Integrity of and damage to wings, feather vanes and serrations in barn owls.

The silent flight of owls is well known. It has served as role model for the designs of new airplane wings and ventilators. One of the structural features that underlies silent flight is the serrated leading edge of the wing that is mainly formed by the tenth primary flight feather (P10). We examined here how much the wings, the P10 feather and the serrations in different populations of barn owls reflect the intact situation. First, when the P10 feather molts, no or fewer serrations are present. Furthermore, damage to feathers and serrations may occur. Damage may be due to several factors like broken feather tips, lost rami, barbules, or broken tips of serrations. The latter may cause a narrowing of the outer vane of the P10 feather. We quantitatively assessed damage by counting the number of wings with missing or broken primary feathers, the number of wings with a narrowed outer vane of the P10 feather, and the number of serrations with reduced length. Considerable damage occurred on wings and feathers on both the macroscopic and microscopic levels. The observed damage most likely influences flight performance. More damage occurred in Galapagos barn owls than in North American and European barn owls. The Galapagos population may be more vulnerable than the other populations because it may at least temporarily be in a bad nutritional state and, thus, postpone molt.

Ear asymmetry in Tengmalm's owl (Aegolius funereus): Two phases of asymmetrical development of the squamoso-occipital wing.

Ear asymmetry is an adaptive characteristic present in the order of owls (Strigiformes). It developed independently up to seven times in this taxon and is accompanied by morphological adaptations in bones or soft tissues around or at the ear openings. Within all strigiform species, the Boreal or Tengmalm's owl (Aegolius funereus) possesses a particularly complex bilateral ear asymmetry that results from modifications of the neurocranium and some cartilaginous elements. While the ear asymmetry in adult birds has been described in detail, data on its development is scarce. Here we describe the development of the asymmetric squamoso-occipital wing of A. funereus from its embryonic origin up to adulthood. The asymmetry of the squamoso-occipital wing develops in two phases. Firstly, it originates as a cartilaginous structure in the last ten days before hatching. Its frontal margin shows a bilateral asymmetry from the beginning of its development while the rostro-ventral process stays symmetrical up to post-hatching day 35. Secondly, when the fledglings have already left the nest, the squamoso-occipital wing ossifies. Moreover, the rostro-ventral process on the right side grows towards the eyeball, while there is no relative displacement on the left side. Thus, the developmental process in A. funereus differs from that in the barn owl that develops its soft tissue asymmetry in one phase and completes the asymmetry before hatching. The new data presented here extend our knowledge of the mechanisms underlying the asymmetric skull development in owls.

A heterochronic shift in skeletal development in the barn owl (Tyto furcata): A description of the ocular skeleton and tubular eye shape formation.

BACKGROUND: The eyes of some birds of prey (Strigiformes and some eagles) are tubular in shape, which contrasts strongly with those in others, which are more globose (e.g., Galliformes) or flat (most diurnal birds). Regardless, all birds have an ocular skeleton composed of a ring of ossicles (annulus ossicularis sclerae) and a cartilage cup within the sclera. RESULTS: We show that the tubular eye of the barn owl, Tyto furcata, grows substantially in length to achieve its long axial length several weeks after hatching, well after the period when the visual input adjusts the optical system and when the scleral ossicles mineralize. This is in contrast to the chicken. The conjunctival papillae are morphologically different in each species, however, they are present for about 3 days in both birds before they degenerate. CONCLUSIONS: Our data shows a heterochronic shift in the timing of scleral cartilage development and ossicle mineralization (but not induction) to later in development compared to in the chicken. These shifts likely relate to the altricial vs precocial nature of these birds and suggests that the scleral ossicles are likely functionally important bones for vision in owls and possibly other altricial species.

EvoDevo in owl ear asymmetry-The little owl (Athene noctua).

