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Adaptations to different diets represent a hallmark of animal diversity. The diets of birds are highly variable, making them an excellent model system for studying adaptive evolution driven by dietary changes. To test whether molecular adaptations to diet have occurred during the evolution of birds, we examined a dietary enzyme alanine-glyoxylate aminotransferase (AGT), which tends to target mitochondria in carnivorous mammals, peroxisomes in herbivorous mammals, and both mitochondria and peroxisomes in omnivorous mammals. A total of 31 bird species were examined in this study, which included representatives of most major avian lineages. Of these, 29 have an intact mitochondrial targeting sequence (MTS) of AGT. This finding is in stark contrast to mammals, which showed a number of independent losses of the MTS. Our cell-based functional assays revealed that the efficiency of AGT mitochondrial targeting was greatly reduced in unrelated lineages of granivorous birds, yet it tended to be high in insectivorous and carnivorous lineages. Furthermore, we found that proportions of animal tissue in avian diets were positively correlated with mitochondrial targeting efficiencies that were experimentally determined, but not with those that were computationally predicted. Adaptive evolution of AGT mitochondrial targeting in birds was further supported by the detection of positive selection on MTS regions. Our study contributes to the understanding of how diet drives molecular adaptations in animals, and suggests that caution must be taken when computationally predicting protein subcellular targeting.

Flight is a key feature in the evolution of birds. Wing anatomy reflects many aspects of avian biology such as flight ability. However, our knowledge of the flight musculature has many gaps still, particularly for the distal wing. Therefore, the aim of this work was to investigate the form-function relationship of the forelimb myology of birds to understand the role of individual muscles during flight. Dissections of six species of birds of prey were performed to collect numerical data of muscle architecture, which is the primary determinant of muscle function and force-generation capacity. Birds of prey are a highly diverse group that presents different flight styles throughout the taxa, making them a good model for our purposes. Wing muscle mass (MM) isometrically scaled with body mass1.035 , muscle length to MM0.343 , and fascicle length (FL) scaled allometrically to MM0.285 . The shoulder musculature scaled differently than the other regions where the FL increases more slowly than would be expected in geometrically similar animals, which affects flight mechanics. A proximal-to-distal reduction of MM occurs, which helps to minimize the wing moment of inertia during flight while allowing precise control of the distal wing. Interestingly, the distribution of MM appeared to be species-specific, suggesting a functional signal. This study provides numerical information of muscle architecture of the avian wing that helps us to understand muscle function and its implication in flight, and can be used in future studies of flight mechanics. Anat Rec, 302:1808-1823, 2019. © 2019 American Association for Anatomy.

Since the sequencing of the zebra finch genome it has become clear the avian genome, while largely stable in terms of chromosome number and gene synteny, is more dynamic at an intrachromosomal level. A multitude of intrachromosomal rearrangements and significant variation in transposable element content have been noted across the avian tree. Transposable elements (TEs) are a source of genome plasticity, because their high similarity enables chromosomal rearrangements through non-allelic homologous recombination, and they have potential for exaptation as regulatory and coding sequences. Previous studies have investigated the activity of the dominant TE in birds, CR1 retrotransposons, either focusing on their expansion within single orders, or comparing passerines to non-passerines. Here we comprehensively investigate and compare the activity of CR1 expansion across orders of birds, finding levels of CR1 activity vary significantly both between and with orders. We describe high levels of TE expansion in genera which have speciated in the last 10 million years including kiwis, geese and Amazon parrots; low levels of TE expansion in songbirds across their diversification, and near inactivity of TEs in the cassowary and emu for millions of years. CR1s have remained active over long periods of time across most orders of neognaths, with activity at any one time dominated by one or two families of CR1s. Our findings of higher TE activity in species-rich clades and dominant families of TEs within lineages mirror past findings in mammals.Author SummaryTransposable elements (TEs) are mobile, self replicating DNA sequences within a species’ genome, and are ubiquitous sources of mutation. The dominant group of TEs within birds are chicken repeat 1 (CR1) retrotransposons, making up 7-10% of the typical avian genome. Because past research has examined the recent inactivity of CR1s within model birds such as the chicken and the zebra finch, this has fostered an erroneous view that all birds have low or no TE activity on recent timescales. Our analysis of numerous high quality avian genomes across multiple orders identified both similarities and significant differences in how CR1s expanded. Our results challenge the established view that TEs in birds are largely inactive and instead suggest that their variation in recent activity may contribute to lineage-specific changes in genome structure. Many of the patterns we identify in birds have previously been seen in mammals, highlighting parallels between the evolution of birds and mammals.

Evolution of birds from non-flying theropod dinosaurs is a classic evolutionary transition, but a deeper understanding of early flight has been frustrated by disagreement on the relationships between birds (Avialae) and their closest theropod relatives. We address this through a larger, more resolved evolutionary hypothesis produced by a novel automated analysis pipeline tailored for large morphological datasets. We corroborate the grouping of dromaeosaurids + troodontids (Deinonychosauria) as the sister taxon to birds (Paraves), as well as the recovery of Anchiornithidae as basalmost avialans. Using these phylogenetic results and available data for vaned feathered paravians, maximum and minimum estimates of wing loading and specific lift calculated using ancestral state reconstruction analysis are used as proxies for the potential for powered flight through this transition. We found a broad range of paravian ancestors with estimates approaching values that are indicative of powered flight potential. This suggests that prior to the evolution of flight there was a wider extent of experimentation with wing-assisted locomotion among paravians than previously appreciated. We recovered wing loading and specific lift estimates indicating the potential for powered flight among fossil birds as well as unenlagiine and microraptorine dromaeosaurids. In the context of our phylogeny and of Mesozoic palaeogeography, our results suggest that the potential for powered flight originated three or more times from a broad range of ancestors already nearing this potential, providing a well-supported scenario for the origin of theropod flight to further explore.

