Neognathous Birds Research Articles

Hindlimb bones texture through postnatal ages of Rhea americana (Aves: Palaeognathae).

The bone texture of Rhea americana was evaluated through the examination of a postnatal ontogenetic series. The hind limb bone surfaces of specimens of one, three and five months old, and adults were compared to characterize each stage according to the morphological features generated by their differential ossification. The results suggest a similar process of tissue maturation concerning neognathous birds, although with some differences. A spongy or striated surface with abundant pores in the femur and longitudinal grooves in the tibiotarsus and tarsometatarsus characterizes chicks. Vascularity decreases, and the bone texture gradually changes acquiring a more homogeneous surface, to eventually reach the smooth appearance of adult bones. The establishment of particular textures corresponding to different bones and ontogenetic stages permits the accurate interpretation of remains in ecological, archaeological and paleontological contexts in which bones are fragmented and/or incomplete.

Tinamiphilopsis temmincki sp. n., a New Quill Mite Species from Tataupa Tinamou, and the Early History of Syringophilid Mites.

The quill mite fauna of the Syringophilidae family (Acariformes: Prostigmata), which is associated with palaeognathous birds of the Tinamiformes order, remains poorly studied. Thus far, only two species of syringophilid mites have been documented on four species of tinamous. In this study, we present a description of a new species, Tinamiphilopsis temmincki sp. n., which was found on the Tataupa Tinamou (Crypturellus tataupa) in South America. This newly identified species differs from others in the genus due to the short hysteronotal setae d2 in females, unlike the long setae d2 found in females of other Tinamiphilopsis species. In addition to describing the new species, we conducted a phylogenetic analysis of the primitive syringophilid genera. The results reveal that the Tinamiphilopsis genus does not emerge as a sister group to all other syringophilids. Instead, it is deeply embedded within the radiation of quill mites associated with neognathous birds. This study provided evidence that mites belonging to the genus Tinamiphilopsis initially parasitised Neoavian birds before host switching to tinamous birds. This placement carries significant implications for our understanding of the evolution of quill mites and their relationship with their avian hosts.

Description of natal down of the ostrich (Struthio camelus) and comparison with common quail (Coturnix coturnix): Developmental and evolutionary implications.

Natal down is a feather stage that differs in both form and function from the definitive feathers of adult birds. It has a simpler structure that has been speculated to be similar to the body coverings of non-avian dinosaurs. However, inference of the evolution of natal down has been limited by our understanding of its structural variation in extant birds. Most descriptive work has focused on neognathous birds, limiting our knowledge of the full diversity of feathers in extant taxa. Here, we describe the natal down of a post-hatch ostrich (Struthio camelus) and compare it to that of a post-hatch quail (Coturnix coturnix). We confirm the presence of featherless spaces (apteria) in S. camelus and the lack of barbules on the tips of natal down in both species. We also find differences between dorsal and ventral natal down structures, such as barbule density in S. camelus and the extent of the bare portion of the barb in both species. Surprisingly, we do not find that the neoptiles of either species follow the ideal morphologies for increasing insulation. Finally, we hypothesize that the different barb types present in S. camelus natal down result from a large addition of new barb ridges during development, which is not known except in feathers with a rachis. These results have implications for our understanding of how structure informs function and development in understudied feather types, such as those shared by non-avian dinosaurs.

Osteological comparison of casque ontogeny in palaeognathous and neognathous birds: insights for selecting modern analogues in the study of cranial ornaments from extinct archosaurs

Abstract Birds, along with their dinosaurian precursors, possess a variety of bony cranial expansions. A deep understanding of the phenotypic complexity of these structures would be useful for addressing the development, evolution, and function of hard-tissue cranial ornamentation. Yet, the evolutionary significance and function of these structures have gone largely unaddressed because no unifying conceptual framework for interpreting bony cranial expansions currently exists. To provide such a framework, we examine osseous ornament variation in modern birds, using µ-CT imaging to examine the cranial casque components, structural composition, and developmental changes of two neognathous (Numida meleagris, Macrocephalon maleo) and one palaeognathous species (Casuarius casuarius) and survey the avian osteology literature of the 11 orders containing members with osseous cranial ornamentation. Our anatomical analyses suggest two broad configuration categories: (i) geminal, in which ornaments consist of paired elements only (i.e. within Neognathae) and (ii) disunited, in which ornaments consist of unpaired, midline elements along with paired bones (i.e. within Palaeognathae). Ornament bones contribute to casque elevation (proximal ornament support), elaboration (distal ornament shape), or both. Our results hold utility for unravelling the selection processes, particularly in difficult-to-decipher display roles, that shaped modern avian casques, as well as for the use of extant avians as comparative analogues of non-avian dinosaurs with ornamental head structures.

