ABSTRACT Hovering insects are divided into two categories: ‘normal’ hoverers that move the wing symmetrically in a horizontal stroke plane, and those with an inclined stroke plane. Normal hoverers have been suggested to support their weight during both downstroke and upstroke, shedding vortex rings each half-stroke. Insects with an inclined stroke plane should, according to theory, produce flight forces only during downstroke, and only generate one set of vortices. The type of hovering is thus linked to the power required to hover. Previous efforts to characterize the wake of hovering insects have used low-resolution experimental techniques or simulated the flow using computational fluid dynamics, and so it remains to be determined whether insect wakes can be represented by any of the suggested models. Here, we used tomographic particle image velocimetry, with a horizontal measurement volume placed below the animals, to show that the wake shed by hovering hawkmoths is best described as a series of bilateral, stacked vortex ‘rings’. While the upstroke is aerodynamically active, despite an inclined stroke plane, it produces weaker vortices than the downstroke. In addition, compared with the near wake, the far wake lacks structure and is less concentrated. Both near and far wakes are clearly affected by vortex interactions, suggesting caution is required when interpreting wake topologies. We also estimated induced power ( P ind ) from downwash velocities in the wake. Standard models predicted a P ind more than double that from our wake measurements. Our results thus question some model assumptions and we propose a reevaluation of the model parameters.

ABSTRACT Cost of flight at various speeds is a crucial determinant of flight behaviour in birds. Aerodynamic models, predicting that mechanical power ( P mech ) varies with flight speed in a U -shaped manner, have been used together with an energy conversion factor (efficiency) to estimate metabolic power ( P met ) . Despite few empirical studies, efficiency has been assumed constant across flight speeds at 23%. Ideally, efficiency should be estimated from measurements of both P mech and P met in un-instrumented flight . Until recently, progress has been hampered by methodological constraints. The main aim of this study was to evaluate recently developed techniques and estimate flight efficiency across flight speeds. We used the 13 C-labelled sodium bicarbonate method (NaBi) and particle image velocimetry (PIV) to measure P met and P mech in blackcaps flying in a wind tunnel. We also cross-validated measurements made by NaBi with quantitative magnetic resonance (QMR) body composition analysis in yellow-rumped warblers. We found that P met estimated by NaBi was ∼12% lower than corresponding values estimated by QMR. P met varied in a U -shaped manner across flight speeds in blackcaps, but the pattern was not statistically significant. P mech could only be reliably measured for two intermediate speeds and estimated efficiency ranged between 14% and 22% (combining the two speeds for raw and weight/lift-specific power, with and without correction for the ∼12% difference between NaBi and QMR), which were close to the currently used default value. We conclude that NaBi and PIV are viable techniques, allowing researchers to address some of the outstanding questions regarding bird flight energetics.

ABSTRACT All echolocating bats and whales measured to date emit a directional bio-sonar beam that affords them a number of advantages over an omni-directional beam, i.e. reduced clutter, increased source level and inherent directional information. In this study, we investigated the importance of directional sound emission for navigation through echolocation by measuring the sonar beam of brown long-eared bats, Plecotus auritus . Plecotus auritus emits sound through the nostrils but has no external appendages to readily facilitate a directional sound emission as found in most nose emitters. The study shows that P. auritus , despite lacking an external focusing apparatus, emits a directional echolocation beam (directivity index=13 dB) and that the beam is more directional vertically (−6 dB angle at 22 deg) than horizontally (−6 dB angle at 35 deg). Using a simple numerical model, we found that the recorded emission pattern is achievable if P. auritus emits sound through the nostrils as well as the mouth. The study thus supports the hypothesis that a directional echolocation beam is important for perception through echolocation and we propose that animals with similarly non-directional emitter characteristics may facilitate a directional sound emission by emitting sound through both the nostrils and the mouth.

ABSTRACT Gliding flight is a relatively inexpensive mode of flight used by many larger bird species, where potential energy is used to cover the cost of aerodynamic drag. Birds have great flexibility in their flight configuration, allowing them to control their flight speed and glide angle. However, relatively little is known about how this flexibility affects aerodynamic drag. We measured the wake of a jackdaw ( Corvus monedula ) gliding in a wind tunnel, and computed the components of aerodynamic drag from the wake. We found that induced drag was mainly affected by wingspan, but also that the use of the tail has a negative influence on span efficiency. Contrary to previous work, we found no support for the separated primaries being used in controlling the induced drag. Profile drag was of similar magnitude to that reported in other studies, and our results suggest that profile drag is affected by variation in wing shape. For a folded tail, the body drag coefficient had a value of 0.2, rising to above 0.4 with the tail fully spread, which we conclude is due to tail profile drag.