Ugolyakia kaluginae gen. et sp. nov. is described from Ugolyak (Santonian Taimyr amber), based primarily on its unbranched Rs and developed katepisternal sulcus. It is attributed to the tribe Simuliini, although it lacks calcipala and spiniform seta on the costal vein characteristic of most genera of the tribe. Possession of a claw with a large subbasal tooth and absence of significant sclerotization of the sternites suggest that U. kaluginae females were blood-sucking avian parasites. Black flies make up 3% of all insect inclusions and 5% of all identifiable dipterans in Ugolyak amber. Only two Late Cretaceous black fly specimens were previously known: a poorly preserved female from Yantardakh (Santonian Taimyr amber) and a complete one from Turonian New Jersey amber. Feathers found at nearly all Cretaceous black fly sites (and at all formations with records of Simuliini) were younger than the Hauterivian. Ugolyak black flies are thought to have inhabited the same environments as Cretaceous ornithurine birds and most likely fed on them. These insects can then be used as an indicator of this bird community, allowing a better understanding of the Late Cretaceous forest ecology of Northern Asia. The inferred presence of Ornithuromorpha at high latitudes by the Early Cretaceous implies that their high growth rate may have evolved as an adaptation to a short yearly period of productivity (probably as a compensation for the poor flight ability of their young). This further implies that advanced ornithuromorphs might have originated at higher latitudes; later, aquatic ornithuromorphs occupied niches in lower latitude regions with tropical climates such as the Chinese Jehol biota, to which they were preadapted. The inferred seasonality at higher latitudes during cold spells of the Early Cretaceous could further be viewed as a prerequisite for the evolutionary origin of the granivory.

The evolution of birds from dinosaurs is a subject that has received great attention among vertebrate paleontologists. Nevertheless, the early evolution of the paravians, the group that contains birds and their closest non-avian dinosaur relatives, remains very poorly known. Even the most basal members of one paravian lineage, the Dromaeosauridae, already show a body plan that differs substantially from their closest non-paravian relatives. Recently, the dromaeosaurid Halszkaraptor escuilliei was described from the Cretaceous of Mongolia. Halszkaraptor possesses numerous unserrated premaxillary teeth, a platyrostral rostrum with a developed neurovascular system, an elongate neck, bizarrely-proportioned forearms, and a foreword-shifted center of mass, differing markedly from other paravians. A reevaluation of the anatomy, taphonomy, environmental setting, and phylogenetic position of H. escuilliei based on additional comparisons with other maniraptorans suggests that, rather than indicating it was a semiaquatic piscivore, the body plan of this dinosaur bears features widely distributed among maniraptorans and in some cases intermediate between the conditions in dromaeosaurids and related clades. I find no evidence for a semiaquatic lifestyle in Halszkaraptor. A phylogenetic reevaluation of Halszkaraptorinae places it as the sister clade to Unenlagiinae, indicating the bizarre features of unenlagiines previously interpreted as evidence of piscivory may also represent a mosaic of plesiomorphic, derived, and intermediate features. The anatomy of Halszkaraptor reveals that dromaeosaurids still possessed many features found in more basal maniraptoran and coelurosaur clades, including some that may have been tied to herbivory. Rather than being a semiaquatic piscavore, Halszkaraptor was a basal dromaeosaurid showing transitional features.

‘Exceptional fossils’ of dinosaurs preserving feathers have radically changed the way we view their paleobiology and the evolution of birds. Understanding how such soft tissues preserve is imperative to accurately interpreting the morphology of fossil feathers. Experimental taphonomy has been integral to such investigations. One such experiment used a printing press to mimic compaction, done subaerially and without sediment burial, and concluded that the leaking of bodily fluid could lead to the clumping of feathers by causing barbs to stick together such that they superficially resemble simpler, less derived, filamentous structures. Here we use a novel, custom-built experimental setup to more accurately mimic subaqueous burial and compaction under low-energy, fine-grain depositional environments applicable to the taphonomic settings most plumage-preserving ‘exceptional fossils’ are found in. We find that when submerged and subsequently buried and compacted, feathers do not clump together and they maintain their original arrangement. Submersion in fluid in and of itself does not lead to clumping of barbs; this would only occur upon pulling feathers out from water into air. Furthermore, sediment encases the feathers, fixing them in place during compaction. Thus, feather clumping that leads to erroneously plesiomorphic morphological interpretations may not be a taphonomic factor of concern when examining fossil feathers. Our current methodology is amenable to further improvements that will continue to more accurately mimic subaqueous burial and compaction, allowing for various hypothesis testing. Electronic supplementary material The online version of this article (10.1007/s12542-018-0411-y) contains supplementary material, which is available to authorized users.

Though a tradeoff between growth rate and locomotor performance has been proposed, empirical data on this relationship are still limited. Here we statistically analyze the associations of growth rate and flight ability in birds by assessing how growth rate is correlated with three wing parameters of birds: flight muscle ratio, wing aspect ratio, and wing loading. We find that fast-growing birds tended to have higher flight muscle ratios and higher wing loadings than slow-growing birds, which suggests that fast-growing birds may have better takeoff performance, but lower efficiency in maneuvering flight. Accordingly, our findings suggest that the relationship between growth rate and flight ability is more complex than a simple tradeoff. Since the hindlimbs also contribute greatly to the locomotion of birds, future investigations on the relationship between growth rate and hindlimb performance will provide more insights into the evolution of birds.

The Early Cretaceous basal birds were known largely from just two-dimensionally preserved specimens from north-eastern China (Jehol Biota), which has hindered our understanding of the early evolution of birds. Here, we present a three-dimensionally-preserved skeleton (FPDM-V-9769) of a basal bird from the Early Cretaceous of Fukui, central Japan. Unique features in the pygostyle and humerus allow the assignment of FPDM-V-9769 to a new taxon, Fukuipteryx prima. FPDM-V-9769 exhibits a set of features comparable to that of other basalmost birds including Archaeopteryx. Osteohistological analyses indicate that FPDM-V-9769 is subadult. Phylogenetic analyses resolve F. prima as a non-ornithothoracine avialan basal to Jeholornis and outgroup of the Pygostylia. This phylogenetic result may imply a complex evolutionary history of basal birds. To our knowledge, FPDM-V-9769 represents the first record of the Early Cretaceous non-ornithothoracine avialan outside of the Jehol Biota and increases our understanding of their diversity and distribution during the time., A new fossil bird from the Early Cretaceous of Japan provides evidence that the evolutionary history of early birds was more complex than previously realised. This new species also has important implications for the distribution of early birds, demonstrating that they inhabited temperate, lowland regions.

