Comparative osteology of the skull of cowbirds (Icteridae: Molothrus)

Detailed osteological descriptions of the craniomandibular complex of passerine birds are lacking for most species, limiting our understanding of their diversity and evolution. Cowbirds (genus Molothrus) are a small but widespread group of New World nine-primaried songbirds, well-known for their unique brooding parasitic behavior. However, detailed osteological data for cowbirds and other Icteridae are currently scarce and several features of their skulls remain undescribed or poorly known. To address this issue, a detailed comparative osteology of cowbird skulls is presented here for the first time based on data from x-ray microcomputed tomography, dry skeletal data, and multivariate analyses of linear morphometric data. Cowbird skulls offer some functional insights, with many finch-like features probably related to a seed-rich diet that distinguishes them from most other icterids. In addition, features previously overlooked in earlier studies might provide valuable phylogenetic information at different levels of passerine phylogeny (Passerida, Emberizoidea, Icteridae, and Agelaiinae), including some of the otic region and nasal septum. Comparisons among cowbirds show that there is substantial cranial variation within the genus, with M. oryzivorus being the most divergent cowbird species. Within the genus, distantly related species share similar overall skull morphology and proportions, but detailed osteological data allow species identification even in cases of strong convergence. Further efforts are warranted to furnish baseline data for future studies of this iconic group of Neotropical birds and to fully integrate it into phylogenetic comparative frameworks.

Paleoneurology of stem palaeognaths clarifies the plesiomorphic condition of the crown bird central nervous system

Lithornithidae, an assemblage of volant Palaeogene fossil birds, provide our clearest insights into the early evolutionary history of Palaeognathae, the clade that today includes the flightless ratites and volant tinamous. The neotype specimen of Lithornis vulturinus, from the early Eocene (approximately 53 million years ago) of Europe, includes a partial neurocranium that has never been thoroughly investigated. Here, we describe these cranial remains including the nearly complete digital endocasts of the brain and bony labyrinth. The telencephalon of Lithornis is expanded and its optic lobes are ventrally shifted, as is typical for crown birds. The foramen magnum is positioned caudally, rather than flexed ventrally as in some crown birds, with the optic lobes, cerebellum, and foramen magnum shifted further ventrally. The overall brain shape is similar to that of tinamous, the only extant clade of flying palaeognaths, suggesting that several aspects of tinamou neuroanatomy may have been evolutionarily conserved since at least the early Cenozoic. The estimated ratio of the optic lobe's surface area relative to the total brain suggests a diurnal ecology. Lithornis may provide the clearest insights to date into the neuroanatomy of the ancestral crown bird, combining an ancestrally unflexed brain with a caudally oriented connection with the spinal cord, a moderately enlarged telencephalon, and ventrally shifted, enlarged optic lobes.

Brain surface morphology and ecological and macroevolutionary inferences of avian New World suboscines (Aves, Passeriformes, Tyrannides)

The New World suboscines (Passeriformes and Tyrannides) are one of the biggest endemic vertebrate radiations in South America, including the families Furnariidae and Tyrannidae. Avian brain morphology is a reliable proxy to study their evolution. The aim of this work is to elucidate whether the brains of these families reflect the ecological differences (e.g., feeding behavior) and to clarify macroevolutionary aspects of their neuroanatomy. Our hypotheses are as follows: Brain size is similar between both families and with other Passeriformes; brain morphology in Tyrannides is the result of the pressure of ecological factors; and brain disparity is low since they share ecological traits. Skulls of Furnariidae and Tyrannidae were micro-computed tomography-scanned, and three-dimensional models of the endocast were generated. Regression analyses were performed between brain volume and body mass. Linear and surface measurements were used to build phylomorphospaces and to calculate the amount of phylogenetic signal. Tyrannidae showed a larger brain disparity than Furnariidae, although it is not shaped by phylogeny in the Tyrannides. Furnariidae present enlarged Wulsts (eminentiae sagittales) but smaller optic lobes, while in Tyrannidae, it is the opposite. This could indicate that in Tyrannides there is a trade-off between the size of these two visual-related brain structures.

