Muscle architecture of the hindlimb of Tyto furcata (Aves Strigiformes): Highlights in owl morphology

The American barn owl is a nocturnal bird of prey in which hind limb movements are a key factor in obtaining food; however, the architectural properties of its hind limb muscles have not been studied. This study sought to identify functional trends in the Tyto furcata hindlimb muscles by studying muscular architecture. The architectural parameters of the selected hip, knee, ankle, and digit muscles were studied in three specimens of the Tyto furcata and joint muscular proportions with an additional dataset were calculated. Previously published information on Asio otus was used for comparison. The flexor muscles of the digits had the highest muscle mass. Regarding architectural parameters, the main flexor of the digits (flexor digitorum longus) and the muscles that extend the knee and ankle joints (femorotibialis and gastrocnemius) showed a high physiological cross-sectional area (PCSA) and short fibers, allowing powerful digit flexion and knee and ankle extension. These mentioned features are in accordance with hunting behavior, in which prey capture is not only closely linked to the flexion of the digits but also to the movements of the ankle. During hunting, the distal hind limb is flexed and then fully extended at the moment of contact with the prey, whereas the digits are close to grasping the prey. The hip muscles showed a predominance of extensors over flexors, which were more massive, with parallel fibers and without tendons or short fibers. These features lead to a higher capacity to generate velocity to the detriment of forces, as indicated by the high values of the architectural index, their relatively low PCSA, and short or intermediate fiber length, which enhance the control of the joint positions and muscle length. Compared to Asio otus, Tyto furcata showed longer fibers; however, the relationship between fiber length and PCSA was similar for both species.

Cranial evolution in the extinct Rodrigues Island owl Otus murivorus (Strigidae), associated with unexpected ecological adaptations

Island birds that were victims of anthropic extinctions were often more specialist species, having evolved their most distinctive features in isolation, making the study of fossil insular birds most interesting. Here we studied a fossil cranium of the 'giant' extinct scops owl Otus murivorus from Rodrigues Island (Mascarene Islands, southwestern Indian Ocean), to determine any potential unique characters. The fossil and extant strigids were imaged through X-ray microtomography, providing 3D views of external and internal (endocast, inner ear) cranial structures. Geometric morphometrics and analyses of traditional measurements yielded new information about the Rodrigues owl's evolution and ecology. Otus murivorus exhibits a 2-tier "lag behind" phenomenon for cranium and brain evolution, both being proportionately small relative to increased body size. It also had a much more developed olfactory bulb than congeners, indicating an unexpectedly developed olfactory sense, suggesting a partial food scavenging habit. In addition, O. murivorus had the eyes placed more laterally than O. sunia, the species from which it was derived, probably a side effect of a small brain; rather terrestrial habits; probably relatively fearless behavior; and a less vertical posture (head less upright) than other owls (this in part an allometric effect of size increase). These evolutionary features, added to gigantism and wing reduction, make the extinct Rodrigues owl's evolution remarkable, and with multiple causes.

Effect of trailing-edge serrations on noise reduction in a coupled bionic aerofoil inspired by barn owls

Noise reduction is an important development direction for aircrafts and wind turbines. Owl wings have three unique morphological characteristics (leading-edge serrations, trailing-edge serrations and velvet-like surfaces) that effectively suppress aerodynamic noise in low Reynolds numbers. Among them, trailing-edge serrations are widely considered the most effective noise-reduction method. Although different serrations have been studied, the quantitative relationship and influence mechanism between the serration shape, wavelength and amplitude are poorly understood. The acoustic characteristics of asymmetrical aerofoils with different trailing-edge serrations have not been fully studied. This work investigates the flow characteristics and acoustic scattering mechanisms of novel owl-based aerofoils with different trailing-edge serrations. A sensitivity analysis is utilized to quantitatively investigate the influence and interaction mechanisms of the shape, wavelength and amplitude in trailing-edge noise reduction. Numerical simulations of the transient flow over the aerofoil are performed via the large eddy simulation method, and the acoustic far-field is obtained by solving the Ffowcs Williams and Hawkings equation. The results indicate that the sawtooth and sinusoidal serrations provide the most significant noise reduction effects; the maximum noise reduction is 8.74 dB. The wavelength and amplitude play similar roles, but the amplitude has relatively greater influence. For the sawtooth and sinusoidal serrations, the large-scale vortex structures are broken into many small-scale spiral vortex structures due to the presence of the sharp serration tip. The serrations can effectively reduce the coherence of the turbulent fluctuations due to spanwise variations in the edge and may be the main reason for noise suppression. The original owl-based aerofoil generates more low-frequency noise and less high-frequency noise than aerofoils with trailing-edge serrations. The peak noise frequencies of all aerofoils are approximately 400 Hz; hence, low-frequency noise is a dominant influence in noise generation. Furthermore, the acoustic sources generated by transient pressure fluctuations are mainly located on the serration root.