The diverse array of phenotypes and courtship displays exhibited by birds-of-paradise have long fascinated scientists and nonscientists alike. Remarkably, almost nothing is known about the genomics of this iconic radiation. There are 41 species in 16 genera currently recognized within the birds-of-paradise family (Paradisaeidae), most of which are endemic to the island of New Guinea. In this study, we sequenced genomes of representatives from all five major clades within this family to characterize genomic changes that may have played a role in the evolution of the group's extensive phenotypic diversity. We found genes important for coloration, morphology, and feather and eye development to be under positive selection. In birds-of-paradise with complex lekking systems and strong sexual dimorphism, the core birds-of-paradise, we found Gene Ontology categories for “startle response” and “olfactory receptor activity” to be enriched among the gene families expanding significantly faster compared to the other birds in our study. Furthermore, we found novel families of retrovirus-like retrotransposons active in all three de novo genomes since the early diversification of the birds-of-paradise group, which might have played a role in the evolution of this fascinating group of birds.

Most extant species are in clades with poor fossil records, and recent studies of comparative methods show they have low power to infer even highly simplified models of trait evolution without fossil data. Birds are a well-studied radiation, yet their early evolutionary patterns are still contentious. The fossil record suggests that birds underwent a rapid ecological radiation after the end-Cretaceous mass extinction, and several smaller, subsequent radiations. This hypothesized series of repeated radiations from fossil data is difficult to test using extant data alone. By uniting morphological and phylogenetic data on 604 extant genera of birds with morphological data on 58 species of extinct birds from 50 million years ago, the "halfway point" of avian evolution, I have been able to test how well extant-only methods predict the diversity of fossil forms. All extant-only methods underestimate the disparity, although the ratio of within- to between-clade disparity does suggest high early rates. The failure of standard models to predict high early disparity suggests that recent radiations are obscuring deep time patterns in the evolution of birds. Metrics from different models can be used in conjunction to provide more valuable insights than simply finding the model with the highest relative fit.

Birds are maniraptoran theropod dinosaurs. The evidence supporting the systematic position of Avialae as a derived clade within Dinosauria is voluminous and derived from multiple independent lines of evidence. In contrast, a paucity of selectively chosen data weakly support, at best, alternative proposals regarding the origin of birds and feathers. Opponents of the theory that birds are dinosaurs have frequently based their criticisms on unorthodox interpretations of paleontological data and misrepresentation of phylogenetic systematic methods. Moreover, arguments against the nested position of Avialae in Dinosauria have often conflated the logically distinct questions of avian origins, the evolution of flight, and the phylogenetic distribution of feathers. Motivated by a Perspectives article with numerous factual inaccuracies that recently appeared in The Auk, we provide a review of the full complement of facts pertaining to the avian origins debate and address the misplaced criticisms raised in...

Major transformations in brain size and proportions, such as the enlargement of the brain during the evolution of birds, are accompanied by profound modifications to the skull roof. However, the hypothesis of concerted evolution of shape between brain and skull roof over major phylogenetic transitions, and in particular of an ontogenetic relationship between specific regions of the brain and the skull roof, has never been formally tested. We performed 3D morphometric analyses to examine the deep history of brain and skull-roof morphology in Reptilia, focusing on changes during the well-documented transition from early reptiles through archosauromorphs, including nonavian dinosaurs, to birds. Non-avialan taxa cluster tightly together in morphospace, whereas Archaeopteryx and crown birds occupy a separate region. There is a one-to-one correspondence between the forebrain and frontal bone and the midbrain and parietal bone. Furthermore, the position of the forebrain–midbrain boundary correlates significantly with the position of the frontoparietal suture across the phylogenetic breadth of Reptilia and during the ontogeny of individual taxa. Conservation of position and identity in the skull roof is apparent, and there is no support for previous hypotheses that the avian parietal is a transformed postparietal. The correlation and apparent developmental link between regions of the brain and bony skull elements are likely to be ancestral to Tetrapoda and may be fundamental to all of Osteichthyes, coeval with the origin of the dermatocranium. Brain and skull development are intimately related across tetrapods. Here, the authors show a close relationship between brain and skull roof across evolutionary transitions and ontogenetic stages of reptiles.

The highly organized morphogenesis of bird feathers holds important phylo- and ontogenetic information on the evolution of birds, organogenesis, tissue regeneration, and the health status of individual animals. Altered topobiological patterns are regularly used as retrospective evidence for disturbed developmental trajectories due to the past exposure to environmental stressors. Using the most advanced high-resolution (5–70 µm) X-ray fluorescence microscopy (XFM), we describe in the feathers from three species of Procellariiformes hitherto unknown, depositions of elements (Zn, Ca, Br, Cu, Fe) that are independent of pigmentation or any underlying variation in density or polymer structure. In the case of Zn, the pattern across several species of Procellariiformes, but not other species, consisted of highly regular bands of Zn numbering 30–32, which may reflect the estimated number of days of active feather growth or the duration of the moult period. Thus, speculatively, the highly consistent Zn pattern might be the result of a so far unknown diurnal systemic regulation rather than local heterogeneity amongst the follicular stem cells.

Several flight modes are thought to have evolved during the early evolution of birds. Here, we use a combination of computational modelling and morphofunctional analyses to infer the flight properties of the raven-sized, Early Cretaceous birdSapeornis chaoyangensis—a likely candidate to have evolved soaring capabilities. Specifically, drawing information from (i) mechanical inferences of the deltopectoral crest of the humerus, (ii) wing shape (i.e. aspect ratio), (iii) estimations of power margin (i.e. difference between power required for flight and available power from muscles), (iv) gliding behaviour (i.e. forward speed and sinking speed), and (v) palaeobiological evidence, we conclude thatS. chaoyangensiswas a thermal soarer with an ecology similar to that of living South American screamers. Our results indicate that as early as 125 Ma, some birds evolved the morphological and aerodynamic requirements for soaring on continental thermals, a conclusion that highlights the degree of ecological, functional and behavioural diversity that resulted from the first major evolutionary radiation of birds.