Evolution of Avian Eye Size Is Associated with Habitat Openness, Food Type and Brain Size

The eye is the primary sensory organ that obtains information from the ecological environments and specifically bridges the brain with the extra environment. However, the coevolutionary relationships between eye size and ecological factors, behaviours and brain size in birds remain poorly understood. Here, we investigate whether eye size evolution is associated with ecological factors (e.g., habitat openness, food type and foraging habitat), behaviours (e.g., migration and activity pattern) and brain size among 1274 avian species using phylogenetically controlled comparative analyses. Our results indicate that avian eye size is significantly associated with habitat openness, food type and brain size. Species living in dense habitats and consuming animals exhibit larger eye sizes compared to species living in open habitats and consuming plants, respectively. Large-brained birds tend to possess larger eyes. However, migration, foraging habitat and activity pattern were not found to be significantly associated with eye size in birds, except for nocturnal birds having longer axial lengths than diurnal ones. Collectively, our results suggest that avian eye size is primarily influenced by light availability, food need and cognitive ability.

The relationship between hard and soft tissue structures of the eye in extant lizards

The sizes of the eye structures, such as the lens diameter and the axial length, are important factors for the visual performance and are considered to be related to the mode of life. Although the size of these soft structures cannot be directly observed in fossil taxa, such information may be obtained from measuring size and morphology of the bony scleral ossicle ring, which is present in the eyes of extant saurospids, excluding crocodiles and snakes, and is variously preserved in fossil taxa. However, there have been only a few studies investigating the relationships between the size, the scleral ossicle ring, and soft structures of the eye. We investigated such relationships among the eye structures in extant Squamata, to establish the basis for inferring the size of the soft structures in the eye in fossil squamates. Three‐dimensional morphological data on the eye and head region of 59 lizard species covering most major clades were collected using micro‐computed tomography scanners. Strong correlations were found between the internal and external diameters of the scleral ossicle ring and soft structures. The tight correlations found here will allow reliable estimations of the sizes of soft structures and inferences on the visual performance and mode of life in fossil squamates, based on the diameters of their preserved scleral ossicle rings. Furthermore, the comparison of the allometric relationships between structures in squamates eyes with those in avian eyes suggest the possibility that the similarities of these structures closely reflect the mechanism of accommodation.

The sizes of the eye structures are important factors for the visual performance. Strong correlations were found between the scleral ossicle ring and soft structures in extant squamates eyes. These correlations will allow reliable estimations of soft structures and inferences on the visual performance and mode of life in fossil squamates.

Not like night and day: the nocturnal letter-winged kite does not differ from diurnal congeners in orbit or endocast morphology

Nocturnal birds display diverse adaptations of the visual system to low-light conditions. The skulls of birds reflect many of these and are used increasingly to infer nocturnality in extinct species. However, it is unclear how reliable such assessments are, particularly in cases of recent evolutionary transitions to nocturnality. Here, we investigate a case of recently evolved nocturnality in the world's only nocturnal hawk, the letter-winged kite Elanus scriptus. We employed phylogenetically informed analyses of orbit, optic foramen and endocast measurements from three-dimensional reconstructions of micro-computed tomography scanned skulls of the letter-winged kite, two congeners, and 13 other accipitrid and falconid raptors. Contrary to earlier suggestions, the letter-winged kite was not unique in any of our metrics. However, all species of Elanus have significantly higher ratios of orbit versus optic foramen diameter, suggesting high visual sensitivity at the expense of acuity. In addition, visual system morphology varies greatly across accipitrid species, likely reflecting hunting styles. Overall, our results suggest that the transition to nocturnality can occur rapidly and without changes to key hard-tissue indicators of vision, but also that hard-tissue anatomy of the visual system may provide a means of inferring a range of raptor behaviours, well beyond nocturnality.

New Remains of Scandiavis mikkelseni Inform Avian Phylogenetic Relationships and Brain Evolution

Although an increasing number of studies are combining skeletal and neural morphology data in a phylogenetic context, most studies do not include extinct taxa due to the rarity of preserved endocasts. The early Eocene avifauna of the Fur Formation of Denmark presents an excellent opportunity for further study of extinct osteological and endocranial morphology as fossils are often exceptionally preserved in three dimensions. Here, we use X-ray computed tomography to present additional material of the previously described taxon Scandiavis mikkelseni and reassess its phylogenetic placement using a previously published dataset. The new specimen provides novel insights into the osteological morphology and brain anatomy of Scandiavis. The virtual endocast exhibits a morphology comparable to that of modern avian species. Endocranial evaluation shows that it was remarkably similar to that of certain extant Charadriiformes, yet also possessed a novel combination of traits. This may mean that traits previously proposed to be the result of shifts in ecology later in the evolutionary history of Charadriiformes may instead show a more complex distribution in stem Charadriiformes and/or Gruiformes depending on the interrelationships of these important clades. Evaluation of skeletal and endocranial character state changes within a previously published phylogeny confirms both S. mikkelseni and a putative extinct charadriiform, Nahmavis grandei, as charadriiform. Results bolster the likelihood that both taxa are critical fossils for divergence dating and highlight a biogeographic pattern similar to that of Gruiformes.

