonishingly, I was even able to start a new personal project last year! That was the blog New Dinosaur Alert, on which I write a brief post for each new genus or species of dinosaur described (including extant birds). Despite everything else going on, I've managed to stay on top of that blog for the most part, so I intend to continue it in the foreseeable future.

When I picked Velociraptor as the primary basis of the logo for New Dinosaur Alert, I did not know that the first Mesozoic dinosaur to be described in 2021 would be the velociraptorine dromaeosaurid Shri devi. That was a happy coincidence!

Lastly, I didn't expect to enjoy rewatching a show from my childhood as much as I did, but I'm glad that I was inspired to do so.

... And that's more than enough about me. Let's take a look at what 2021 had to offer in the world of maniraptoran research. As always, my coverage of papers about modern birds is necessarily going to be incomplete, so I put more focus on those that have more direct connections to paleontology, such as studies on anatomy, ontogeny, and higher-order phylogeny.

In January, oilbirds were found to disperse seeds across longer average distances than megafauna. A specimen of Pachystruthio from the Nihewan Formation was described. The skull morphology of phorusrhacids and the language-like capabilities of Japanese tits were reviewed. New studies came out on the hindlimb musculature of Nothronychus, the evolution of tooth shape in avialans and coloration and song in American warblers, and the phylogeny of neoavians and shearwaters. Newly-named maniraptors included the dromaeosaurid Shri devi, the Pliocene petrel Procellaria altirostris, the Eocene possible stem-coracioid Ueekenkcoracias tambussiae, the Pleistocene woodpeckers Bitumenpicus minimus, Breacopus garretti, and Melanerpes shawi, and the psittacopedid Parapsittacopes bergdahli.

Skull of the holotype of Parapsittacopes bergdahli, from Mayr (2020). (The print version of the journal retroactively dates the paper to 2020, but really, the paper was first released in 2021.)

In February, southern giant petrels were reported preying on Atlantic yellow-nosed albatrosses. Purported gastroliths in Bohaiornis were reinterpreted as mineral precipitate (as had been previously suggested). The mineralization of avian eggshells was reviewed. A tinamou egg from the Dolores Formation was described. Macrornis was redescribed as a possible phorusrhacid (though a dubious taxon). Male superb lyrebirds were found to mimic the sounds of mobbing flocks during courtship. New studies came out on the evolution of disparity in Mesozoic avialans, the diversification of avialans, atavisms in the avian hindlimb, the structure of kiwi eggshells, the phylogenetic position of Brontornis (favoring galloanseran affinities), recent extinctions of eastern North American birds, the phylogeny of galliforms and potoos, the ontogeny of locomotion in chukars and hindlimb muscle mass in Cabot's tragopans, skeletal pneumaticity in cuckoos, the biogeography of rails, the cranial anatomy of Spheniscus urbinai, the offshore behavior of Whenua Hou diving petrels, the relationship between male-biased sexual selection and speciation in passeriforms, and the use of alarm calls in yellow warblers. Newly-named maniraptors included the Oligocene passeriform Crosnoornis nargizia.

Displaying male superb lyrebird and spectrograms comparing the sounds of a mobbing flock to a lyrebird's mimicry thereof, from Dalziell et al. (2021).

In March, the phylogenetic position of Nesotrochis was evaluated based on ancient DNA, recovering it as a stem-flufftail. The regionalization of avian integument was reviewed. Soft tissues were reported from an ostrich from the Liushu Formation. Vegavis and Columba congi were redescribed. The name Feraequornithes was coined for the clade uniting most aequornitheans other than loons. A raven skull from the Pleistocene of China was described. New studies came out on the pelvic musculature of maniraptors, the tail anatomy of alvarezsaurs, the forelimb musculature of Nothronychus and aquatic birds, the evolution of dentition in avialans, the bone histology of Mirarce, the evolutionary versatility of the avian neck, the factors influencing the ease of puncturing avian eggshells, the endocranial anatomy of dromornithids and piciforms, the phylogeny of sea ducks and leaf warblers, the skeletal elements of penguin eyes, and the correlation between cooperative breeding and longevity in birds. Newly-named maniraptors included the troodontid Tamarro insperatus, the Alagoas screech-owl (Megascops alagoensis), the Xingu screech-owl (Megascops stangiae), the Alagoas black-throated trogon (Trogon muriciensis), and the messelasturid Tynskya waltonensis.

Phylogenetic tree showing the position of Nesotrochis, from Oswald et al. (2021).