Nest sanitation-related traits have often been explained at the intraspecific level as reducing the probability of infection or detection by predators and parasites, but its evolution within the avian phylogeny is still poorly understood. We compiled detailed information of such traits for more than 400 bird species and, by means of modern comparative methodologies, we reconstructed the evolution of adults' contribution to removing their offspring's faeces and the production of faecal sacs by nestlings. Furthermore, because the functional hypotheses used to explain nest sanitation behaviour assume potential effects of brood size, body mass, nestling period and diet, we explored the association between these traits and those related to nest sanitation in a phylogenetically controlled framework. Our results suggest that parental removal of nestling faeces has driven the evolution of faecal sacs, while the ancestral states involved birds with faecal sacs removed by parents. These results support the long-held idea that faecal sacs facilitate the removal of faeces by parents. Moreover, we found that animal diets and small body sizes have favoured the evolution of faecal sacs suggesting the existence of some chemical and physical constraints in relation to the evolution of the mucous covering. Our results highlight the importance of nest sanitation in the evolution of birds and their life history characteristics, J.D.I. was financed by a postdoctoral contract (TAHUB-104) from the program ‘Andalucía Talent Hub’ (cofunded by the European Union's Seventh Framework Program Marie Skłodowska-Curie actions (COFUND) and the regional Government of Andalucía). Funding was partially provided by the Spanish Ministerio de Economía y Competitividad (European funds (FEDER)) (CGL2013-48193-C3-1-P).

A new basal Avialae, Jeholornis curvipes sp. nov., from the Yixian Formation (Lower Cretaceous) of Liaoning Province (north-eastern China) is described. A revision of long-tailed birds from China and a phylogenetic analysis of basal Avialae suggest that Jeholornithiformes were paraphyletic, with Jixiangornis orientalis being the sister-taxon of pygostylia. The phylogenetic analysis also recovered that the tail reduction is a unique event in the evolution of birds. Jeholornis species were cursorial, nonperching, and seed-eating birds.

Hybridization is increasingly seen as an important source of adaptive genetic variation and biotic diversity. Recent phylogenetic studies on the early evolution of birds suggest that the early diversification of neoavian orders perhaps involved a period of extensive hybridization or incomplete lineage sorting. Phylogenetic error, saturation, long-branch attraction, and convergence make it difficult to detect ancient hybridization events and differentiate them from incomplete lineage sorting using sequence data. We used recently published retroposon marker data to visualize the early radiation of Neoaves within a phylogenetic network approach, and found that the most basal neoavian taxa indeed show a complex pattern of reticulated relationships. Moreover, the reticulation levels of different parts of the network are consistent with the insertion pattern of the retroposon elements. The use of network-based analyses on homoplasy-free data shows true conflicting signals and the taxa involved that are not represented in trees.

Passerines, taxonomic group that comprises about two-thirds of the world's existing avian species, originated in the part of Gondwana that today forms Australia (Edwards and Boles, 2002). The ancient supercontinent Gondwana broke up about 180 million years ago, generating the landmasses that we recognize today as Africa, South America, Australia, Antarctica, the Indian subcontinent and the Arabian Peninsula. The book's subtitle makes reference to these major events, which are presented with great accuracy by the author. However, the aim of this book goes well beyond the topic of Australia as one of the cradles of avian evolution. More generally, and interesting not only for comparative psychologists and neuroscientists but also for the general public, this is a book on avian brains and intelligence. I suspect that, in a sense, Australian birds are an excuse for the author, an eminent animal behavior researcher who has made important contributions to the study of Australian (and non-Australian) animal cognition, to discuss some of the more general and central topics of animal intelligence in a broad perspective. In this sense, I believe the book is of interest to readers of any Hemisphere.

Assessment of competing theories for the evolution of avian flight is problematic, and tends to rest too heavily on reconstruction of the mode of life of one or a few specimens representing still fewer species. A more powerful method is to compare the sequence of character acquisition predicted by the various theories with the empirical sequence provided by cladistic phylogeny. Arboreal and cursorial theories incorrectly predict the sequence of character acquisition for several key features of avian evolution. We propose an alternative 'pouncing proavis' model for the evolution of flight. As well as being both biologically and evolutionarily plausible, the pouncing proavis model correctly predicts the evolutionary sequence of all five key features marking the evolution of birds.

Coelurosauria is the most diverse clade of theropod dinosaurs. Much of this diversity is present in Paraves—the clade of dinosaurs containing dromaeosaurids, troodontids, and avialans. Paraves has over 160 million years of evolutionary history that continues to the present day. The clade represents the most diverse living tetrapod group (there are over 9000 extant species of Aves—a word used here as synonomous with “bird”), and it is at the root of the paravian radiation, when dromaeosaurids, troodontids, and avialans were diverging from one another, that we find the morphology and soft tissue changes associated with the origin of modern avian flight. Within the first 15 million years of known paravian evolutionary history members of this clade exhibited a difference of nearly four orders of magnitude in body size, a value that is similar to the extreme body size disparity present today in mammalian carnivorans, avians, and varanoid squamates. In this respect, Paraves is an important case study i...

It has been suggested that relative egg size in living birds is strongly correlated with the developmental mode of the young; "altricial" (helpless) or "precocial" (independent). Using a data set of extant taxa we show that altricial birds lay relatively larger eggs than their precocial counterparts but that this may be due to the small size of most altricial species. Smaller birds tend to lay relatively small eggs compared to large species. Nonetheless, a predictive egg mass-body mass relationship extends into the avian fossil record. Such a relationship is important to our understanding of avian evolution because relative egg size (and thus available developmental mode) was constrained in many early birds—oviduct diameter was limited by the presence of pubic fusion. Therefore we document the evolution of avian developmental strategies using morphology-based phylogenies for Mesozoic and extant avians and corroborate correlations between developmental strategies, egg weight and female body mass. The sequential loss of precocial features in hatchlings characterises the evolution of birds while altriciality is derived within Neoaves. A set of precocial strategies is seen in earlier lineages, including basal Neornithes (modern birds) and are implied in their Mesozoic counterparts—skeletal constraints on egg size, present in many Jurassic and Early Cretaceous birds (Archaeopteryx, Confuciusornis, Enantiornithes) were lost in later diverging lineages. Attributes of precociality were already present in a number of lineages of non-avian maniraptoran theropods. We propose that the evolution of "unrestricted egg size" may have precipitated subsequent development of the diverse reproductive strategies seen in living birds.