Owl-inspired leading-edge serrations play a crucial role in aerodynamic force production and sound suppression

Owls are widely known for silent flight, achieving remarkably low noise gliding and flapping flights owing to their unique wing morphologies, which are normally characterized by leading-edge serrations, trailing-edge fringes and velvet-like surfaces. How these morphological features affect aerodynamic force production and sound suppression or noise reduction, however, is still not well known. Here we address an integrated study of owl-inspired single feather wing models with and without leading-edge serrations by combining large-eddy simulations (LES) with particle-image velocimetry (PIV) and force measurements in a low-speed wind tunnel. With velocity and pressure spectra analysis, we demonstrate that leading-edge serrations can passively control the laminar-turbulent transition over the upper wing surface, i.e. the suction surface at all angles of attack (0°  <  AoA  <  20°), and hence play a crucial role in aerodynamic force and sound production. We find that there exists a tradeoff between force production and sound suppression: serrated leading-edges reduce aerodynamic performance at lower AoAs  <  15° compared to clean leading-edges but are capable of achieving both noise reduction and aerodynamic performance at higher AoAs  >  15° where owl wings often reach in flight. Our results indicate that the owl-inspired leading-edge serrations may be a useful device for aero-acoustic control in biomimetic rotor designs for wind turbines, aircrafts, multi-rotor drones as well as other fluid machinery.

Low frequency eardrum directionality in the barn owl induced by sound transmission through the interaural canal

The middle ears of birds are typically connected by interaural cavities that form a cranial canal. Eardrums coupled in this manner may function as pressure difference receivers rather than pressure receivers. Hereby, the eardrum vibrations become inherently directional. The barn owl also has a large interaural canal, but its role in barn owl hearing and specifically in sound localization has been controversial so far. We discuss here existing data and the role of the interaural canal in this species and add a new dataset obtained by laser Doppler vibrometry in a free-field setting. Significant sound transmission across the interaural canal occurred at low frequencies. The sound transmission induces considerable eardrum directionality in a narrow band from 1.5 to 3.5 kHz. This is below the frequency range used by the barn owl for locating prey, but may conceivably be used for locating conspecific callers.

Combined particle-image velocimetry and force analysis of the three-dimensional fluid-structure interaction of a natural owl wing

Low-speed aerodynamics has gained increasing interest due to its relevance for the design process of small flying air vehicles. These small aircraft operate at similar aerodynamic conditions as, e.g. birds which therefore can serve as role models of how to overcome the well-known problems of low Reynolds number flight. The flight of the barn owl is characterized by a very low flight velocity in conjunction with a low noise emission and a high level of maneuverability at stable flight conditions. To investigate the complex three-dimensional flow field and the corresponding local structural deformation in combination with their influence on the resulting aerodynamic forces, time-resolved stereoscopic particle-image velocimetry and force and moment measurements are performed on a prepared natural barn owl wing. Several spanwise positions are measured via PIV in a range of angles of attack [Formula: see text] 6° and Reynolds numbers 40 000 [Formula: see text] 120 000 based on the chord length. Additionally, the resulting forces and moments are recorded for -10° ≤ α ≤ 15° at the same Reynolds numbers. Depending on the spanwise position, the angle of attack, and the Reynolds number, the flow field on the wing's pressure side is characterized by either a region of flow separation, causing large-scale vortical structures which lead to a time-dependent deflection of the flexible wing structure or wing regions showing no instantaneous deflection but a reduction of the time-averaged mean wing curvature. Based on the force measurements the three-dimensional fluid-structure interaction is assumed to considerably impact the aerodynamic forces acting on the wing leading to a strong mechanical loading of the interface between the wing and body. These time-depending loads which result from the flexibility of the wing should be taken into consideration for the design of future small flying air vehicles using flexible wing structures.

Morphological Variations of Leading-Edge Serrations in Owls (Strigiformes)

Background

Owls have developed serrations, comb-like structures, along the leading edge of their wings. Serrations were investigated from a morphological and a mechanical point of view, but were not yet quantitatively compared for different species. Such a comparative investigation of serrations from species of different sizes and activity patterns may provide new information about the function of the serrations.

Results

Serrations on complete wings and on tenth primary remiges of seven owl species were investigated. Small, middle-sized, and large owl species were investigated as well as species being more active during the day and owls being more active during the night. Serrations occurred at the outer parts of the wings, predominantly at tenth primary remiges, but also on further wing feathers in most species. Serration tips were oriented away from the feather rachis so that they faced into the air stream during flight. The serrations of nocturnal owl species were higher developed as demonstrated by a larger inclination angle (the angle between the base of the barb and the rachis), a larger tip displacement angle (the angle between the tip of the serration and the base of the serration) and a longer length. Putting the measured data into a clustering algorithm yielded dendrograms that suggested a strong influence of activity pattern, but only a weak influence of size on the development of the serrations.

Conclusions

Serrations are supposed to be involved in noise reduction during flight and also depend on the aerodynamic properties that in turn depend on body size. Since especially nocturnal owls have to rely on hearing during prey capture, the more pronounced serrations of nocturnal species lend further support to the notion that serrations have an important function in noise reduction. The differences in shape of the serrations investigated indicate that a silent flight requires well-developed serrations.