Artificial night light and anthropogenic noise interact to influence bird abundance over a continental scale

The extent of artificial night light and anthropogenic noise (i.e., "light" and "noise") impacts is global and has the capacity to threaten species across diverse ecosystems. Existing research involving impacts of light or noise has primarily focused on noise or light alone and single species; however, these stimuli often co-occur and little is known about how co-exposure influences wildlife and if and why species may vary in their responses. Here, we had three aims: (1) to investigate species-specific responses to light, noise, and the interaction between the two using a spatially explicit approach to model changes in abundance of 140 prevalent bird species across North America, (2) to investigate responses to the interaction between light exposure and night length, and (3) to identify functional traits and habitat affiliations that explain variation in species-specific responses to these sensory stimuli with phylogenetically informed models. We found species that responded to noise exposure generally decreased in abundance, and the additional presence of light interacted synergistically with noise to exacerbate its negative effects. Moreover, the interaction revealed negative emergent responses for several species that only reacted when light and noise co-occurred. Additionally, an interaction between light and night length revealed 47 species increased in abundance with light exposure during longer nights. In addition to modifying behavior with optimal temperature and potential foraging opportunities, birds might be attracted to light, yet suffer inadvertent physiological consequences. The trait that most strongly related to avian response to light and noise was habitat affiliation. Specifically, species that occupy closed habitat were less tolerant of both sensory stressors compared to those that occupy open habitat. Further quantifying the contexts and intrinsic traits that explain how species respond to noise and light will be fundamental to understanding the ecological consequences of a world that is ever louder and brighter.

Endocranial morphology of the piciformes (Aves, Coraciimorphae): Functional and ecological implications

We used three-dimensional digital models to investigate the brain and endosseous labyrinth morphology of selected Neotropical Piciformes (Picidae, Ramphastidae, Galbulidae and Bucconidae). Remarkably, the brain morphology of Galbulidae clearly separates from species of other families. The eminentiae sagittales of Galbulidae and Bucconidae (insectivorous with high aerial maneuverability abilities) are smaller than those of the toucans (scansorial frugivores). Galbula showed the proportionally largest cerebellum, and Ramphastidae showed the least foliated one. Optic lobes ratio relative to the telencephalic hemispheres showed a strong phylogenetic signal. Three hypotheses were tested: (a) insectivorous taxa that need precise and fast movements to catch their prey, have well developed eminentiae sagittales compared to fruit eaters, (b) species that require high beak control would show larger cerebellum compared to other brain regions and higher number of visible folia and (c) there are marked differences between the brain shape of the four families studied here that bring valuable information of this interesting bird group. Hypotheses H1 and H2 are rejected, meanwhile H3 is accepted.

Endocranial Anatomy of the Giant Extinct Australian Mihirung Birds (Aves, Dromornithidae)

Dromornithids are an extinct group of large flightless birds from the Cenozoic of Australia. Their record extends from the Eocene to the late Pleistocene. Four genera and eight species are currently recognised, with diversity highest in the Miocene. Dromornithids were once considered ratites, but since the discovery of cranial elements, phylogenetic analyses have placed them near the base of the anseriforms or, most recently, resolved them as stem galliforms. In this study, we use morphometric methods to comprehensively describe dromornithid endocranial morphology for the first time, comparing Ilbandornis woodburnei and three species of Dromornis to one another and to four species of extant basal galloanseres. We reveal that major endocranial reconfiguration was associated with cranial foreshortening in a temporal series along the Dromornis lineage. Five key differences are evident between the brain morphology of Ilbandornis and Dromornis, relating to the medial wulst, the ventral eminence of the caudoventral telencephalon, and morphology of the metencephalon (cerebellum + pons). Additionally, dromornithid brains display distinctive dorsal (rostral position of the wulst), and ventral morphology (form of the maxillomandibular [V2+V3], glossopharyngeal [IX], and vagus [X] cranial nerves), supporting hypotheses that dromornithids are more closely related to basal galliforms than anseriforms. Functional interpretations suggest that dromornithids were specialised herbivores that likely possessed well-developed stereoscopic depth perception, were diurnal and targeted a soft browse trophic niche.