The skeletons of birds are universally described as lightweight as a result of selection for minimizing the energy required for flight. From a functional perspective, the weight (mass) of an animal relative to its lift-generating surfaces is a key determinant of the metabolic cost of flight. The evolution of birds has been characterized by many weight-saving adaptations that are reflected in bone shape, many of which strengthen and stiffen the skeleton. Although largely unstudied in birds, the material properties of bone tissue can also contribute to bone strength and stiffness. In this study, I calculated the density of the cranium, humerus and femur in passerine birds, rodents and bats by measuring bone mass and volume using helium displacement. I found that, on average, these bones are densest in birds, followed closely by bats. As bone density increases, so do bone stiffness and strength. Both of these optimization criteria are used in the design of strong and stiff, but lightweight, manmade airframes. By analogy, increased bone density in birds and bats may reflect adaptations for maximizing bone strength and stiffness while minimizing bone mass and volume. These data suggest that both bone shape and the material properties of bone tissue have played important roles in the evolution of flight. They also reconcile the conundrum of how bird skeletons can appear to be thin and delicate, yet contribute just as much to total body mass as do the skeletons of terrestrial mammals.

The paper summarizes the current understanding of the evolution and diversification of birds. New insights into this field have mainly come from two fundamentally different, but complementary sources of information: the many newly discovered Mesozoic bird fossils and the wealth of genetic analyses of living birds at various taxonomic levels. The birds have evolved from theropod dinosaurs from which they can be defined by but a few morphological characters. The early evolutionary history of the group is characterized by the extinctions of many major clades by the end of the Cretaceous, and by several periods of rapid radiations and speciation. Recent years have seen a growing consensus about the higher-level relationships among living birds, at least as can be deduced from genetic data.

Fossil evidence for changes in dinosaurs near the lineage leading to birds and the origin of flight has been sparse. A dinosaur from Mongolia represents the basal divergence within Dromaeosauridae. The taxon's small body size and phylogenetic position imply that extreme miniaturization was ancestral for Paraves (the clade including Avialae, Troodontidae, and Dromaeosauridae), phylogenetically earlier than where flight evolution is strongly inferred. In contrast to the sustained small body sizes among avialans throughout the Cretaceous Period, the two dinosaurian lineages most closely related to birds, dromaeosaurids and troodontids, underwent four independent events of gigantism, and in some lineages size increased by nearly three orders of magnitude. Thus, change in theropod body size leading to flight's origin was not unidirectional.

Although the dinosaurian hypothesis of bird origins is widely accepted, debate remains about how the ancestor of birds first learned to fly. Here we provide new evidence suggesting that basal dromaeosaurid dinosaurs were four-winged animals and probably could glide, representing an intermediate stage towards the active, flapping-flight stage. The new discovery conforms to the predictions of early hypotheses that proavians passed through a tetrapteryx stage.

Birds, especially those on islands, have contributed disproportionately to the development of theories that explain the origin and maintenance of organic diversity. This essay surveys the major ideas of how bird species are thought to evolve, speciate and multiply in adaptive radiations on islands. I begin with the nineteenth century view that speciation is the result of natural selection slowly causing divergence of geographically separated populations. I then trace the course of discovery and deeper understanding during the twentieth century, from the influence of Mendelian genetics at the beginning to the modern, molecular era at the end. The dominant and pivotal evolutionary question has been how new species are formed. An initial, nearly exclusive, attention to adaptive processes has been replaced by a search for additional reasons for allopatric divergence, coupled with the conditions that permit coexistence in sympatry after allopatric divergence has occurred. Recent studies have shown that pre-mating barriers to gene exchange caused by natural selection and sexual selection often arise before post-mating genetic incompatibilities have evolved. Therefore behavioural and ecological factors play a predominant role in this stage of speciation. Imprinting and imprinting-like processes of early learning contribute to pre-mating barriers by constraining mate choice. Ecological factors determine coexistence potential and the fates of hybrids. Genetical factors at all stages, from the founding of a new population to the evolution of genetic incompatibilities, are still poorly known. But molecular genetics is now helping us to reconstruct evolutionary history, thereby transforming ideas about systematic relationships, colonization routes and both the tempo and mode of evolution. As patterns of evolution within radiations become better known, more attention needs to be given to the task of explaining the sequential build-up of avian communities; by evolutionary radiation and additional immigration, offset to some extent by extinction. The new and substantial challenge is to understand evolutionary radiations in relation to changing environments. Until that challenge is met we cannot claim that the problem of explaining adaptive radiations of birds or any other group of organisms is solved.

The evolution of birds and feathers are examined in terms of the aerodynamic constraints imposed by the arboreal and cursorial models of flight evolution. The cursorial origin of flight is associated with the putative coelurosaurian ancestry of birds. As presently known, coelurosaurs have a center of mass located in the pelvic region and an elongated pubis that is ventrally or anteriorly directed. Both of these characteristics make it difficult to postulate an origin of flight that would involve a gliding phase because the abdomen cannot be flattened into an aerodynamic shape. Moreover, the cursorial model must counteract gravity using the hindlimb and, thus, selection for the power requirement for lift-off would not focus on the forelimb. Therefore, if the hypothesis proposing a coelurosaurian ancestry of birds is to remain viable, it must be via an as yet undiscovered taxon that is compatible with the morphological and aerodynamic constraints imposed by flight evolution. The arboreal model, cur...

A selective regime favoring a streamlining of body contours and surfaces is proposed as having been instrumental in driving the morphological and functional transformations of an unfeathered reptilian integument into a feather-bearing avian one. This hypothesis is consistent with a new, structurally and functionally coherent analysis of the microanatomy of the avian feather-bearing integument as a complex, integrated organ system that includes an intricate, hydraulic skeleto-muscular apparatus of the feathers, a dermo-subcutaneous muscle system of the integument, and a subcutaneous hydraulic skeletal system formed by fat bodies. Key elements of the evidence supporting the new hypothesis are (1) the presence of depressor feather muscles that are not needed as antagonists for the erector feather muscles, but can counteract external forces, such as air currents; (2) the fact that the highly intricate feather-bearing integument represents a machinery to move feathers or to stabilize them against external forces; (3) the crucial role of the coat of feathers in streamlining the body contours and surfaces of birds; (4) the aerodynamic role of feathers as pressure and turbulence sensors and as controllable temporary turbulators; and (5) the critical role that a streamlined body plays in avian flight and is likely to have played in the evolutionary transformations from ecologically and locomotorily versatile quadrupedal reptiles to volant bipedal birds without passing through parachuting or gliding stages. These transformations are likely to have occurred more than once. The ancestral birds were probably small, arboreal, hopping, and using flap-bounding, or intermittent bounding, flight.