Histological and Ultrastructural Studies on the Conjunctiva of the Barred Owl (Strix varia)

This report is the first characterization of the histology and ultrastructure of the barred owl conjunctiva. The inferior eyelid was dominated by a large disk-shaped plate covered by a non-keratinized stratified squamous or cuboidal epithelium of variable thickness. The apical surface of the plate epithelium varied from flat to long microvilli or even short cytoplasmic extensions similar to those seen in the third eyelid. All specimens had a few goblet cells filled with mucous secretory granules in the plate region. The underlying connective tissue was a dense fibroelastic stroma. Eosinophils were surprisingly common in the epithelial layer and underlying connective tissue in the plate and more distal orbital mucosal region. The orbital mucosa contained goblet cells with heterogeneous glycosylation patterns. The leading edge and marginal plait of the third eyelid are designed to collect fluid and particulate matter as they sweep across the surface of the eye. The palpebral conjunctival surface of the third eyelid was covered by an approximately five-cell-deep stratified squamous epithelium without goblet cells. The bulbar surface of the third eyelid was a bilayer of epithelial cells whose superficial cells have elaborate cytoplasmic tapering extensions reaching out 25 μm. Narrow cytofilia radiated outwards up to an additional 15–20 μm from the cytoplasmic extensions. Lectin labeling demonstrated heterogeneous glycosylation of the apical membrane specializations but only small amounts of glycoprotein-filled secretory granules in the third eyelid.

Particle-image velocimetry investigation of the fluid-structure interaction mechanisms of a natural owl wing

The increasing interest in the development of small flying air vehicles has given rise to a strong need to thoroughly understand low-speed aerodynamics. The barn owl is a well-known example of a biological system that possesses a high level of adaptation to its habitat and as such can inspire future small-scale air vehicle design. The combination of the owl-specific wing geometry and plumage adaptations with the flexibility of the wing structure yields a highly complex flow field, still enabling the owl to perform stable and at the same time silent low-speed gliding flight. To investigate the effects leading to such a characteristic flight, time-resolved stereoscopic particle-image velocimetry (TR-SPIV) measurements are performed on a prepared natural owl wing in a range of angles of attack 0° ≤ α ≤ 6° and Reynolds numbers 40,000 ≤ Re(c) ≤ 120,000 based on the chord length at a position located at 30% of the halfspan from the owl's body. The flow field does not show any flow separation on the suction side, whereas flow separation is found on the pressure side for all investigated cases. The flow field on the pressure side is characterized by large-scale vortices which interact with the flexible wing structure. The good agreement of the shedding frequency of the pressure side vortices with the frequency of the trailing-edge deflection indicates that the structural deformation is induced by the flow field on the pressure side. Additionally, the reduction of the time-averaged mean wing curvature at high Reynolds numbers indicates a passive lift-control mechanism that provides constant lift in the entire flight envelope of the owl.

Muscular Arrangement and Muscle Attachment Sites in the Cervical Region of the American Barn Owl (Tyto furcata pratincola)

Owls have the largest head rotation capability amongst vertebrates. Anatomical knowledge of the cervical region is needed to understand the mechanics of these extreme head movements. While data on the morphology of the cervical vertebrae of the barn owl have been provided, this study is aimed to provide an extensive description of the muscle arrangement and the attachment sites of the muscles on the owl’s head-neck region. The major cervical muscles were identified by gross dissection of cadavers of the American barn owl (Tyto furcata pratincola), and their origin, courses, and insertion were traced. In the head-neck region nine superficial larger cervical muscles of the craniocervical, dorsal and ventral subsystems were selected for analysis, and the muscle attachment sites were illustrated in digital models of the skull and cervical vertebrae of the same species as well as visualised in a two-dimensional sketch. In addition, fibre orientation and lengths of the muscles and the nature (fleshy or tendinous) of the attachment sites were determined. Myological data from this study were combined with osteological data of the same species. This improved the anatomical description of the cervical region of this species. The myological description provided in this study is to our best knowledge the most detailed documentation of the cervical muscles in a strigiform species presented so far. Our results show useful information for researchers in the field of functional anatomy, biomechanical modelling and for evolutionary and comparative studies.

Microstructure and cross-sectional shape of limb bones in Great Horned Owls and Red-tailed Hawks: how do these features relate to differences in flight and hunting behavior?

The Red-tailed Hawk and Great Horned Owl are two species of raptor that are similar in body size, diet, and habitat. Both species use their hindlimbs during hunting, but differ in foot morphology, how they approach and immobilize prey, and the average size of prey captured. They also differ in primary flight style: the Red-tailed Hawk uses static soaring and the Great Horned Owl uses flap-gliding. The objectives of this study were to characterize the microstructure and cross-sectional shape of limb bones of these species and examine the relationship with flight and hunting behaviors. The mid-shaft of six limb bones from six individuals of each species was sampled. The degree of bone laminarity (proportion of circular primary vascular canals) and cross-sectional geometric parameters were calculated. In both species, the humerus and femur exhibited features that suggest high resistance to torsional loading, whereas the tibiotarsus and phalanges had a shape more likely to resist compression and bending in a specific plane. The femur of the Red-tailed Hawk exhibited higher laminarity and larger polar moment of area than that of the Great Horned Owl. The tibiotarsus was more elliptical than that of the Great Horned Owl. The hawk approaches prey from a more horizontal axis, takes prey of greater mass, and is more likely to pursue prey on the ground, which could potentially be causing more torsional loads on the femur and bending loads on the tibiotarsus. In addition, differences in polar moment of area of the phalanges between the species could relate to differences in foot morphology or digit length. The humerus and ulna of the flap-gliding Great Horned Owl are more elliptical than the static soaring Red-tailed Hawk, a shape that may better resist the bending loads associated with a larger amount of flapping.