In April, a juvenile specimen of Archaeorhynchus was reported. A large caenagnathid from the Hell Creek Formation, a giant euornithean from the Tremp Formation, birds from the Nanjemoy Formation, a pheasant from the Chi-Ting Formation, and a petrel from the Gaiman Formation were described. The evolution of the avian chondrocranium and species limits in birds were reviewed. Red blood cell mitochondria in birds were shown to produce more heat in winter than in fall. The ecological consequences of the extinction of Chendytes were investigated. The feather microstructure of male Ramphocelus tanagers was found to amplify their plumage signals. New studies came out on the evolution of eggshell thickness in birds (and other dinosaurs), the osteology of Unenlagia and Dryornis, the scapulocoracoid bone histology of Confuciusornis, the hindlimb muscle function and jumping performance of elegant crested tinamous, the craniofacial development of strisoreans, cultural evolution in great tits, the cranial musculature of the black-throated finch, and the diversification of tanagers. Newly-named maniraptors included the Pleistocene kiwi Apteryx littoralis.

Juvenile specimen of Archaeorhynchus, from Foth et al. (2021).

In May, research on the evolution of hearing and vision in theropods suggested that alvarezsaurs were likely to have been nocturnal. Larger neuron numbers were found to correlate with longer yawn duration in birds (and mammals). The global abundance of birds was estimated. The genetics of avian coloration were reviewed. Eggs of extinct emus and coprolites of little bush moa were described. Supposed tooth sockets in a juvenile gastornithid were reevaluated. Ant-following birds were found to have a higher probability of being infested by ticks. The genome of the California condor was published. Siberian jays were shown to use social knowledge to avoid being deceived. Great reed warblers were reported flying at extreme altitudes during migration. Great-tailed grackles were documented to be able to direct their eyes independently towards different targets. New studies came out on the evolution of the inner ear in maniraptors (and other reptiles), pectoral girdle morphology in paravians, competition as a driver of trait divergence in birds, convergent evolution in avian mitochondria, the effects of environmental lighting on avian eye evolution, the coracoscapular joint of birds, hybridization in kiwi, the morphometrics of wing shape in aquatic birds, the macroevolutionary stability of fruit-eating birds, the bone histology of Genyornis, migration speeds in common swifts, the correlation between speciation and plumage color evolution in hummingbirds, variation in echo parakeets, the phylogeny of fieldwrens and Afro-Eurasian sparrows, the persistence of song culture in zebra finches, the diversification of Afro-Eurasian buntings, and feather coloration in swallow tanagers. Newly-named maniraptors included the presbyornithid Bumbalavis anatoides, the Oligocene gruiform Palaeogeranos tourmenti, the Pleistocene magpie Pica praepica, the white-tailed cisticola (Cisticola anderseni), and the Kilombero cisticola (Cisticola bakerorum). The new genus Radinopsyche was coined for the caatinga antwren ("Herpsilochmus" sellowi).

Comparison of maniraptoran skulls with sclerotic rings highlighted, including the nocturnal Australian owlet-nightjar (B), the potentially nocturnal Haplocheirus (C), the diurnal Finsch's pygmy parrot (D), and the potentially diurnal Erlikosaurus (E), from Choiniere et al. (2021).

In June, isotope analysis was used to infer that the Chatham Island duck primarily ate marine invertebrates. An alvarezsaurid from the Qiupa Formation and a juvenile enantiornithean from the Jiufotang Formation were described. The life history of troodontids and divergent foraging strategies in hummingbirds were reviewed. Flocks of rock pigeons were found not to exhibit "selfish herd" behavior when under threat. Great snipes were reported to make extreme changes in flight altitude during migration. Malar stripe prominence in peregrine falcons was found to correlate with solar radiation. New studies came out on vertebral pneumaticity in Unenlagia, the quadrate of Longipteryx, the diversity of avian olfactory receptor genes, the development of avian wing digits, the effects of flight efficiency on dispersal distances in birds, the phylogenetic positions of the bee hummingbird and the Whenua Hou diving petrel, the diet of the tiny hawk, the systematics of sharp-shinned hawks, diversity patterns in tyrant flycatchers, perceptual inabilities in Eurasian jays, avian defenses against brood parasites, magnetic sensitivity in European robins, and the innervation of vocal muscles in zebra finches. Newly-named maniraptors included the enantiornithean Fortipesavis prehendens (based on a Burmese amber specimen, albeit one already previously described), the Eocene pelican Eopelecanus aegyptiacus, and the satin berrypecker (Melanocharis citreola).

Changes in flight altitude of great snipes, from Lindström et al. (2021).