Sensory pollutants alter bird phenology and fitness across a continent

Expansion of anthropogenic noise and night lighting across our planet^1,2 is of increasing conservation concern^3-6. Despite growing knowledge of physiological and behavioural responses to these stimuli from single-species and local-scale studies, whether these pollutants affect fitness is less clear, as is how and why species vary in their sensitivity to these anthropic stressors. Here we leverage a large citizen science dataset paired with high-resolution noise and light data from across the contiguous United States to assess how these stimuli affect reproductive success in 142 bird species. We find responses to both sensory pollutants linked to the functional traits and habitat affiliations of species. For example, overall nest success was negatively correlated with noise among birds in closed environments. Species-specific changes in reproductive timing and hatching success in response to noise exposure were explained by vocalization frequency, nesting location and diet. Additionally, increased light-gathering ability of species' eyes was associated with stronger advancements in reproductive timing in response to light exposure, potentially creating phenological mismatches⁷. Unexpectedly, better light-gathering ability was linked to reduced clutch failure and increased overall nest success in response to light exposure, raising important questions about how responses to sensory pollutants counteract or exacerbate responses to other aspects of global change, such as climate warming. These findings demonstrate that anthropogenic noise and light can substantially affect breeding bird phenology and fitness, and underscore the need to consider sensory pollutants alongside traditional dimensions of the environment that typically inform biodiversity conservation.

The endocast of the Night Parrot (Pezoporus occidentalis) reveals insights into its sensory ecology and the evolution of nocturnality in birds

The Night Parrot (Pezoporus occidentalis) is a rare, nocturnal parrot species that has largely escaped scientific investigation due to its behaviour and habitat preferences. Recent field studies have revealed some insights into Night Parrot behaviour, but nothing is known of its sensory abilities. Here, we used μCT scans of an intact Night Parrot specimen to determine if its visual system shares similarities with other nocturnal species. The endocast of the Night Parrot revealed relatively small optic lobes and optic foramina, especially compared with closely related grass parakeets, but no apparent differences in orbit dimensions. Our data suggests that the Night Parrot likely has lower visual acuity than most other parrots, including its congener, the Eastern Ground Parrot (P. wallicus). We propose that the visual system of the Night Parrot might represent a compromise between the need to see under low light conditions and the visual acuity required to detect predators, forage, and fly. Based on the endocast and optic foramen measurements, the Night Parrot fits into a common pattern of decreased retinal input to the optic lobes in birds that should be explored more thoroughly in extant and extinct species.

Avian palaeoneurology: Reflections on the eve of its 200th anniversary

In birds, the brain (especially the telencephalon) is remarkably developed, both in relative volume and complexity. Unlike in most early-branching sauropsids, the adults of birds and other archosaurs have a well-ossified neurocranium. In contrast to the situation in most of their reptilian relatives but similar to what can be seen in mammals, the brains of birds fit closely to the endocranial cavity so that their major external features are reflected in the endocasts. This makes birds a highly suitable group for palaeoneurological investigations. The first observation about the brain in a long-extinct bird was made in the first quarter of the 19th century. However, it was not until the 2000s and the application of modern imaging technologies that avian palaeoneurology really took off. Understanding how the mode of life is reflected in the external morphology of the brains of birds is but one of several future directions in which avian palaeoneurological research may extend. Although the number of fossil specimens suitable for palaeoneurological explorations is considerably smaller in birds than in mammals and will very likely remain so, the coming years will certainly witness a momentous strengthening of this rapidly growing field of research at the overlap between ornithology, palaeontology, evolutionary biology and neurosciences.