Perching of Tengmalm's owl (Aegolius funereus) nestlings at the nest box entrance: effect of time of the day, age, wing length and body weight

The behaviour of the nestlings of nocturnal cavity-nesting species has relatively rarely been studied in detail because of problems connected with use of the technical devices required to provide long-term monitoring of individuals. However, long-term observation of nestling behaviour is crucial in order to identify different types of behaviour which may be caused by sibling competition at the end of nesting period. We studied behaviour of 43 Tengmalm's owl (Aegolius funereus) nestlings at 14 nests using a camera and a chip system. The nestlings perched at the nest box entrance from an average age of 28 days from hatching (range 24–34 days) until fledging, spending around 2 hours per day here in total, in periods ranging from a few seconds to 147 min (7.6±10.9 min, mean ± SD). We found that individual duration of perching at the nest box entrance was significantly influenced by nestlings' age and wing length and that the duration of perching at the nest box entrance significantly decreased with time of night. However, during daylight hours, time of day had no effect on either probability or duration of nestlings' perching. We suggest daylight perching at the nest box entrance results from nestlings' preparation for fledging, while individuals perching here during the night may gain an advantageous position for obtaining food from the parents; another possibility at all times of day is that nestlings can reaffirm their social dominance status by monopolizing the nest box entrance.

The cervical spine of the American barn owl (Tyto furcata pratincola): I. Anatomy of the vertebrae and regionalization in their S-shaped arrangement

BACKGROUND

Owls possess an extraordinary neck and head mobility. To understand this mobility it is necessary to have an anatomical description of cervical vertebrae with an emphasis on those criteria that are relevant for head positioning. No functional description specific to owls is available.

METHODOLOGY/PRINCIPAL FINDINGS

X-ray films and micro-CT scans were recorded from American barn owls (Tyto furcata pratincola) and used to obtain three-dimensional head movements and three-dimensional models of the 14 cervical vertebrae (C1-C14). The diameter of the vertebral canal, the zygapophyseal protrusion, the distance between joint centers, and the pitching angle were quantified. Whereas the first two variables are purely osteological characteristics of single vertebrae, the latter two take into account interactions between vertebrae. These variables change in characteristic ways from cranial to caudal. The vertebral canal is wide in the cranial and caudal neck regions, but narrow in the middle, where both the zygapophyseal protrusion and the distance between joint centers are large. Pitching angles are more negative in the cranial and caudal neck regions than in the middle region. Cluster analysis suggested a complex regionalization. Whereas the borders (C1 and C13/C14) formed stable clusters, the other cervical vertebrae were sorted into 4 or 5 additional clusters. The borders of the clusters were influenced by the variables analyzed.

CONCLUSIONS/SIGNIFICANCE

A statistical analysis was used to evaluate the regionalization of the cervical spine in the barn owl. While earlier measurements have shown that there appear to be three regions of flexibility of the neck, our indicators suggest 3-7 regions. These many regions allow a high degree of flexibility, potentially facilitating the large head turns that barn owls are able to make. The cervical vertebral series of other species should also be investigated using statistical criteria to further characterize morphology and the potential movements associated with it.

Scanning electron microscopic study of the tongue in the Oriental scops owl (Otus scops)

The dorsal lingual surface of an adult owl (Otus scops) was examined by scanning electron microscopy. The tongue of the adult owl was about 1 cm long. Three parts were distinguished in the dorsal surface of the tongue: the apex, the body and the root of the tongue. The conical region between the lingual apex and lingual root was very wide area. The conical papillae of the lingual body were inclined toward the posterior of the tongue. At low magnification of scanning electron microscopy, the desquamated cells were observed in the entire dorsal surface of the lingual apex. The connective tissue cores of the epithelium of the lingual apex showed the rod-shaped protrusions. The border between the lingual apex and body was clear and the small conical papillae were observed in the lingual body. The small and large conical papillae were observed on the lingual body. The many openings of the lingual glands existed in the lingual body and lingual root.