Why and how do Y chromosome stop recombining?

Why and how do Y chromosome stop recombining?

Dynamic Patterns of Sex Chromosome Evolution in Neognath Birds: Many Independent Barriers to Recombination at the ATP5F1A Locus

Avian sex chromosomes evolved after the divergence of birds and crocodilians from their common ancestor, so they are younger than the better-studied chromosomes of mammals. It has long been recognized that there may have been several stages to the evolution of avian sex chromosomes. For example, the CHD1 undergoes recombination in paleognaths but not neognaths. Genome assemblies have suggested that there may be variation in the timing of barriers to recombination among Neognathae, but there remains little understanding of the extent of this variability. Here, we look at partial sequences of ATP5F1A, which is on the avian Z and W chromosomes. It is known that recombination of this gene has independently ceased in Galliformes, Anseriformes, and at least five neoavian orders, but whether there are other independent cessations of recombination among Neoaves is not understood. We analyzed a combination of data extracted from published chromosomal-level genomes with data collected using PCR and cloning to identify Z and W copies in 22 orders. Our results suggest that there may be at least 19 independent cessations of recombination within Neognathae, and 3 clades that may still be undergoing recombination (or have only recently ceased recombination). Analyses of ATP5F1A protein sequences revealed an increased amino acid substitution rate for W chromosome gametologs, suggesting relaxed purifying selection on the W chromosome. Supporting this hypothesis, we found that the increased substitution rate was particularly pronounced for buried residues, which are expected to be more strongly constrained by purifying selection. This highlights the dynamic nature of avian sex chromosomes, and that this level of variation among clades means they should be a good system to understand sex chromosome evolution.

The impact of allometry on vomer shape and its implications for the taxonomy and cranial kinesis of crown-group birds

Crown birds are subdivided into two main groups, Palaeognathae and Neognathae, that can be distinguished, among others, by the organization of the bones in their pterygoid-palatine complex (PPC). Shape variation to the vomer, which is the most anterior part of the PPC, was recently analysed by Hu et al. (2019) with help of geometric morphometrics to discover morphological differences between palaeognath and neognath birds. Based on this study, the vomer was identified as sufficient to distinguish the two main groups (and even more inclusive neognath groups) and their cranial kinetic system. As there are notable size differences between the skulls of palaeognaths and neognaths, we here investigate the impact of allometry on vomeral shape and its implication for taxonomic classification by re-analysing the data of the previous study. Different types of multivariate statistical analyses reveal that taxonomic identification based on vomeral shape is strongly impaired by allometry, as the error of correct identification is high when shape data is corrected for size. This finding is evident by a great overlap between palaeognath and neognath subclades in morphospace. The correct identification is further influenced by the convergent presence of a flattened vomeral morphotype in multiple neognath subclades. As the evolution of cranial kinesis has been linked to vomeral shape in the original study, the existing correlation between shape and size of the vomer across different bird groups found in the present study questions this conclusion. In fact, cranial kinesis in crown birds results from the loss of the jugal-postorbital bar in the temporal region and ectopterygoid in the PPC and the combination of a mobilized quadrate-zygomatic arch complex and a flexible PPC. Therefore, we can conclude that the vomer itself is not a suitable proxy for exploring the evolution of cranial kinesis in crown birds and their ancestors.