Since the description of Confuciusornis (the oldest beaked bird) in1995, based on three partial specimens, large numbers of complete skeletons have been recovered1,2. Most new material of Confuciusornis3,4 can be assigned to a single sexually dimorphic species, C. sanctus . Here we report a new species based on a remarkably well preserved skeleton with feathers and, for the first time in the Mesozoic record, direct evidence of the shape of a horny beak. It has a complete and large preserved postorbital that has a broad contact with the jugal bone. This character is presently only known in Confuciusornis, and may confirm previous suggestions of a postorbital in Archaeopteryx5. The squamosal is in tight contact with the postorbital. These two bones form an arch dividing the upper and lower temporal fenestrae, as in other diapsid reptiles6. The presence of a typical diapsid cheek region with two openings in Confuciusornis may preclude the presence of prokinesis (upper jaw mobility against the braincase and orbital area), a feeding adaptation found in most modern birds. The presence of a horny beak, characteristic of modern birds, coupled with a primitive temporal region provides new evidence for a mosaic pattern in the early evolution of birds.

The skull and jaws of extant birds possess secondary cartilage, a tissue that arises after bone formation during embryonic development at articulations, ligamentous and muscular insertions. Using histological analysis, we discovered secondary cartilage in a non-avian dinosaur embryo, Hypacrosaurus stebingeri (Ornithischia, Lambeosaurinae). This finding extends our previous report of secondary cartilage in post-hatching specimens of the same dinosaur species. It provides the first information on the ontogeny of avian and dinosaurian secondary cartilages, and further stresses their developmental similarities. Secondary cartilage was found in an embryonic dentary within a tooth socket where it is hypothesized to have arisen due to mechanical stresses generated during tooth formation. Two patterns were discerned: secondary cartilage is more restricted in location in this Hypacrosaurus embryo, than it is in Hypacrosaurus post-hatchlings; secondary cartilage occurs at far more sites in bird embryos and nestlings than in Hypacrosaurus. This suggests an increase in the number of sites of secondary cartilage during the evolution of birds. We hypothesize that secondary cartilage provided advantages in the fine manipulation of food and was selected over other types of tissues/articulations during the evolution of the highly specialized avian beak from the jaws of their dinosaurian ancestors.

AVIAN flight is one of the most remarkable achievements of vertebrate evolution, yet there is little evidence of its early phases. Specimens of Archaeopteryx shed important (albeit controversial) light on this evolutionary phenomenon, but the large morphological (and almost certainly functional) gap between Archaeopteryx and modern avians remained virtually empty until recently. Here we report a new, exquisitely preserved, bird from the Lower Cretaceous Konservat-Lagerstatte of Las Hoyas (Cuenca, Spain) which provides evidence for the oldest known alula (bastard wing). Crustacean remains found inside its belly also provide the oldest direct evidence of feeding habits in birds. The new specimen has numerous synapomorphies with the Enantiornithes, but its unique sternal morphology, along with other autopomorphies in the furcula and vertebral centra, support the recognition of a new enantiornithine taxon, Eoalulavishoyasi. The combination in Eoalulavis of a decisive aerodynamic feature, such as the alula, with the basic structures of the modern flight apparatus indicates that as early as 115 million years ago, birds had evolved a sophisticated structural system that enabled them to fly at low speeds and to attain high manoeuvrability.

Archaeopteryx is widely accepted as being the most basal bird, and accordingly it is regarded as central to understanding avialan origins; however, recent discoveries of derived maniraptorans have weakened the avialan status of Archaeopteryx. Here we report a new Archaeopteryx-like theropod from China. This find further demonstrates that many features formerly regarded as being diagnostic of Avialae, including long and robust forelimbs, actually characterize the more inclusive group Paraves (composed of the avialans and the deinonychosaurs). Notably, adding the new taxon into a comprehensive phylogenetic analysis shifts Archaeopteryx to the Deinonychosauria. Despite only tentative statistical support, this result challenges the centrality of Archaeopteryx in the transition to birds. If this new phylogenetic hypothesis can be confirmed by further investigation, current assumptions regarding the avialan ancestral condition will need to be re-evaluated.