Transcriptome changes associated with instructed learning in the barn owl auditory localization pathway

Owls reared wearing prismatic spectacles learn to make adaptive orienting movements. This instructed learning depends on re-calibration of the midbrain auditory space map, which in turn involves the formation of new synapses. Here we investigated whether these processes are associated with differential gene expression, using longSAGE. Newly fledged owls were reared for 8-36 days with prism or control lenses at which time the extent of learning was quantified by electrophysiological mapping. Transciptome profiles were obtained from the inferior colliculus (IC), the major site of synaptic plasticity, and the optic tectum (OT), which provides an instructive signal that controls the direction and extent of plasticity. Twenty-two differentially expressed sequence tags were identified in IC and 36 in OT, out of more than 35,000 unique tags. Of these, only four were regulated in both structures. These results indicate that regulation of two largely independent gene clusters is associated with synaptic remodeling (in IC) and generation of the instructive signal (in OT). Real-time PCR data confirmed the changes for two transcripts, ubiquitin/polyubiquitin and tyrosine 3-monooxgenase/tryotophan 5-monooxygenase activation protein, theta subunit (YWHAQ; also referred to as 14-3-3 protein). Ubiquitin was downregulated in IC, consistent with a model in which protein degradation pathways act as an inhibitory constraint on synaptogenesis. YWHAQ was up-regulated in OT, indicating a role in the synthesis or delivery of instructive information. In total, our results provide a path towards unraveling molecular cascades that link naturalistic experience with synaptic remodeling and, ultimately, with the expression of learned behavior.

Preservation of spectrotemporal tuning between the nucleus laminaris and the inferior colliculus of the barn owl

Performing sound recognition is a task that requires an encoding of the time-varying spectral structure of the auditory stimulus. Similarly, computation of the interaural time difference (ITD) requires knowledge of the precise timing of the stimulus. Consistent with this, low-level nuclei of birds and mammals implicated in ITD processing encode the ongoing phase of a stimulus. However, the brain areas that follow the binaural convergence for the computation of ITD show a reduced capacity for phase locking. In addition, we have shown that in the barn owl there is a pooling of ITD-responsive neurons to improve the reliability of ITD coding. Here we demonstrate that despite two stages of convergence and an effective loss of phase information, the auditory system of the anesthetized barn owl displays a graceful transition to an envelope coding that preserves the spectrotemporal information throughout the ITD pathway to the neurons of the core of the central nucleus of the inferior colliculus.

Embryonic and posthatching development of the barn owl (Tyto alba): Reference data for age determination

The normal development of the barn owl was documented with the intent of providing a guideline for determining the maturational stage of embryos and posthatching individuals. Embryonic development up to stage 39 could be well described using the well-known developmental atlas for the chicken (Hamburger and Hamilton [1951] J. Morphol. 88:49-92). For later stages, limb size was established as a suitable indicator. In addition, measuring the egg's vascularized area through candling was found to be a useful, noninvasive method for staging very early embryos, up to stage 25. An average relationship between incubation period and embryonic stage was derived, which showed that development in the barn owl initially lags that in the chicken. For posthatching individuals, skeletal measures (tarsal and ulnar length, skull width and length) were the most reliable parameters for judging maturation, up to 1 month. For older individuals, feather development (e.g., length of primary wing feathers) provided the only cue.

Distribution within the barn owl's inferior colliculus of neurons projecting to the optic tectum and thalamus

Behavioral studies in barn owls indicate that both the optic tectum (OT) and the auditory arcopallium (AAr) mediate sound localization through the presence of neurons that respond only when sound comes from a circumscribed direction in space. The early stages of the computations leading to these so-called space-specific neurons are shared in a common brainstem pathway, which then splits at the level of the inferior colliculus (IC) such that the last computational stage is thought to be duplicated. The study presented here addresses whether the space-specific neurons in OT and AAr are indeed partially independent of each other by using anatomical methods more precise than those used in previous studies. Specifically, projection neurons in IC were retrogradely labelled with injections of fluorescein- and rhodamine-conjugated dextran amines into OT and nucleus ovoidalis (OV), the thalamic nucleus leading to AAr. By labelling the OT-projecting and OV-projecting neurons in the same owl, it was confirmed that neurons in IC project to either OV or OT but not both. However, although a segregation was generally observed between the medially positioned OV-projecting neurons and the laterally positioned OT-projecting neurons, there was also a slight overlap between the two populations. Moreover, electrolytic lesions demarcating physiological tuning properties indicate that many OV-projecting neurons are within the area containing space-specific neurons. These results highlight the need for more detailed studies elucidating the microcircuitry and corresponding physiology of IC, such as have been done in the cortices of the mammalian cerebellum and cerebrum.

Low density of membrane particles in auditory hair cells of lizards and birds suggests an absence of somatic motility

Hair cells are the mechanoreceptive cells of the vertebrate lateral line and inner ear. In addition to their sensory function, hair cells display motility and thus themselves generate mechanical energy, which is thought to enhance sensitivity. Two principal cellular mechanism are known that can mediate hair-cell motility in vitro. One of these is based on voltage-dependent changes of an intramembrane protein and has so far been demonstrated only in outer hair cells of the mammalian cochlea. Correlated with this, the cell membranes of outer hair cells carry an extreme density of embedded particles, as revealed by freeze fracturing. The present study explored the possibility of membrane-based motility in hair cells of nonmammals, by determining their density of intramembrane particles. Replicas of freeze-fractured membrane were prepared from auditory hair cells of a lizard, the Tokay gecko, and a bird, the barn owl. These species were chosen because of independent evidence for active cochlear mechanics, in the form of spontaneous otoacoustic emissions. For quantitative comparison, mammalian inner and outer hair cells, as well as vestibular hair, cells were reevaluated. Lizard and bird hair cells displayed median densities of 2,360 and 1,880 intramembrane particles/microm2, respectively. This was not significantly different from the densities in vestibular and mammalian inner hair cells; however, it was about half the density in of mammalian outer hair cells. This suggests that nonmammalian hair cells do not possess high densities of motor protein in their membranes and are thus unlikely to be capable of somatic motility.