ESTUDO DESCRITIVO E MORFOMÉTRICO DO CRÂNIO DO Ramphastos toco (AVES: PICIFORMES)

Resumo O estudo de caracteres morfológicos e suas funções biomecânicas na estrutura craniana das aves levanta debates de cunho filogenético, taxonômico e morfofuncional. O objetivo deste trabalho foi descrever e analisar caracteres morfológicos e morfométricos do esqueleto do Ramphastos toco e confrontá-los com a bibliografia disponível. Foram analisados 13 crânios de indivíduos adultos e de um espécime dissecado fixado em formol (4%). As carcaças foram submetidas à técnica de maceração para o preparo dos esqueletos, com a retirada dos tecidos moles, desengorduramento e clareamento em água oxigenada (50%) e hipoclorito de sódio (50%). O formato e amplitude da sutura naso-frontal e zona flexora craniofacial, a presença dos sesamóides intracapsulares rostral e caudal na articulação quadrática e a presença do ligamento pós-orbital foram fatores que desafiaram a bibliografia existente e os modelos de transferência de força propostos durante estudos da cinética craniana da espécie. A discussão que emerge é a possibilidade dessas características limitadoras da cinética craniana descritas nos espécimes possuírem, em conjunto, a função de promover estabilidade funcional para uma estrutura cuja natureza dimensional, em si, gera instabilidade. Os resultados morfométricos também apresentaram divergências significativas quando comparados à bibliografia disponível, o que pode estar relacionado a variáveis como especiação alopátrica, hibridização e amostragem.

3.0 Tesla magnetic resonance imaging anatomy of the central nervous system, eye, and inner ear in birds of prey

Despite the increasing interest in the clinical neurology of birds, little is known about the magnetic resonance imaging (MRI) appearance of the avian central nervous system, eye, and inner ear. The objective of this cadaveric study was to document the MRI anatomic features of the aforementioned structures using a high-resolution 3.0 Tesla MRI system. The final study group consisted of 13 cadavers of the diurnal birds of prey belonging to six species. Images were acquired in sagittal, dorsal, and transverse planes using T1-weighted and T2-weighted turbo spin echo sequences. A necropsy with macroscopic analysis of the brain and spinal cord was performed on all cadavers. Microscopic examination of the brain was performed on one cadaver of each species; the spinal cord was examined in three subjects. Anatomic structures were identified on the magnetic resonance images based on histologic slices and available literature. Very good resolution of anatomic detail was obtained. The olfactory bulbs; cerebral hemispheres; diencephalon; optic lobe; cerebellum; pons; ventricular system; optic, trigeminal, and facial nerves; pineal and pituitary glands; as well as the semicircular canals of the inner ear were identified. Exquisite detail was achieved on the ocular structures. In the spinal cord, the gray and white matter differentiation and the glycogen body were identified. This study establishes normal MRI anatomy of the central nervous system, eye, and inner ear of the birds of prey; and may be used as a reference in the assessment of neurologic disorders or visual impairment in this group of birds.

Best practices for digitally constructing endocranial casts: examples from birds and their dinosaurian relatives

The rapidly expanding interest in, and availability of, digital tomography data to visualize casts of the vertebrate endocranial cavity housing the brain (endocasts) presents new opportunities and challenges to the field of comparative neuroanatomy. The opportunities are many, ranging from the relatively rapid acquisition of data to the unprecedented ability to integrate critically important fossil taxa. The challenges consist of navigating the logistical barriers that often separate a researcher from high-quality data and minimizing the amount of non-biological variation expressed in endocasts - variation that may confound meaningful and synthetic results. Our purpose here is to outline preferred approaches for acquiring digital tomographic data, converting those data to an endocast, and making those endocasts as meaningful as possible when considered in a comparative context. This review is intended to benefit those just getting started in the field but also serves to initiate further discussion between active endocast researchers regarding the best practices for advancing the discipline. Congruent with the theme of this volume, we draw our examples from birds and the highly encephalized non-avian dinosaurs that comprise closely related outgroups along their phylogenetic stem lineage.

A reappraisal of Cerebavis cenomanica (Aves, Ornithurae), from Melovatka, Russia

The evolution of the avian brain is of crucial importance to studies of the transition from non-avian dinosaurs to modern birds, but very few avian fossils provide information on brain morphological development during the Mesozoic. An isolated specimen from the Cenomanian of Melovatka in Russia was described by Kurochkin and others as a fossilized brain, designated the holotype of Cerebavis cenomanica Kurochkin and Saveliev and tentatively referred to Enantiornithes. We have previously highlighted that this specimen is an incomplete skull, rendering the diagnostic characters invalid and Cerebavis cenomanica a nomen dubium. We provide here a revised diagnosis of Cerebavis cenomanica based on osteological characters, and a reconstruction of the endocranial morphology (= brain shape) based on μCT investigation of the braincase. Absence of temporal fenestrae indicates an ornithurine affinity for Cerebavis. The brain of this taxon was clearly closer to that of modern birds than to Archaeopteryx and does not represent a divergent evolutionary pathway as originally concluded by Kurochkin and others. No telencephalic wulst is present, suggesting that this advanced avian neurological feature was not recognizably developed 93 million years ago.