​TheThe deliberate or accidental introduction of species beyond their natural range has been one of the major consequences of the growth of trade, travel, and tourism in recent times. Many introduced species fail to establish or to spread significantly, but many others become successful invaders. Invasive alien species can transform natural ecosystems and cause extinctions of native species. As a result, invasive species are now recognized in the IUCN Red List as one of the main threats to native biodiversity worldwide. In some parts of the world, especially on oceanic islands and previously isolated landmasses, they are arguably now a more important threat than factors such as habitat loss or overexploitation, which tend to preoccupy conservationists in continental landscapes that humans have modified over thousands of years. For example, in New Zealand there is now a major “biosecurity” focus on preventing new invasions, eradicating introduced mammals from offshore islands, and managing a suite of invasive species on the mainland. Blackburn TM, Lockwood JL, Cassey P (2009) Avian Invasions: The Ecology and Evolution of Exotic Birds. New York: Oxford University Press. 320p. ISBN (paperback): 978-0-19-923255-0. US$55.00. To deal effectively with invasive species, it is clearly important to understand the processes of biological invasion. If, by studying the “unnatural experiments” of past species introductions, insights can also gained into fundamental ecological processes, so much the better. Although birds are very well-studied in many other contexts and are a common cause of conservation concern, they have received relatively little attention as biological invaders, compared with mammals, for example. This is despite the fact that the history and success or failure of avian introductions around the world is relatively well-documented. Since there is also a large amount of information on the distribution, ecology, and evolution of birds in general, this means that exotic birds provide excellent opportunities for examining the major stages of biological invasion and addressing important questions about the factors that facilitate invasions. The new book Avian Invasions: The Ecology and Evolution of Exotic Birds, by Tim Blackburn, Julie Lockwood, and Phillip Cassey, is therefore very timely and topical in its synthesis of the hitherto neglected subjects of the ecology and evolution of exotic birds. The book is structured around widely accepted stages of the invasion pathway: transport and introduction, establishment, spread, and invasive impact. It concludes with chapters on the ecology, genetics, and evolution of exotic birds, and lessons learned from the study of avian invasions. Throughout the book, the authors not only critically review the existing literature on biological invasions in general and avian invasions in particular; they also make several excellent suggestions for future research. The book starts with a brief but interesting history of bird introductions and a summary of the general invasion pathway. From the outset, the authors point out some of the problems in trying to draw scientific conclusions from historical data, which are often biased in terms of taxon, location, and time period. The bird species chosen for introduction reflected societal demands at the time, with the result that more than half of all known avian introductions come from only five families, which contain less than 15% of extant birds. Gamebirds, parrots, and passerines are over-represented among introduced birds because they were disproportionately selected for introduction as hunting quarry, as ornamental species, or as songbirds. Common bird species may also have been more likely to be successfully introduced; for example, common British birds were introduced to New Zealand in larger numbers. Overall, more than two thirds of bird introductions have been to islands, which comprise only a small fraction of the world's land area, and a disproportionate number of introductions have been made to Australia, New Zealand, and the Pacific Islands. Despite such shortcomings in the historical data, the authors illustrate throughout the book how the rigorous analysis of past introductions can still yield some powerful insights into invasion processes. The first two stages of the invasion pathway are transport and introduction. These are considered together as the subject of the second chapter, with a focus on evidence for non-randomness in both stages. The authors point out that a consistent thread running through analyses of character selectivity is that the “availability” of a species is important for its success; large geographic range size and/or population size are predictors of transport and introduction probability. The third stage of invasion is the crucial one of establishment, and this is the subject of chapters 3–5, which progressively consider the roles of contingency, species traits, and location in relation to establishment success. It is widely agreed that “propagule pressure” (usually defined as the number of individuals introduced) is an important determinant of success. The authors argue why this would be expected, relating it to the challenges faced by small populations in general and presenting evidence for the relationship between propagule pressure and establishment success. They then go on to consider the role of species-level traits, where previous studies have generally failed to find associations with establishment success. By framing this issue in the context of small population biology, they successfully argue that general conclusions can be drawn about the importance of factors such as population growth rates, Allee effects (reduced population growth at low densities), and abilities to cope with novelty, in relation to the likelihood of successful establishment. Chapter 5 discusses the importance of recipient location, including the historically controversial topics of invasibility and the role of competition in determining establishment success. The authors successfully revisit these arguments and also address the other important biotic interactions of predation, parasitism, and mutualisms (such as pollination and seed dispersal). After establishment, the next phase of the invasion process is spread (which is discussed in Chapter 6). The authors review the general literature on geographic range expansion of invasive species and note the strong bias in substantial data from a few widespread and highly successful bird species (such as starlings and sparrows). Despite the taxonomic bias, this is a key area in which data on exotic birds have contributed substantially to our current understanding of one of the major stages of biological invasion. In the second part of the book, the authors move beyond the invasion pathway to consider the ecology, genetics, and evolution of exotic birds, lessons learned from the study of avian invasions, and some of the exciting research opportunities that they present. They review information on interactions between exotic birds and native species and consider how comparing data on phenomena, such as species-area relationships and latitudinal gradients of exotic and native birds, can contribute to general ecological understanding. For example, whereas native bird species richness on islands is a negative function of distance to the mainland, exotic bird species richness is a positive function of this distance because of where exotic species were introduced. The authors then summarise what is known about the impacts of exotic birds on native species and communities (i.e., effectively the final stage of the invasion pathway). These impacts include competition with native birds, predation, mutualisms (such as pollination and seed dispersal), and the role of exotic birds in disease transmission. Classic examples include the pollination and dispersal of native and introduced plant species by exotic birds in many ecosystems and the devastating transmission of avian malaria and avian pox from exotic birds to vulnerable native bird species in Hawaii. Chapters 8 and 9 take the longer-term, evolutionary perspective on the effects of successful avian invasions. Exotic birds have sometimes lost significant amounts of genetic variation during the introduction process, but in many cases there is no evidence of any such loss. There is, however, some evidence of evolution in phenotypes of exotic birds since they have established at new sites, despite the relatively short time frames involved. As is stressed by the authors, the “experiments in nature” represented by avian invasions, provide multiple (but so far under-utilized) opportunities for addressing basic questions in evolutionary ecology. The authors conclude their book by stating that a coherent picture is beginning to emerge of how introduced bird species move through stages of the invasion process. Significant and consistent themes include the role of humans; interactions between event, species, and location; and the importance of both the stochastic and the deterministic in the establishment and spread of exotic birds. They strongly advocate the use of transport/introduction patterns as null hypotheses in invasion ecology, suggesting that ecological explanations are needed when there is deviation from these human-mediated patterns. Overall, this is a comprehensive, up-to-date, and exciting book. It provides a much-needed stimulus for a greater focus on avian invasions and their effects on native biodiversity, but it also illustrates how the study of exotic birds can help to advance our general understanding of invasion ecology. I predict that it will become a classic text in invasion ecology, and I strongly commend it to all who are interested in this important and growing field.

Recent coelurosaurian discoveries have greatly enriched our knowledge of the dinosaur-bird transition, but all reported taxa close to this transition are from relatively well-known coelurosaurian groups^1-3^. Here we report a new basal avialan, Epidexipteryx hui gen. et sp. nov., from the Middle-Late Jurassic of Inner Mongolia, China. This new species is characterized by an unexpected combination of characters seen in several different theropod groups, particularly the Oviraptorosauria. Phylogenetic analysis shows it to be the sister taxon to Epidendrosaurus^4,5^, forming a new clade at the base of Avialae^6^. Epidexipteryx also possesses two pairs of elongate ribbon-like tail feathers (ETFs), and its limbs lack contour feathers for flight. This finding shows that a member of the avialan lineage experimented with integumentary ornamentation as early as the Middle-Late Jurassic, and provides further evidence relating to this important aspect of the transition from non-avian theropods to birds.