In July, possible troodontid pellets were reported. A new specimen of Elmisaurus (suggesting that "Nomingia" is a junior synonym), the wishbone of Halszkaraptor, a troodontid from the Wulansuhai Formation, a new skull of Ichthyornis, and teratornithids from the Pleistocene of Argentina were described. The diet and bone growth variability in Mesozoic avialans and the theft of mammal hair by birds were reviewed. Potential evidence of molting in Archaeopteryx was disputed. The innovation and spread of bin-opening behavior in sulfur-crested cockatoos were documented. The origin of sweet taste perception in songbirds was investigated. New studies came out on the evolution of body size in alvarezsaurs, brain shape in birds, and sex chromosomes in paleognaths, the morphometrics of avialan limbs, patterns of skeletal integration in birds, factors correlating with extinction in Quaternary birds, the bone histology of North Island brown kiwi, phylogenetic conflict in galliforms, the effects of dark wings on flight efficiency in seabirds, the taxonomic status of the Canary Islands oystercatcher, wing morphing in raptors, the phylogeny of white-eyes and Campylorhynchus wrens, the origin of the Sulawesi babbler, and morphological signatures of introgression in Darwin's finches. Newly-named maniraptors included the dromaeosaurid Kansaignathus sogdianus, the Eocene galliforms Bumbanortyx transitoria and Bumbanipodius magnus, the Eocene gruiforms Bumbanipes aramoides and Bumbaniralla walbeckornithoides, the archaeotrogonid Archaeodromus anglicus (suggesting that archaeotrogonids are stem-nightjars), and the Eocene stem-penguin Marambiornopsis sobrali. The new genus Aptenorallus was coined for the Calayan rail ("Gallirallus" calayanensis).

Charts showing that the taste receptors of many songbirds respond to sugars, whereas those of suboscines (the two leftmost species) only respond to amino acids, from Toda et al. (2021).

In August, eogruids and ergilornithids were reinterpreted as stem-ostriches instead of gruiforms. The preservation of cartilage in Confuciusornis and Yanornis was examined. Male-like ornamentation in female white-necked jacobins was shown to function in reducing social harassment. Tool manufacture was documented in wild Tanimbar corellas. Passeriforms from the Miocene of Austria were described. An evolutionary trade-off between song and plumage complexity was found in antwrens. New studies came out on the role of locomotor modularity in avian origins, the bone histology of Yanornis and Gansus, the (limited) correlation between latitude and evolutionary dynamics in birds, lateral openings and depressions in avian back vertebrae, the cerebellar anatomy of birds, the relationship between avian sternal variation and locomotion, the challenges of flying through gaps for birds, variation in the postcranial skeleton of ostriches, the morphology of the femoral nutrient foramen and nutrient artery in chickens, the migratory routes of Arctic terns, the genomic bases of telluravian diversification, the mitochondrial genomes of condors, the reproductive benefits of cooperative polygamy to acorn woodpeckers, the sensitivity of Eurasian jays to cognitive illusions, the diversification of the common chaffinch species complex, and the evolution of the skull of the giant cowbird. Newly-named maniraptors included the unenlagiine Ypupiara lopai. The new genus Microspizias was coined for the semicollared hawk ("Accipiter" collaris) and the tiny hawk ("Accipiter" superciliosus).

Partial eogruid or ergilornithid skull (A) compared to those of a common ostrich (C) and a limpkin (a gruiform, D), from Mayr and Zelenkov (2021).

In September, a special issue on vocal learning in birds (and other animals) was published, including a report of vocal learning in musk ducks. Nuclear preservation in the cartilage of Caudipteryx was examined. An enantiornithean from the Jiufotang Formation, plotopterids from the Paleogene of the United States, and a specimen of Septencoracias from the London Clay Formation were described. Recent advances in avian genomics were reviewed. Evidence of humans harvesting and rearing cassowaries in the Pleistocene and early Holocene was presented. Island colonization was found to facilitate diversification in pigeons. Tool innovation by a disabled kea was documented. Cockatiels were shown to be able to sing in synchrony with human music. New studies came out on the postcranial osteology of Beipiaosaurus, the body mass of Anzu, dental replacement in enantiornitheans, the effects of topographic uplift on avian (and mammalian) speciation, the role of brain size and allometry in avian craniofacial evolution, the relationship between avian forelimb proportions and flight capability, phylogenetic patterns of ultraviolet vision in birds, the diversity of eggshell thicknesses in moa, signatures of coevolution between hosts and brood parasites in the avian visual system, the use of olfactory cues by hummingbirds, species delimitation in rockhopper penguins, the population genomics of kākāpō, the perception of virtual stimuli by kea, constraints on skull shape in passeriforms, introgression in suboscines, and the diversification of bulbuls in South Asia. Newly-named maniraptors included the non-pygostylian avialan Neimengornis rectusmim, the enantiornitheans Yuanchuavis kompsosoura and Yuornis junchangi, the Oligocene stem-penguin Kairuku waewaeroa, the Oligocene hawk Archaehierax sylvestris, and the Pleistocene hawk Buteogallus irpus (with "Amplibuteo" considered a junior synonym of Buteogallus).