Comparative study of visual pathways in owls (Aves: Strigiformes)

Although they are usually regarded as nocturnal, owls exhibit a wide range of activity patterns, from strictly nocturnal, to crepuscular or cathemeral, to diurnal. Several studies have shown that these differences in the activity pattern are reflected in differences in eye morphology and retinal organization. Despite the evidence that differences in activity pattern among owl species are reflected in the peripheral visual system, there has been no attempt to correlate these differences with changes in the visual regions in the brain. In this study, we compare the relative size of nuclei in the main visual pathways in nine species of owl that exhibit a wide range of activity patterns. We found marked differences in the relative size of all visual structures among the species studied, both in the tectofugal and the thalamofugal pathway, as well in other retinorecipient nuclei, including the nucleus lentiformis mesencephali, the nucleus of the basal optic root and the nucleus geniculatus lateralis, pars ventralis. We show that the barn owl (Tyto alba), a species widely used in the study of the integration of visual and auditory processing, has reduced visual pathways compared to strigid owls. Our results also suggest there could be a trade-off between the relative size of visual pathways and auditory pathways, similar to that reported in mammals. Finally, our results show that although there is no relationship between activity pattern and the relative size of either the tectofugal or the thalamofugal pathway, there is a positive correlation between the relative size of both visual pathways and the relative number of cells in the retinal ganglion layer.

Anatomical specializations for nocturnality in a critically endangered parrot, the Kakapo (Strigops habroptilus)

The shift from a diurnal to nocturnal lifestyle in vertebrates is generally associated with either enhanced visual sensitivity or a decreased reliance on vision. Within birds, most studies have focused on differences in the visual system across all birds with respect to nocturnality-diurnality. The critically endangered Kakapo (Strigops habroptilus), a parrot endemic to New Zealand, is an example of a species that has evolved a nocturnal lifestyle in an otherwise diurnal lineage, but nothing is known about its' visual system. Here, we provide a detailed morphological analysis of the orbits, brain, eye, and retina of the Kakapo and comparisons with other birds. Morphometric analyses revealed that the Kakapo's orbits are significantly more convergent than other parrots, suggesting an increased binocular overlap in the visual field. The Kakapo exhibits an eye shape that is consistent with other nocturnal birds, including owls and nightjars, but is also within the range of the diurnal parrots. With respect to the brain, the Kakapo has a significantly smaller optic nerve and tectofugal visual pathway. Specifically, the optic tectum, nucleus rotundus and entopallium were significantly reduced in relative size compared to other parrots. There was no apparent reduction to the thalamofugal visual pathway. Finally, the retinal morphology of the Kakapo is similar to that of both diurnal and nocturnal birds, suggesting a retina that is specialised for a crepuscular niche. Overall, this suggests that the Kakapo has enhanced light sensitivity, poor visual acuity and a larger binocular field than other parrots. We conclude that the Kakapo possesses a visual system unlike that of either strictly nocturnal or diurnal birds and therefore does not adhere to the traditional view of the evolution of nocturnality in birds.

Morphometrics of the eyes and orbits of the nocturnal Swallow-tailed Gull (Creagrus furcatus)

The Swallow-tailed Gull ( Creagrus furcatus (Neboux, 1846)) is known for its nocturnal feeding habits and apparently large eyes. Despite frequent observations of its large eyes, detailed measurements of its eyes and orbits are wanting. Here, we provide a detailed analysis of the size and shape of the eye and orbits of this unique species in relation to a range of other gull species. Although the C. furcatus does have a slightly enlarged cornea and optical axis, neither the transverse orbit diameter nor the shape of its eye differs significantly from other larids. In addition, we found no significant difference between C. furcatus and other gulls in terms of its orbit dimensions and orbit orientation. We therefore conclude that C. furcatus does not possess a transversely enlarged eye, but rather a slightly larger cornea and longer eye. Our results do not, however, preclude the presence of other changes in the visual system, such as retinal morphology or neurophysiology, that could be adaptive for nocturnal feeding.