The spate of recent discoveries of Mesozoic birds has substantially improved our understanding of the early evolution of birds and flight1–5, but has failed to close the morphological gap between the Upper Jurassic Archaeopteryx lithographica, the earliest known bird, and the Dromaeosauridae, the group of non-avian theropod dinosaurs regarded as most closely related to birds6,7. Here we describe a theropod dinosaur from Patagonia, Unenlagia comahuensis gen. et sp. nov., which partially fills this gap. Despite the relatively late appearance of this dinosaur in the fossil record (Upper Cretaceous), several features of Unenlagia are more bird-like than in any other non-avian theropod so far discovered.Unenlagia resembles Archaeopteryx in the morphology of the scapula, pelvis and hindlimb. But several shared, primitive features of the pubis, ischium and hindlimb proportions suggest that Unenlagia may represent the sister taxon of the Avialae (=Aves). The structure of the forelimb suggests that the avian mode of forelimb folding, and the extensive forelimb elevation necessary for powered, flapping flight, was already present in cursorial, non-flying theropod dinosaurs.

Natural selection may favour sexually similar brain size owing to similar selection pressures in males and females, while sexual selection may lead to sexually dimorphic brains. For example, sperm competition involves clear–cut sex differences in behaviour, as males display, mate guard and copulate with females, while females choose among males, and solicit or reject copulations. These behaviours may require fundamentally different neural government in the two sexes leading to sex–dependent brain evolution. Using two phylogenetic approaches in a comparative study, we tested for roles of both natural and sexual–selection pressures on brain size evolution of birds. In accordance with the natural–selection theory, relative brain size of males coevolved with that of females, which may be the result of adaptation to similar environmental constraints such as feeding innovation. However, the mode of brain size evolution differed between the sexes, and factors associated with sperm competition as reflected by extra–pair paternity may give rise to sexually size dimorphic brains. Specifically, species in which females have larger brains than males were found to have a higher degree of extra–pair paternity independently of potentially confounding factors, whereas species in which males have relatively larger brains than females appeared to have lower rates of extra–pair paternity. Hence, the evolution of sperm competition may select for complex behaviours together with the associated neural substrates in the sex that has a higher potential to control extra–pair copulations at the observed levels. Brain function may thus be affected differently in males and females by sexual selection.

The author provides a general overview of the molecular data used to reconstruct the avian tree of life, summarizes some highlights of the ensuing controversies, and reveals those taxonomic relationships that remain largely unchanged by molecular data.

Zhongornis refers to a ‘middle’ or ‘intermediate’ bird because its tail was claimed to be transitional between primitive birds with a long bony tail (such as Archaeopteryx and Jeholornis) and those with a short tail ending in a fused tip called the pygostyle (such as Confuciusornis) [1]. This finding is significant, as it appeared to fill a ‘gap’ at a critical stage in the early evolution of birds. However, after a detailed restudy, O’Connor and Sullivan [2] suggest that this juvenile bird from the Early Cretaceous Jehol fauna of China actually possesses 20, rather than 14, tail vertebrae; further, this tail is very similar to those of Epidexipteryx (a scansoriopterygid) and Caudipteryx (an oviraptorosaur) among the theropods. Based on this and other features, the authors reinterpretZhongornis as the sister taxon of scansoriopterygids, and further suggest that this clade (Zhongornis + Scansoriopterygidae) is the sister group of Oviraptorosauria. Surprisingly, despite the discovery of so many beautifully preserved feathered dinosaurs in China, the relationships among them, especially those close to the origin of birds, are still intriguing. This intrigue applies to Oviraptorosauria, the ‘egg thief lizards’, and Scansoriopterygidae, a recently recognized rare arboreal clade. Consider this: you have four tails, one each from an oviraptorosaur dinosaur, a dromaeosaur dinosaur, a primitive bird (Archaeopteryx), and a true bird (Confuciusornis). Like many others, you may have immediately seen (1) a long bony tail similar in dromaeosaurs and primitive birds, (2) a very short tail with a pygostyle unique in true birds, and (3) a reduced tail, an intermediate condition seen in oviraptorosaurs. However, most recent cladistic analyses reveal Oviraptorosauria as the sister group of Paraves, which includes dromaeosaurs and birds [3]. This forces us to interpret the tail similarities between oviraptorosaurs and birds as having evolved convergently. The robust and flexible oviraptorosaur tails, together with their broad tail-feather fans, were uniquely adapted to serve as dynamic intraspecific display structures [4]. However, the origin of oviraptorosaurs remains a mystery, with no Jurassic record. O’Connor and Sullivan’s study indicates that oviraptorosaurs’ cousins (scansoriopterygids) once lived in the Jurassic trees in the north-eastern China, and their differences might result from adaptation to contrasting ecological niches. Will taking Zhongornis’s tail out of the ‘middle’ bird tail allow us to eventually discover the real ‘middle’ creature someday? Perhaps not! A recent genetic and evolutionary development study shows that a single genetic mutation could have occurred in paravian dinosaurs, which both truncated its tail and fused its distal caudal vertebrae into a pygostyle [5]. Moreover, this potential mutation has pleiotropic effects also leading to fusions inmore anterior vertebrae, ribs, and even digits. These fusions have been observed in mice and modern birds, as well as in Confuciusornis and more advanced birds, but not in Jeholornis and Archaeopteryx [5]. The co-occurrence of Confuciusornis and Jeholornis about125millionyears ago also supports this sudden appearance of short-tailed birds. Therefore, the search for the fantastical middle bird may prove to be futile.

SummaryThe origin and early evolution of birds has been a major topic in evolutionary biology. In the 20th century, evolutionary scenarios posited either ground-based bird ancestors or tree-dwelling ancestors. This has since been recognised as a false dichotomy [1]. We suggest that part of the problem is the loose categorisation of many extant bird species as either ground or tree locomotors when considering hind-limb function [2–7]. In reality these are not mutually exclusive alternatives. Many extant birds exhibit different degrees of ground- and tree-based behaviours. We thus propose they can be better placed on a spectrum — rather than a dichotomy — according to the extent of ground and/or tree foraging they exhibit. To test this system we analysed the toe claws of 249 species of Holocene birds, revealing that claw curvature increases as tree foraging becomes more predominant. Improved claw morphometrics allow more direct comparisons between extant and extinct birds in order to infer the behaviours of the latter. In contrast to previous studies [2–6], we find that claw curvatures of Mesozoic birds and closely related non-avian theropod dinosaurs, differ significantly from Holocene arboreal birds and more closely resemble those of Holocene ‘ground-foraging’ birds.