Asymmetrical Force Production in the Maneuvering Flight of Pigeons

Downstroke force produced by Rock Doves (Columba livia) as they negotiated an obstacle course was measured using in vivo recordings of delto-pectoral crest strain. During this slow $(\u3c6\ {\rm m}\ {\rm s}^{-1})$ , maneuvering flight, pigeons produced a series of four to six successive wingbeats in which the wing on the outside of the turn produced greater peak force than the wing on the inside of the turn, suggesting that the birds maneuvered in a saltatory manner during slow flight. This asymmetrical downstroke force may be used to increase or reestablish bank lost during upstroke, or it may be directed as thrust to compensate for adverse yaw or create excess yaw to alter the bird\u27s direction of flight. Continuous production of asymmetrical downstroke force through a turn differs from the traditional model of maneuvering flight, in which asymmetrical force is used only to initiate a bank, the forces are briefly reversed to arrest the momentum of the roll and then equalized to maintain the established bank, and the redirected lift of the wings then effects a turn. Although this traditional model probably describes most turns initiated during fast and gliding flight in birds, it underestimates the complexity of maneuvering during slow, flapping flight, where sophisticated kinematics and neuromuscular control are needed to change direction effectively

Effects of Flight Speed upon Muscle Activity in Hummingbirds

Hummingbirds have the smallest body size and highest wingbeat frequencies of all flying vertebrates, so they represent one endpoint for evaluating the effects of body size on sustained muscle function and flight performance. Other bird species vary neuromuscular recruitment and contractile behavior to accomplish flight over a wide range of speeds, typically exhibiting a U-shaped curve with maxima at the slowest and fastest flight speeds. To test whether the high wingbeat frequencies and aerodynamically active upstroke of hummingbirds lead to different patterns, we flew rufous hummingbirds (Selasphorus rufus, 3 g body mass, 42 Hz wingbeat frequency) in a variable-speed wind tunnel $(\textrm{0–10 m s}^{-1})$. We measured neuromuscular activity in the pectoralis (PECT) and supracoracoideus (SUPRA) muscles using electromyography (EMG, $N=4$ birds), and we measured changes in PECT length using sonomicrometry ($N=1$). Differing markedly from the pattern in other birds, PECT deactivation occurred before the start of downstroke and the SUPRA was deactivated before the start of upstroke. The relative amplitude of EMG signal in the PECT and SUPRA varied according to a U-shaped curve with flight speed; additionally, the onset of SUPRA activity became relatively later in the wingbeat at intermediate flight speeds $(\textrm{4 and 6 m s}^{−1})$. Variation in the relative amplitude of EMG was comparable with that observed in other birds but the timing of muscle activity was different. These data indicate the high wingbeat frequency of hummingbirds limits the time available for flight muscle relaxation before the next half stroke of a wingbeat. Unlike in a previous study that reported single-twitch EMG signals in the PECT of hovering hummingbirds, across all flight speeds we observed 2.9±0.8 spikes per contraction in the PECT and 3.8±0.8 spikes per contraction in the SUPRA. Muscle strain in the PECT was 10.8±0.5%, the lowest reported for a flying bird, and average strain rate was 7.4±0.2 muscle lengths $s^{−1}$. Among species of birds, PECT strain scales proportional to body mass to the 0.2 power $(\infty M_b^{0.2})$ using species data and $\infty M_b^{0.3}$ using independent contrasts. This positive scaling is probably a physiological response to an adverse scaling of mass-specific power available for flight.Organismic and Evolutionary BiologyOther Research Uni

Respiratory Evaporative Water Loss During Hovering and Forward Flight in Hummingbirds

Hummingbirds represent an end point for small body size and water flux in vertebrates. We explored the role evaporative water loss (EWL) plays in management of their large water pool and its use in dissipating metabolic heat. We measured respiratory evaporative water loss (REWL) in hovering hummingbirds in the field (6 species) and over a range of speeds in a wind tunnel (1 species) using an open-circuit mask respirometry system. Hovering REWL during the active period was positively correlated with operative temperature (Te) likely due to some combination of an increase in the vapor-pressure deficit, increase in lung ventilation rate, and reduced importance of dry heat transfer at higher Te. In rufous hummingbirds (Selasphorus rufus; 3.3 g) REWL during forward flight at 6 and 10 m/s was less than half the value for hovering. The proportion of total dissipated heat (TDH) accounted for by REWL during hovering at Te\u3e40 °C was b40% in most species. During forward flight in S. rufus the proportion of TDH accounted for by REWL was ~35% less than for hovering. REWL in hummingbirds is a relatively small component of the water budget compared with other bird species (b20%) so cutaneous evaporative water loss and dry heat transfer must contribute significantly to thermal balance in hummingbirds

Three-Dimensional Kinematics of Hummingbird Flight

Hummingbirds are specialized for hovering flight, and substantial research has explored this behavior. Forward flight is also important to hummingbirds, but the manner in which they perform forward flight is not well documented. Previous research suggests that hummingbirds increase flight velocity by simultaneously tilting their body angle and stroke-plane angle of the wings, without varying wingbeat frequency and upstroke: downstroke span ratio. We hypothesized that other wing kinematics besides stroke-plane angle would vary in hummingbirds. To test this, we used synchronized highspeed (500·Hz) video cameras and measured the threedimensional wing and body kinematics of rufous hummingbirds (Selasphorus rufus, 3·g, N=5) as they flew at velocities of 0–12·m·s–1 in a wind tunnel. Consistent with earlier research, the angles of the body and the stroke plane changed with velocity, and the effect of velocity on wingbeat frequency was not significant. However, hummingbirds significantly altered other wing kinematics including chord angle, angle of attack, anatomical strokeplane angle relative to their body, percent of wingbeat in downstroke, wingbeat amplitude, angular velocity of the wing, wingspan at mid-downstroke, and span ratio of the wingtips and wrists. This variation in bird-centered kinematics led to significant effects of flight velocity on the angle of attack of the wing and the area and angles of the global stroke planes during downstroke and upstroke. We provide new evidence that the paths of the wingtips and wrists change gradually but consistently with velocity, as in other bird species that possess pointed wings. Although hummingbirds flex their wings slightly at the wrist during upstroke, their average wingtip–span ratio of 93% revealed that they have kinematically ‘rigid’ wings compared with other avian species

Aerodynamics of the Hovering Hummingbird

Despite profound musculoskeletal differences, hummingbirds (Trochilidae) are widely thought to employ aerodynamic mechanisms similar to those used by insects. The kinematic symmetry of the hummingbird upstroke and downstroke has led to the assumption that these halves of the wingbeat cycle contribute equally to weight support during hovering, as exhibited by insects of similar size. This assumption has been applied, either explicitly or implicitly, in widely used aerodynamic models, and in a variety of empirical tests. Here we provide measurements of the wake of hovering rufous hummingbirds (Selasphorus rufus) obtained with digital particle image velocimetry that show force asymmetry: hummingbirds produce 75% of their weight support during the downstroke and only 25% during the upstroke. Some of this asymmetry is probably due to inversion of their cambered wings during upstroke. The wake of hummingbird wings also reveals evidence of leading-edge vortices created during the downstroke, indicating that they may operate at Reynolds numbers sufficiently low to exploit a key mechanism typical of insect hovering. Hummingbird hovering approaches that of insects, yet remains distinct because of effects resulting from an inherently dissimilar—avian—body plan