Anatomy and histochemistry of spread-wing posture in birds. I. Wing drying posture in the double-crested cormorant,Phalacrocorax auritus

Anatomy and histochemistry of spread-wing posture inbirds. 4. Eagles soar with fast, not slow muscle fibres

Slow fibers are typically characterized as functioning in avian postural behaviors such as soaring flight, and are described for a number of elite soarers such as vultures, pelicans and albatrosses. Golden Eagles and Bald Eagles also display soaring behavior and we examined their flight muscles for the presence of slow fibers. Surprisingly, eagles lack a deep layer to the pectoralis found in other soaring species. Additionally, the pectoralis as well as other shoulder muscles had few to no slow muscle fibers. The lack of functionally meaningful numbers of slow muscle fibers in eagle flight muscles indicates that they must rely on fast fibers for posture; these can function in that role due to their high aerobic capacity and also perhaps a "tuning" of muscle contraction frequency to function more efficiently at isometric contractions.

Fiber type homogeneity of the flight musculature in small birds

Studies of medium- and large-bodied avian species have suggested that variation in flight muscle composition is related to differences in flight behavior. For example, slow-twitch or tonic fibers are generally found only in the flight muscles of non-volant or soaring/gliding birds. However, we know comparatively little about fiber composition of the muscles of the smallest birds. Here we describe the fiber composition of muscles from the wings, shoulders, and legs of two small avian species, which also display very high wingbeat frequencies: Anna's hummingbirds (Calypte anna) and zebra finches (Taeniopygia guttata). All flight muscles examined in both species contained exclusively fast oxidative glycolytic (FOG) fibers. These unique results suggest that fast oxidative fibers are both necessary and sufficient for the full range of flight behaviors in these small-bodied birds. Like all other studied birds, the zebra finch gastrocnemius, a tarsometatarsal extensor, contained a mixture of FOG (27.1%), slow oxidative (SO, 12.7%), and fast glycolytic (FG, 60.2%) fibers. By contrast, the hummingbird gastrocnemius lacked FG fibers (85.5% FOG, 14.5% SO), which may reflect the reduced role of the hindlimb during take-off. We further hypothesize that thermogenic requirements constrain fiber type heterogeneity in these small endothermic vertebrates.

Anatomy and histochemistry of hindlimb flight posture in birds. I. The extended hindlimb posture of shorebirds

Birds utilize one of two hindlimb postures during flight: an extended posture (with the hip and knee joints flexed, while the ankle joint is extended caudally) or a flexed posture (with the hip, knee, and ankle joints flexed beneath the body). American Avocets (Recurvirostra americana) and Black-necked Stilts (Himantopus mexicanus) extend their legs caudally during flight and support them for extended periods. Slow tonic and slow twitch muscle fibers are typically found in muscles functioning in postural support due to the fatigue resistance of these fibers. We hypothesized that a set of small muscles composed of high percentages of slow fibers and thus dedicated to postural support would function in securing the legs in the extended posture during flight. This study examined the anatomy and histochemical profile of eleven hindlimb muscles to gain insight into their functional roles during flight. Contrary to our hypothesis, all muscles possessed both fast twitch and slow twitch or slow tonic fibers. We believe this finding is due to the versatility of dynamic and postural functions the leg muscles must facilitate, including standing, walking, running, swimming, and hindlimb support during flight. Whether birds use an extended or flexed hindlimb flight posture may be related to the aerodynamic effect of leg position or may reflect evolutionary history.