Extinct crane-like birds (Eogruidae and Ergilornithidae) from the Cenozoic of Central Asia are indeed ostrich precursors

AbstractWe describe new fossils from the late Eocene of Mongolia, which show that the crane-like Eogruidae and Ergilornithidae are stem group representatives of the Struthioniformes (ostriches). Currently, both taxa are unanimously assigned to the neognathous Gruiformes (cranes and allies). However, ergilornithids show a progressive reduction of the second toe, and a few earlier authors likened these birds to ostriches, which are the only extant birds with just 2 toes. So far, eogruids and ergilornithids were mainly known from hindlimb bones from the Cenozoic of Asia, and here we provide important new data on the skeletal anatomy of these birds. A partial skull exhibits characteristic features of palaeognathous birds, and ostriches in particular. In its distinctive shape, it furthermore closely resembles the skull of the Eocene palaeognathous Palaeotididae, which are here also considered to be stem group representatives of the Struthioniformes. A femur from the late Eocene of Mongolia likewise corresponds to that of ostriches in derived traits, whereas cervical vertebrae exhibit features of neognathous birds. The fossils suggest that true ostriches (crown group Struthionidae) originated in Asia, and the Neognathae-like morphology of some bones opens a new perspective on the evolution of skeletal characteristics of palaeognathous birds.

Long-term stability of sex chromosome gene content allows accurate qPCR-based molecular sexing across birds.

Embryos, juveniles, and even adults of many bird species lack pronounced external sexually dimorphic characteristics. Accurate identification of sex is crucial for research (e.g., developmental, population, and evolutionary studies), management of wildlife species, and captive breeding programmes for both conservation and poultry. An accurate molecular sexing method applicable across the entire bird radiation is theoretically possible thanks to the long-term stability of their ZZ/ZW sex chromosomes, but current methods are not applicable in a wide range of bird lineages. Here, we developed a novel molecular sexing method based on the comparison of gene copy number variation by quantitative real-time PCR (qPCR) in conserved Z-specific genes (CHRNA6, DDX4, LPAR1, TMEM161B, VPS13A), i.e. genes linked to Z but absent from W chromosomes. We tested the method across three paleognath and 70 neognath species covering the avian phylogeny. In addition, we designed primers for four Z-specific genes (DOCK8, FUT10, PIGG and PSD3) for qPCR-based molecular sexing in three paleognath species. We have demonstrated that the genes DOCK8, FUT10, PIGG and PSD3 can identify sex in paleognath birds and the genes CHRNA6, DDX4, TMEM161B, and VPS13A can reveal sex in neognath birds. The gene LPAR1 can be used to accurately identify sex in both paleognath and neognath species. Along with outlining a novel method of practical importance for molecular sexing in birds, our study also documents in detail the conservation of sex chromosomes across the avian phylogeny.

Putative adult neurogenesis in palaeognathous birds: The common ostrich (Struthio camelus) and emu (Dromaius novaehollandiae).

In the current study, we examined adult neurogenesis throughout the brain of the common ostrich (Struthio camelus) and emu (Dromaius novaehollandiae) using immunohistochemistry for the endogenous markers PCNA which labels proliferating cells, and DCX, which stains immature and migrating neurons. The distribution of PCNA and DCX labelled cells was widespread throughout the brain of both species. The highest density of cells immunoreactive to both markers was observed in the olfactory bulbs and the telencephalon, especially the subventricular zone of the lateral ventricle. Proliferative hot spots, identified with strong PCNA and DCX immunolabelling, were identified in the dorsal and ventral poles of the rostral aspects of the lateral ventricles. The density of PCNA immunoreactive cells was less in the telencephalon of the emu compared to the common ostrich. Substantial numbers of PCNA immunoreactive cells were observed in the diencephalon and brainstem, but DCX immunoreactivity was weaker in these regions, preferentially staining axons and dendrites over cell bodies, except in the medial regions of the hypothalamus where distinct DCX immunoreactive cells and fibres were observed. PCNA and DCX immunoreactive cells were readily observed in moderate density in the cortical layers of the cerebellum of both species. The distribution of putative proliferating cells and immature neurons in the brain of the common ostrich and the emu is widespread, far more so than in mammals, and compares with the neognathous birds, and suggests that brain plasticity and neuronal turnover is an important aspect of cognitive brain functions in these birds.

Distal spinal nerve development and divergence of avian groups

The avian transition from long to short, distally fused tails during the Mesozoic ushered in the Pygostylian group, which includes modern birds. The avian tail embodies a bipartite anatomy, with the proximal separate caudal vertebrae region, and the distal pygostyle, formed by vertebral fusion. This study investigates developmental features of the two tail domains in different bird groups, and analyzes them in reference to evolutionary origins. We first defined the early developmental boundary between the two tail halves in the chicken, then followed major developmental structures from early embryo to post-hatching stages. Differences between regions were observed in sclerotome anterior/posterior polarity and peripheral nervous system development, and these were consistent in other neognathous birds. However, in the paleognathous emu, the neognathous pattern was not observed, such that spinal nerve development extends through the pygostyle region. Disparities between the neognaths and paleognaths studied were also reflected in the morphology of their pygostyles. The ancestral long-tailed spinal nerve configuration was hypothesized from brown anole and alligator, which unexpectedly more resembles the neognathous birds. This study shows that tail anatomy is not universal in avians, and suggests several possible scenarios regarding bird evolution, including an independent paleognathous long-tailed ancestor.