Owls are well adapted to nocturnal hunting. This includes vision tuned to low level light conditions, silent flight, and asymmetrical ears. Asymmetrical ears facilitate sound localization and evolved up to seven times independently in the evolutionary history of owls. However, there are also owl species with a crepuscular or diurnal lifestyle, like the little owl (Athene noctua), that have symmetrical ears as adults. Here we show that a small, but significant ear asymmetry occurs in the embryonic development of little owls, despite the presence of symmetrical ears in adults. In the asymmetric period, the left ear opening is bigger in area than its counterpart on the right. The asymmetry in the little owl occurs in the same stages at which the asymmetry in the barn owl (Tyto furcata) develops, but in the little owl the asymmetry vanishes shortly before hatching. Asymmetries in the size of the ear openings are also found in the adults of other owl species, most of them belonging to the genus Strix. We interpret our finding as an indication of a secondarily evolved diurnal activity in little owls. Further, ear asymmetry might be more deeply rooted in the evolution of owls than previously assumed.

Development of ear asymmetry in the American barn owl (Tyto furcata pratincola).

Owls are known for their nocturnal hunting capability. Many owl species are able to localize prey in complete darkness just by hearing. Sound localization of strictly nocturnal owls is improved by asymmetrically arranged outer ears. According to Norberg (1977), who worked with adult owls, asymmetrical ears evolved at least four times independently among owls. What is unknown so far is how the ear asymmetry develops in the embryo. Here we examine the embryonic development of ear asymmetry in the American barn owl (Tyto furcata pratincola) in the frame of the 42 stages suggested by Köppl et al. (2005). In this species, the left ear opening in the skin is located higher than its counterpart on the right. Micro-CT scans show that in an anatomically defined coordinate system, the ear openings initially appear symmetrically as does the skull as well as the eyes, the nasal openings, the stapes and the squamosum. The ear openings are initially located ventrally in the skull. Soon after their appearance, the ear openings start to move dorso-occipitally. At the developmental stages 36-39, the left ear opening moves faster than the right one. In this way, an ear asymmetry develops within a few developmental stages. The skull and the other anatomical markers remain symmetrical. Thus, these data show that the soft tissue asymmetry in the barn owl develops already before hatching. It will be interesting to compare the time course described here with the time course of development in owl species with bony ear asymmetries.

Barn owls maximize head rotations by a combination of yawing and rolling in functionally diverse regions of the neck.

Owls are known for their outstanding neck mobility: these birds can rotate their heads more than 270°. The anatomical basis of this extraordinary neck rotation ability is not well understood. We used X-ray fluoroscopy of living owls as well as forced neck rotations in dead specimens and computer tomographic (CT) reconstructions to study how the individual cervical joints contribute to head rotation in barn owls (Tyto furcata pratincola). The X-ray data showed the natural posture of the neck, and the reconstructions of the CT-scans provided the shapes of the individual vertebrae. Joint mobility was analyzed in a spherical coordinate system. The rotational capability was described as rotation about the yaw and roll axes. The analyses suggest a functional division of the cervical spine into several regions. Most importantly, an upper region shows high rolling and yawing capabilities. The mobility of the lower, more horizontally oriented joints of the cervical spine is restricted mainly to the roll axis. These rolling movements lead to lateral bending, effectively resulting in a side shift of the head compared with the trunk during large rotations. The joints in the middle of the cervical spine proved to contribute less to head rotation. The analysis of joint mobility demonstrated how owls might maximize horizontal head rotation by a specific and variable combination of yawing and rolling in functionally diverse regions of the neck.

Morphological comparison of five species of poison dart frogs of the genus Ranitomeya (Anura: Dendrobatidae) including the skeleton, the muscle system and inner organs.

The morphology of larvae stages of most amphibians are often completely different than in adults. Tadpole descriptions have historically been based on external characters like morphometrics, color pattern and oral disc structure. Other papers described anatomical details by the use of dissections. The increase in micro-CT scanning technology provides an opportunity to quantify and describe in detail internal characters like skeleton, musculature and organs. To date, no such tadpole descriptions exist for the well-studied Neotropical poison dart frog genus Ranitomeya (Anura: Dendrobatidae). Here we provide descriptions of the internal skeletal, musculature and organ structures of five Ranitomeya species and then provide morphological comparisons. Contrary to previous observations, closely related species display several morphological differences. For example, we observed considerable variation in chondrocranial characters, the extent of cranial ossifications, the appearance of some cranial muscles and the arrangement of inner organs. Further studies on the tadpole morphology of more species of Ranitomeya and other dendrobatid genera are needed to enable us to understand the complete morphological variation in this group.

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.

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.