ABSTRACT Bats evolved the ability of powered flight more than 50 million years ago. The modern bat is an efficient flyer and recent research on bat flight has revealed many intriguing facts. By using particle image velocimetry to visualize wake vortices, both the magnitude and time-history of aerodynamic forces can be estimated. At most speeds the downstroke generates both lift and thrust, whereas the function of the upstroke changes with forward flight speed. At hovering and slow speed bats use a leading edge vortex to enhance the lift beyond that allowed by steady aerodynamics and an inverted wing during the upstroke to further aid weight support. The bat wing and its skeleton exhibit many features and control mechanisms that are presumed to improve flight performance. Whereas bats appear aerodynamically less efficient than birds when it comes to cruising flight, they have the edge over birds when it comes to manoeuvring. There is a direct relationship between kinematics and the aerodynamic performance, but there is still a lack of knowledge about how (and if) the bat controls the movements and shape (planform and camber) of the wing. Considering the relatively few bat species whose aerodynamic tracks have been characterized, there is scope for new discoveries and a need to study species representing more extreme positions in the bat morphospace.

SUMMARY To obtain a full understanding of the aerodynamics of animal flight, the movement of the wings, the kinematics, needs to be connected to the wake left behind the animal. Here the detailed 3D wingbeat kinematics of bats, Glossophaga soricina, flying in a wind tunnel over a range of flight speeds (1–7 m s −1 ) was determined from high-speed video. The results were compared with the wake geometry and quantitative wake measurements obtained simultaneously to the kinematics. The wingbeat kinematics varied gradually with flight speed and reflected the changes observed in the wake of the bats. In particular, several of the kinematic parameters reflected the differences in the function of the upstroke at low and high flight speeds. At lower flight speeds the bats use a pitch-up rotation to produce a backward flick which creates thrust and some weight support. At higher speeds this mechanism disappears and the upstroke generates weight support but no thrust. This is reflected by the changes in e.g. angle of attack, span ratio, camber and downstroke ratio. We also determined how different parameters vary throughout a wingbeat over the flight speeds studied. Both the camber and the angle of attack varied over the wingbeat differently at different speeds, suggesting active control of these parameters to adjust to the changing aerodynamic conditions. This study of the kinematics strongly indicates that the flight of bats is governed by an unsteady high-lift mechanism at low flight speeds and points to differences between birds and bats.

Sustained climb rates and airspeeds in flapping flight were measured by radar tracking fifteen species of migrating birds ranging in body mass between 10 g and 10 kg. There was an inverse correlation between body size and climb rate: the lowest mean climb rate, 0.32 m s −1 , was observed in the mute swan and the highest mean value, 1.63ms −1 , in the dunlin. Some dunlin flocks achieved sustained climb rates exceeding 2ms −1 , up to 2.14ms −1 . Assuming that the migrants expend maximum sustained power during their climbs, the climbing power can be used as a conservative estimate of the power margin. Estimates of climbing power for the species tracked by radar were used, in conjunction with aerodynamic theory, to calculate the amount of extra load the migrants should be able to carry if their power margin was used for load transportation rather than for climbing. Calculated ratios of total body mass with maximum load to lean body mass ranged between 1.28 and 2.75, showing an overall negative correlation with body size. There was a broad agreement with maximum fuel loads observed among freeliving birds, indicating that the upper limits of fuel-carrying capacities and flight ranges in migrating birds are determined by power margin constraints in sustained flapping flight. Markedly reduced climb rates have been recorded for shorebirds departing with very large fuel reserves from W. Africa and Iceland, supporting the calculated trade-off between climb rate and fuel load. Total power consumption was estimated as the sum of calculated aerodynamic power for forward flight and climbing power. The ratio of total power to the expected minimum aerodynamic power was 1.1-1.3 in the three largest species, increasing to 2–4 in the smallest species. Medium- and small-sized species seem to allocate power in excess of the minimum aerodynamic power not only for climbing but also for maintaining a forward speed faster than the minimum power speed. Given provisional estimates of flight muscle masses and wingbeat frequencies, the mass-specific sustained muscle work for the different climbing bird species was calculated to be in the range 16–41 joules per kilogram muscle mass, showing a significant positive correlation with body mass