The otic region of the skull of neognathous birds: on the homology and comparative morphology of some neurovascular and muscular foramina and other external skeletal structures

The otic region of the skull of neognathous birds: on the homology and comparative morphology of some neurovascular and muscular foramina and other external skeletal structures

First discovery of avian egg and bone remains (Presbyornithidae) from the Gour Lazib (Eocene, Algeria)

Numerous avian eggshells and one well-preserved small ovoid-shaped egg (32 × 28 mm) were collected from three localities in the Gour Lazib area (HGL10, HGL51 and HGL62), western Algeria from the Glib Zegdou Formation dated from the late early or early middle Eocene. This material was associated with a rich vertebrate fauna. The study of the eggshells using microscopies (PLM and SEM) led to attribute them to neognathous birds. They are characterized by a thin shell (less than 0.5 mm thick) with a prismatic trilaminate microstructure. The occurrence of an avian carpometacarpus belonging to the Presbyornithidae from HGL50, which is located 1 m below the HGL 51 level, suggests that the egg remains may be referred to this bird family, which is so far unknown in the Palaeogene of the Afro-Arabian continent.

How to walk carryinga huge egg? Trade-offs between locomotion and reproduction explain the special pelvis and leg anatomy in kiwi (Aves; Apteryx spp.).

Kiwi (Aves; genus Apteryx) are famous for laying an enormous egg in comparison with their relatively small body size. Considering the peculiar gait of this flightless bird, we suspected the existence of morpho-functional trade-offs between reproduction and locomotion. To understand how structural constraints, imposed by a large egg size, might influence the terrestrial locomotion of Apteryx, we analysed the anatomy of the limb osteomuscular system in two species of kiwi (Apteryx mantelli and Apteryx owenii). We performed detailed dissections and brought to light specific anatomical features of kiwi, in comparison with other ratites and neognathous birds. Our osteological study revealed a strongly curved pelvis, a rigid tail, and enlarged ribs. Our myology study showed an unusual location of the caudofemoralis muscle origin and insertion. The insertion of the pars pelvica along the entire caudal face of the femur, contrasts with the proximal insertion usually seen in other birds. Additionally, the pars caudalis originates along the entire tail, whereas it only inserts on the uropygium in the other birds. To interpret these specificities from a functional point of view, we built three-dimensional osteomuscular models based on computed tomography scans, radiographies and our dissections. We chose three postures associated with reproductive constraints: the standing position of a gravid compared with a non-gravid bird, as well as the brooding position. The 3D model of the brooding position suggested that the enlarged ribs could support the bodyweight when leaning on the huge egg in both males and females. Moreover, we found that in gravid females, the unusual shape of the pelvis and tail allowed the huge egg to sit ventrally below the pelvis, whereas it is held closer to the rachis in other birds. The specific conformation of the limb and the insertions of the two parses of the caudofemoralis help to maintain the tail flexed, and to keep the legs adducted when carrying the egg. The caudal location of the hip and its flexed position explains the long stance phase during the strange gait of kiwi, revealing the functional trade-off between reproduction and locomotion in this emblematic New Zealand bird.