Error rate of axons at the owl's optic chiasm

BACKGROUND

In owls, the visual pathways from the retina are totally crossed. Attempts to find ganglion cells with uncrossed axons have failed consistently, when retrograde labeling with HRP is used for their identification. In the present investigation we have used retrograde fluorescent tracers of complementary colour in each optic tectum to demonstrate a tiny population of ipsilaterally-projecting retinal ganglion cells in the owl.

METHODS

We studied two species, one from each of the two important owl families: the Southern boobook owl, Ninox boobook: (Family Strigidae); and the Barn Owl, Tyto alba: (Family Tytonidae).

RESULTS

The small numbers, random distribution and heterogeneity of the mis-projecting ganglion cells, taken together, argue against a functional role for them. Instead, they appear to be the result of developmental errors in the specification of laterality. At a number of different eccentricities and ganglion cell densities, the error rate was roughly a constant fraction of the neurons involved, at around 10-4 for the tytonid owl (lacking a fovea) and around 10-5 for the strigid owl (which has a fovea and a higher overall density of ganglion cells).

CONCLUSIONS

These values are close to the error rates of replicating enzymes, such as nucleic acid polymerases. The evolution of a higher retinal ganglion cell density and a fovea in the Strigidae appears to be accompanied by an improvement in the error rate.

Brain architecture and social complexity in modern and ancient birds

Vertebrate brains vary tremendously in size, but differences in form are more subtle. To bring out functional contrasts that are independent of absolute size, we have normalized brain component sizes to whole brain volume. The set of such volume fractions is the cerebrotype of a species. Using this approach in mammals we previously identified specific associations between cerebrotype and behavioral specializations. Among primates, cerebrotypes are linked principally to enlargement of the cerebral cortex and are associated with increases in the complexity of social structure. Here we extend this analysis to include a second major vertebrate group, the birds. In birds the telencephalic volume fraction is strongly correlated with social complexity. This correlation accounts for almost half of the observed variation in telencephalic size, more than any other behavioral specialization examined, including the ability to learn song. A prominent exception to this pattern is owls, which are not social but still have very large forebrains. Interpolating the overall correlation for Archaeopteryx, an ancient bird, suggests that its social complexity was likely to have been on a par with modern domesticated chickens. Telencephalic volume fraction outperforms residuals-based measures of brain size at separating birds by social structure. Telencephalic volume fraction may be an anatomical substrate for social complexity, and perhaps cognitive ability, that can be generalized across a range of vertebrate brains, including dinosaurs.

Anatomical markers for the subdivisions of the barn owl's inferior-collicular complex and adjacent peri- and subventricular structures

The anatomy of the inferior-collicular complex of the barn owl, situated below the fourth ventricle in the tectal lobe, was studied by determining the distribution of antigens with antibodies directed against tyrosine hydroxylase, gamma-aminobutyric acid (GABA)(Abeta), dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32), calretinin, and calbindin. Additionally, the somata were stained with cresyl violet, and fibers were marked according to the Gallyas procedure. These markers were chosen to allow for an easy delineation of the boundaries between the subnuclei of the inferior colliculus. We could discriminate eight structures that belong to the three subnuclei of the inferior colliculus [the central nucleus (ICC), the superficial nucleus (ICS), the external nucleus (ICX)] and to the optic tectum. Periventricular tectal layers 15a and 15b stained well with all the antibodies used. The ICS, embedded in tectal layer 15a, may be divided into a dorsal and a ventral lamina. It does not have direct contact with the other nuclei of the inferior colliculus. The border between tectal layer 15a and ICX was well marked by all antibodies, but less so in Gallyas and cresyl violet stains. The ICC consists of a core and a medial and lateral shell. The core was clearly demarcated with antibodies against calretinin and calbindin. The border between the lateral shell and the ICX was marked less well than the borders between ICX and 15a, but the somata were much more darkly labeled with the DARPP-32 antibody in ICX than in the lateral shell of ICC. None of the markers delineated the border between the medial and lateral shell of ICC.

Bilaterally-projecting efferent neurones to the basilar papilla in the barn owl and the chicken

The efferent innervation of the auditory basilar papilla of birds and mammals is provided by a dedicated population of brainstem neurones that are separate from those supplying the vestibular organs. This study addresses the question whether a population of bilaterally-projecting efferents, contacting hair cells in both basilar papillae, is consistently present in birds. The chicken and the barn owl were chosen, two species where the total number of efferents was already known and which represent two extremes of an auditory generalist and an auditory specialist, respectively. Fluorogold and Choleratoxin, two potent retrograde tracers, were injected into one cochlear duct each of all individuals. Labelled neurones were subsequently identified in the brainstem using standard fluorescence techniques. A small proportion (up to 2% of the total population) of double-labelled cells was found in both species. The great majority of those double-labelled neurones could be assigned to the ventrolateral group of efferents, which has previously been shown to project exclusively to the auditory basilar papilla. Thus, in birds, like in mammals, a small subgroup of auditory efferents innervates both basilar papillae.