Holotype of Yuornis junchangi, from Xu et al. (2021).

In October, parthenogenesis was reported in California condors. The avian altricial–precocial spectrum was quantified. Birds from the Miocene of Spain were revised. Birds from the Pleistocene–Holocene of Tajikistan and a galliform skull from the Makah Formation were described. Frugivory in raptors was reviewed. A new westward migration route was documented in Richard's pipits. New studies came out on the distribution of carotenoid pigments in birds (and other reptiles), the bone histology of birds, ecological drivers of avian eggshell wettability, the evolution of sex chromosomes in paleognaths and egg coloration in Australian songbirds, sensory adaptations in flightless birds, the phylogeny of tinamous and the spectacled thrush species complex, the histology of sutures in chicken skulls, embryo movement in avian brood parasites, the phylogeography of Chalcophaps doves, rates of hybridization in hummingbirds, the relationship between plumage coloration and colonization history in barn owls of the British Isles, correlations between morphology and migratory behavior in kingbirds, dispersal of fungal spores by tapaculos, the maintenance of evolutionary diversity in pale martins, and beak color polymorphism in Darwin's finches. Newly-named maniraptors included the troodontid Papiliovenator neimengguensis and the inti tanager (Heliothraupis oneilli).

Inti tanager, from Lane et al. (2021).

In November, adaptations for wing-propelled diving in dippers were documented. Aposematism in birds was reviewed. A South Island giant moa from Rakiura was described. Ecological shifts were found not to be strongly linked to morphological evolution in Australasian parrots. New Caledonian crows were reported investigating heated objects. Cavity-nesting birds were found to use feathers to dissuade nest usurpers. New studies came out on wing kinematics in Caudipteryx, the loss of functional diversity due to recent island bird extinctions, the evolution of wing feather molt in birds, phylogenetic analyses of avian mitochondrial data, phylogenetic conflict in paleognaths, divergence times of galliforms, the annual cycle of pallid swifts, factors influencing plumage ornamentation in male red-backed fairywrens, the phylogenetic position of the Sulawesi thrush, and the genetic basis of variation in redpolls. Newly-named maniraptors included the alvarezsaur Khulsanurus magnificus, the dromaeosaurid Kuru kulla, the Cretaceous euornithean Kaririavis mater, the Pliocene hawk Buteo dondasi, and the cryptic flatbill (Rhynchocyclus cryptus).

Brown dipper, photographed by Alpsdake, under CC BY-SA 3.0.

In December, evidence of iridescent plumage in Eoconfuciusornis was reported. Putative red blood cells preserved in Beipiaosaurus were reevaluated. An oviraptorid embryo preserved in a bird-like prehatching posture and a new specimen of Scandiavis were described. The fossil record of avian tracks and the morphology of the avian notarium were reviewed. Migratory birds were shown to be generally lighter colored. Ring-billed gulls were documented solving the string-pull test. Zebra finches were shown to use calls to influence mitochondrial function in their developing young. New studies came out on the evolution of feeding mechanics in maniraptors (and other coelurosaurs), iridescent feather nanostructures, and avian beak shape, the morphology of Borogovia, divergence times of Mesozoic avialans, the metabolism of Concornis and Iberomesornis, the ossification of avian respiratory turbinates, pathologies in Genyornis, the diversification of shearwaters, the feeding behavior of the Haast's eagle, the population history of barn owls in the Western Palearctic, the development of parrot pseudoteeth, the correlation between sex roles and sexual dimorphism in fairywrens, the phylogeny of whistlers, the safekeeping of tools by New Caledonian crows, the carpometacarpus morphology of mimids, and the migratory routes of citrine wagtails. Newly-named maniraptors included the dromaeosaurid Vectiraptor greeni, the enantiornithean Brevirostruavis macrohyoideus, the Cretaceous euornitheans Brevidentavis zhangi and Meemannavis ductrix, the Miocene duck Manuherikia primadividua, and the Pleistocene owl Margarobyas abronensis. The new genus Leucoptilon was coined for the white-tailed flycatcher ("Cyornis" concretus).

Skull and foot of Haast's eagle, from Te Papa, under CC BY 4.0.