Avian mitochondrial genomes retrieved from museum eggshell

Avian eggshell is a bio-ceramic material with exceptional properties for preserving DNA within its crystalline structure, presenting an opportunity to retrieve genomic information from extinct or historical populations of birds. However, intracrystalline DNA has only been recovered from the large, thick eggshell of palaeognaths; members of their more-diverse sister group (neognaths) lay smaller, thinner eggs that may not exhibit the same propensity for DNA preservation. Here, we use three 40-60-year-old museum eggshell specimens of Australian neognath birds to determine the minimum mass of eggshell from which intracrystalline DNA can be retrieved, and to characterize the yield and quality of such DNA. In doing so, we describe the first protocol for successful extraction of intracrystalline DNA from neognath eggshells, with the view to unlocking the potential of vast museum egg collections for genetic research. We were able to retrieve DNA fragments over 200bp in length from 10mg of eggshell powder from all three specimens, and demonstrate that expanding the existing blow-hole can allow sufficient material to be collected for DNA extraction while minimizing damage to the appearance and structural integrity of the egg. Furthermore, we were able to reconstruct near-complete mitochondrial genomes at a coverage of 40-83X through shotgun sequencing of these extracts on three NextSeq lanes. Given the current extinction and extirpation rates of many avian species world-wide, genetic data from eggshell could provide a rapid and cost-effective approach to examining temporal changes in avian diversity, which is not only becoming crucial for conservation management, but also serve to deepen our understanding of genome-wide evolutionary processes.

Comparative morphology of the avian maxillary bone (os maxillare) based on an examination of macerated juvenile skeletons

AbstractFor the first time, isolated maxillary bones of juvenile neornithine birds are examined and compared. Contrary to the anatomical terminology currently employed, the avian maxillare exhibits five rather than four processes. In addition to the praemaxillary, jugal, nasal, and maxillopalatine processes, all palaeognathous and many neognathous birds also have a palatine process. The occurrence of these processes is, however, variable across different clades and only few taxa exhibit a pentaradiate maxillare with all five processes. Within Neognathae, a great morphological variability exists in the shape of the maxillopalatine process, which is more easily studied in juvenile individuals, in which the bones of the beak and palate are not co‐ossified. In some Neognathae, a caudally facing recess is situated in the junction of the maxillopalatine and jugal processes, which is likely to be homologous to the pneumatic recess of palaeognathous birds. Several derived morphologies of potential phylogenetic significance for the characterization of neognathous clades are identified and major morphological transformations in the lineage leading towards modern birds are highlighted.

Functional morphology of the cranio-mandibular complex of the Guira cuckoo (Aves).

The cranio-mandibular complex is an important structure involved in food capture and processing. Its morphology is related to the nature of the food item. Jaw muscles enable the motion of this complex and their study is essential for functional and evolutionary analysis. The present study compares available behavioral and dietary data obtained from the literature with novel results from functional morphological analyses of the cranio-mandibular complex of the Guira cuckoo (Guira guira) to understand its relationship with the zoophagous trophic habit of this species. The bite force was estimated based on muscle dissections, measurements of the physiological cross-sectional area, and biomechanical modeling of the skull. The results were compared with the available functional morphological data for other birds. The standardized bite force of G. guira is higher than predicted for exclusively zoophagous birds, but lower than for granivorous and/or omnivorous birds. Guira guira possesses the generalized jaw muscular system of neognathous birds, but some features can be related to its trophic habit. The external adductor muscles act mainly during food item processing and multiple aspects of this muscle group are interpreted to increase bite force, that is, their high values of muscle mass, their mechanical advantage (MA), and their perpendicular orientation when the beak is closed. The m. depressor mandibulae and the m. pterygoideus dorsalis et ventralis are interpreted to prioritize speed of action (low MA values), being most important during prey capture. The supposed ecological significance of these traits is the potential to widen the range of prey size that can be processed and the possibility of rapidly capturing agile prey through changes in the leverage of the muscles involved in opening and closing of the bill. This contributes to the trophic versatility of the species and its ability to thrive in different habitats, including urban areas.

Morphological phylogenetic conflict in the parasitic mite family Harpirhynchidae (Acariformes: Cheyletoidea) correlates with host associations

The mite family Harpirhynchidae (Acariformes: Cheyletoidea) comprises 154 named species from 15 genera, all of which are permanent parasites of birds or snakes. We inferred the phylogeny of the family with maximum parsimony (MP) and Bayesian analyses for 67 ingroup and 3 outgroup species from the families Cheyletidae, Demodecidae, and Psorergatidae based on 177 morphological characters. The analyses revealed uncertainty of monophyly of the subfamily Harpirhynchinae. The subfamilies Harpypalpinae and Ophioptinae constitute a monophyletic group which is either sister to Demodecidae+Psorergatidae (supported by Bayesian analysis and by 59% of MP trees) or Harpirhynchinae (41% of MP trees). Harpirhynchids were shown to be highly host specific, with 69.5% being monoxenous species and 23% species are associated with several closely related hosts. Results of multinomial logistic regression showed that the microhabitat on the host and type of cysts produced by mites do not significantly influence at level of their host specificity. Ancestral host reconstruction shows that mites of the clade Harpypalpinae+Ophioptinae evolved to parasitism on Passeriformes independently from the Harpirhynchinae. Ancestor of ophioptines switched to an ancestor of colubroid snakes from passerines. In contrast, harpirhynchines colonized a common ancestor of neognathous birds. In the subfamily Harpirhynchinae, the ancestral microhabitat was probably the surface of the host's skin, with intracutaneous parasitism being a more derived lifestyle. According to reconstructed phylogenetic pattern, parasitism at the feather bases of passerine hosts arose independently in two genera (Neharpyrhynchus and Trichorhynchiella) from intracutaneous parasitism.