The evolution of plumage polymorphism in birds of prey and owls: the apostatic selection hypothesis revisited

Co-evolution between phenotypic variation and other traits is of paramount importance for our understanding of the origin and maintenance of polymorphism in natural populations. We tested whether the evolution of plumage polymorphism in birds of prey and owls was supported by the apostatic selection hypothesis using ecological and life-history variables in birds of prey and owls and performing both cross taxa and independent contrast analyses. For both bird groups, we did not find any support for the apostatic selection hypothesis being the maintaining factor for the polymorphism: plumage polymorphism was not more common in taxa hunting avian or mammalian prey, nor in migratory species. In contrast, we found that polymorphism was related to variables such as sexual plumage dimorphism, population size and range size, as well as breeding altitude and breeding latitude. These results imply that the most likely evolutionary correlate of polymorphism in both bird groups is population size, different plumage morphs might simply arise in larger populations most likely because of a higher probability of mutations and then be maintained by sexual selection.

Information optimization in coupled audio-visual cortical maps

Barn owls hunt in the dark by using cues from both sight and sound to locate their prey. This task is facilitated by topographic maps of the external space formed by neurons (e.g., in the optic tectum) that respond to visual or aural signals from a specific direction. Plasticity of these maps has been studied in owls forced to wear prismatic spectacles that shift their visual field. Adaptive behavior in young owls is accompanied by a compensating shift in the response of (mapped) neurons to auditory signals. We model the receptive fields of such neurons by linear filters that sample correlated audio-visual signals and search for filters that maximize the gathered information while subject to the costs of rewiring neurons. Assuming a higher fidelity of visual information, we find that the corresponding receptive fields are robust and unchanged by artificial shifts. The shape of the aural receptive field, however, is controlled by correlations between sight and sound. In response to prismatic glasses, the aural receptive fields shift in the compensating direction, although their shape is modified due to the costs of rewiring.

Response properties of neurons in the core of the central nucleus of the inferior colliculus of the barn owl

The central nucleus of the inferior colliculus (ICC) is particularly important for the processing of interaural time differences (ITDs). In the barn owl, neuronal best frequencies in a subnucleus of the ICC, the ICCcore, span the animal's entire hearing range (approximately equal to 200-10 000 Hz). This means that low-frequency ITD-sensitive ICCcore neurons in the owl can be directly compared to ITD-sensitive mammalian ICC neurons with similar best frequencies as well as to the high-frequency ITD-sensitive neurons usually studied in owls. This report represents a first attempt to systematically describe important physiological properties of ICCcore neurons in the barn owl, with particular attention to the low-frequency region (< 2 kHz). Responses were obtained from 133 neurons or small clusters of neurons; recording sites were confirmed by histological reconstruction of electrode tracks based on electrolytic lesions. Iso-intensity frequency response functions were typically approximately equal to 1 octave wide in the low-frequency range and approximately equal to 1/3 octave wide in the high-frequency range. Most neurons were ITD-tuned; both noise and pure tone stimuli yielded periodic ITD tuning curves with several equivalent response maxima. In most cases ITD tuning curves had a response peak within the barn owl's physiological ITD range. ITD tuning widths were inversely correlated with neuronal best frequency. None of the ICCcore neurons studied were sensitive to interaural level differences. Monaural inputs to ICCcore cells were typically binaurally balanced, i.e. they exhibited similar response thresholds, dynamic ranges, slopes and saturation levels, for both left and right ear monaural stimulation.

Morphometry of auricular feathers of barn owls (Tyto alba)

In all owl species, the facial plumage forms a parabolic dish, the facial ruff, which is most conspicuous in the the barn owl (Tyto alba). The center of the ruff is formed by auricular feathers. Such feathers are also found on the preaural flaps which cover the ear openings, and in the region of the beak. In this study, we compare the different types of auricular feathers of the barn owl with contour feathers from the neck. Auricular feathers are characterised by an open vane structure and fewer barbs as compared to contour feathers. Auricular feathers also have fewer distal and proximal barbules than contour feathers. The open vane of the auricular feather results from an acute angle between the barb and the basis of the barbules, and from the extension of the pennula parallel to the barbs. These reductions are differently expressed in the three different types of auricular feathers investigated here and correspond with their function (protecting the ruff from dust).

Tonotopic projections of the auditory nerve to the cochlear nucleus angularis in the barn owl

The nucleus angularis (NA), one of the two cochlear nuclei of birds, plays an important role in the processing of sound intensity. To begin investigating the NA in detail in the barn owl, which is a popular animal model for neural mechanisms of sound localization, a frequency map for this nucleus is presented here. Focal injections of horseradish peroxidase or neurobiotin were placed either in the NA or in the cochlear nucleus magnocellularis, labeling small groups of auditory nerve fibers of known characteristic frequency (CF) from 0.25 to 9.6 kHz. The courses of their axonal branches were used to construct a composite average map of the tonotopic frequency representation in the nucleus angularis. Nucleus angularis in the barn owl, as seen in frontal sections, resembles a sheet of cells bent approximately into an S shape. The lowest frequencies were found represented at the ventromedial extreme. The representation of increasingly higher frequencies then followed the S shape, with the highest frequencies located at the ventrolateral tip. Auditory nerve fibers of a given CF always entered the nucleus angularis within a well-restricted area and then traveled along their isofrequency band within the NA while branching off terminals. The isofrequency bands were typically slanted from caudo-ventro-medial to rostro-dorso-lateral. The basic tonotopic organization is comparable to that found in other birds, the major differences being the large size and unusual shape of the barn owl's nucleus angularis.