Skeletal muscle fiber composition of the English sparrow (Passer domesticus)

Substrate utilization by English sparrow skeletal muscle has been extensively studied in our lab. However, there are few published studies on the muscle fiber composition of English sparrow wing and gastrocnemius muscles. The objective of the present study was to examine the fiber type composition of a variety of muscles in the English sparrow. The classification of a muscle fiber as fast glycolytic, slow oxidative, or fast oxidative glycolytic provides insight into the physiological function of muscles. Therefore, we completed mATPase and NADH stains on four muscles of the sparrow wing, as well as the gastrocnemius muscle, to characterize these muscle fiber types. Results show that the fibers of extensor digitorum communis, extensor metacarpi ulnaris, and extensor metacarpi radialis are homogeneous fast oxidative. The fibers of the supinator are homogeneous fast oxidative in 62.5% of samples, and heterogeneous (45.2% fast oxidative, 54.8% fast nonoxidative) in 37.5% of samples. Whereas the gastrocnemius muscle fibers are heterogeneous (10% fast oxidative, 64% fast nonoxidative, 26% slow oxidative) in all muscles examined.

Anatomy and histochemistry of spread-wing posture in birds. 3. Immunohistochemistry of flight muscles and the ?shoulder lock? in albatrosses

As a postural behavior, gliding and soaring flight in birds requires less energy than flapping flight. Slow tonic and slow twitch muscle fibers are specialized for sustained contraction with high fatigue resistance and are typically found in muscles associated with posture. Albatrosses are the elite of avian gliders; as such, we wanted to learn how their musculoskeletal system enables them to maintain spread-wing posture for prolonged gliding bouts. We used dissection and immunohistochemistry to evaluate muscle function for gliding flight in Laysan and Black-footed albatrosses. Albatrosses possess a locking mechanism at the shoulder composed of a tendinous sheet that extends from origin to insertion throughout the length of the deep layer of the pectoralis muscle. This fascial "strut" passively maintains horizontal wing orientation during gliding and soaring flight. A number of muscles, which likely facilitate gliding posture, are composed exclusively of slow fibers. These include Mm. coracobrachialis cranialis, extensor metacarpi radialis dorsalis, and deep pectoralis. In addition, a number of other muscles, including triceps scapularis, triceps humeralis, supracoracoideus, and extensor metacarpi radialis ventralis, were found to have populations of slow fibers. We believe that this extensive suite of uniformly slow muscles is associated with sustained gliding and is unique to birds that glide and soar for extended periods. These findings suggest that albatrosses utilize a combination of slow muscle fibers and a rigid limiting tendon for maintaining a prolonged, gliding posture.

Evolutionary significance of myosin heavy chain heterogeneity in birds

This article reviews the complexity, expression, genetics, regulation, function, and evolution of the avian myosin heavy chain (MyHC). The majority of pertinent studies thus far published have focussed on domestic chicken and, to a much lesser extent, Japanese quail. Where possible, information available about wild species has also been incorporated into this review. While studies of additional species might modify current interpretations, existing data suggest that some fundamental properties of myosin proteins and genes in birds are unique among higher vertebrates. We compare the characteristics of myosins in birds to those of mammals, and discuss potential molecular mechanisms and evolutionary forces that may explain how avian MyHCs acquired these properties.

Anatomy and histochemistry of spread-wing posture in birds. 2. Gliding flight in the California gull, Larus californicus: a paradox of fast fibers and posture

Gliding flight is a postural activity which requires the wings to be held in a horizontal position to support the weight of the body. Postural behaviors typically utilize isometric contractions in which no change in length takes place. Due to longer actin-myosin interactions, slow contracting muscle fibers represent an economical means for this type of contraction. In specialized soaring birds, such as vultures and pelicans, a deep layer of the pectoralis muscle, composed entirely of slow fibers, is believed to perform this function. Muscles involved in gliding posture were examined in California gulls (Larus californicus) and tested for the presence of slow fibers using myosin ATPase histochemistry and antibodies. Surprisingly small numbers of slow fibers were found in the M. extensor metacarpi radialis, M. coracobrachialis cranialis, and M. coracobrachialis caudalis, which function in wrist extension, wing protraction, and body support, respectively. The low number of slow fibers in these muscles and the absence of slow fibers in muscles associated with wing extension and primary body support suggest that gulls do not require slow fibers for their postural behaviors. Gulls also lack the deep belly to the pectoralis found in other gliding birds. Since bird muscle is highly oxidative, we hypothesize that fast muscle fibers may function to maintain wing position during gliding flight in California gulls.