Sleep-Related Electrophysiology and Behavior of Tinamous (Eudromia elegans): Tinamous Do Not Sleep Like Ostriches

The functions of slow wave sleep (SWS) and rapid eye movement (REM) sleep, distinct sleep substates present in both mammals and birds, remain unresolved. One approach to gaining insight into their function is to trace the evolution of these states through examining sleep in as many taxonomic groups as possible. The mammalian and avian clades are each composed of two extant groups, i.e., the monotremes (echidna and platypus) and therian (marsupial and eutherian [or placental]) mammals, and Palaeognaths (cassowaries, emus, kiwi, ostriches, rheas, and tinamous) and Neognaths (all other birds) among birds. Previous electrophysiological studies of monotremes and ostriches have identified a unique “mixed” sleep state combining features of SWS and REM sleep unlike the well-delineated sleep states observed in all therian mammals and Neognath birds. In the platypus this state is characterized by periods of REM sleep-related myoclonic twitching, relaxed skeletal musculature, and rapid eye movements, occurring in conjunction with SWS-related slow waves in the forebrain electroencephalogram (EEG). A similar mixed state was also observed in ostriches; although in addition to occurring during periods with EEG slow waves, reduced muscle tone and rapid eye movements also occurred in conjunction with EEG activation, a pattern typical of REM sleep in Neognath birds. Collectively, these studies suggested that REM sleep occurring exclusively as an integrated state with forebrain activation might have evolved independently in the therian and Neognath lineages. To test this hypothesis, we examined sleep in the elegant crested tinamou (Eudromia elegans), a small Palaeognath bird that more closely resembles Neognath birds in size and their ability to fly. A 24-h period was scored for sleep state based on electrophysiology and behavior. Unlike ostriches, but like all of the Neognath birds examined, all indicators of REM sleep usually occurred in conjunction with forebrain activation in tinamous. The absence of a mixed REM sleep state in tinamous calls into question the idea that this state is primitive among Palaeognath birds and therefore birds in general.

The centrifugal visual system of a palaeognathous bird, the Chilean Tinamou (Nothoprocta perdicaria).

The avian centrifugal visual system, which projects from the brain to the retina, has been intensively studied in several Neognathous birds that have a distinct isthmo-optic nucleus (ION). However, birds of the order Palaeognathae seem to lack a proper ION in histologically stained brain sections. We had previously reported in the palaeognathous Chilean Tinamou (Nothoprocta perdicaria) that intraocular injections of Cholera Toxin B subunit retrogradely label a considerable number of neurons, which form a diffuse isthmo-optic complex (IOC). In order to better understand how this IOC-based centrifugal visual system is organized, we have studied its major components by means of in vivo and in vitro tracing experiments. Our results show that the IOC, though structurally less organized than an ION, possesses a dense core region consisting of multipolar neurons. It receives afferents from neurons in L10a of the optic tectum, which are distributed with a wider interneuronal spacing than in Neognathae. The tecto-IOC terminals are delicate and divergent, unlike the prominent convergent tecto-ION terminals in Neognathae. The centrifugal IOC terminals in the retina are exclusively divergent, resembling the terminals from "ectopic" centrifugal neurons in Neognathae. We conclude that the Tinamou's IOC participates in a comparable general IOC-retina-TeO-IOC circuitry as the neognathous ION. However, the connections between the components are structurally different and their divergent character suggests a lower spatial resolution. Our findings call for further comparative studies in a broad range of species for advancing our understanding of the evolution, plasticity and functional roles of the avian centrifugal visual system.