The cytoarchitecture of the nucleus angularis of the barn owl (Tyto alba)

The cochlear nucleus angularis (NA) of the barn owl (Tyto alba) was analyzed using Golgi, Nissl, and tract tracing techniques. NA forms a column of cells in the dorsolateral brainstem that partly overlaps with, and is rostral and lateral to, the cochlear nucleus magnocellularis (NM). Highest best frequencies are mapped in lateral NA (NAl), intermediate in medial NA (NAm), and lowest in the foot region (NAf). Cell density followed the tonotopic axis and decreased with decreasing best frequency. NA contained four major cell classes: planar, radiate, vertical, and stubby. Planar and radiate classes were further subdivided into bipolar and multipolar types according to their number of primary dendrites. Planar neurons were confined to an isofrequency band, whereas radiate neurons had dendrites that could extend across an isofrequency band. Vertical cells had long dendrites oriented perpendicularly to isofrequency bands. Stubby cells were the most numerous and were confined to an isofrequency band because of their short dendrites. Neurons in each of these four classes projected to the inferior colliculus and dorsal nucleus of the lateral lemniscus.

An additional bone in the sclera of the eyes of owls and the common potoo (Nictibius griseus) and its role in the contraction of the nictitating membrane

Morphometric, anatomical and histological examinations were made in 10 species of owls of the families Tytonidae and Strigidae and compared with the eyes of other species of nocturnal birds including common potoo (Nictibiidae) and three species of nightjars (Caprimulgidae) and two diurnal species: the roadside hawk (Accipitridae) and the domestic duck (Anatidae). In owls and the common potoo the nictitating membrane is situated on the dorsal edge of the eye. In these birds, the scleral ring bears an additional, previously undescribed bone of various forms and dimensions (1.4-6.8 mm of length and 0.8-3.3 mm of width), located on the trajectory of the tendon of the pyramidal muscle which is attached to the nictitating membrane. This bone has a groove that encloses the tendon of the pyramidal muscle, preventing it from separating from the sclera during contraction, as well as diverting the trajectory of the tendon to impede it from projecting itself over the cornea. In the ferruginous pygmy owl, Nacunda nighthawk, Pauraque, scissor-tailed nightjar, roadside hawk and domestic duck the additional bone was not seen. Based on the morphofunctional characteristics of the bone, we suggest that this bone should be named the scleral sesamoid bone.

Miniature stereo radio transmitter for simultaneous recording of multiple single-neuron signals from behaving owls

Wireless radiotelemetric transmission of neuronal activity is an elegant technique to study brain-behavior interaction in unrestrained animals. In the current study, a miniature FM-stereo radio transmitter is described that permitted simultaneous recordings from two microelectrodes in behaving barn owls. Input from two independent channels is multiplexed to form a stereo composite signal that modulates a radio frequency carrier. The high quality of broadcasted extracellular signals enabled separation of single units based on differences in spike waveforms. Recording several single cells from different electrodes allows the possibility of investigating correlations between small, distributed neuronal ensembles. Multi-channel radiotelemetry that meets the demands of modern electrophysiology might open a new perspective for combined behavioral/neurophysiological approaches in freely-behaving animals.

Topographic projection from the optic tectum to the auditory space map in the inferior colliculus of the barn owl

In the barn owl (Tyto alba), the external nucleus of the inferior colliculus (ICX) contains a map of auditory space that is calibrated by visual experience. The source of the visually based instructive signal to the ICX is unknown. Injections of biotinylated dextran amine and Fluoro-Gold in the ICX retrogradely labelled neurons in layers 8-15 of the ipsilateral optic tectum (OT) that could carry this instructive signal. This projection was point-to-point and in register with the feed-forward, auditory projection from the ICX to the OT. Most labelled neurons were in layers 10-11, and most were bipolar. Tripolar, multipolar, and unipolar neurons were also observed. Multipolar neurons had dendrites that were oriented parallel to the tectal laminae. In contrast, most labelled bipolar and tripolar neurons had dendrites oriented perpendicular to the tectal laminae, extending superficially into the retino-recipient laminae and deep into the auditory recipient laminae. Therefore, these neurons were positioned to receive both visual and auditory information from particular locations in space. Biocytin injected into the superficial layers of the OT labelled bouton-laden axons in the ICX. These axons were generally finer than, but had similar bouton densities as, feed-forward auditory fibers in the ICX, labelled by injections of biocytin into the central nucleus of the inferior colliculus (ICC). These data demonstrate a point-to-point projection from the OT to the ICX that could provide a spatial template for calibrating the auditory space map in the ICX.