Flying in the rain: hovering performance of Anna's hummingbirds under varied precipitation

Flight in rain represents a greater challenge for smaller animals because the relative effects of water loading and drop impact are greater at reduced scales given the increased ratios of surface area to mass. Nevertheless, it is well known that small volant taxa such as hummingbirds can continue foraging even in extreme precipitation. Here, we evaluated the effect of four rain intensities (i.e. zero, light, moderate and heavy) on the hovering performance of Anna's hummingbirds (Calypte anna) under laboratory conditions. Light-to-moderate rain had only a marginal effect on flight kinematics; wingbeat frequency of individuals in moderate rain was reduced by 7 per cent relative to control conditions. By contrast, birds hovering in heavy rain adopted more horizontal body and tail positions, and also increased wingbeat frequency substantially, while reducing stroke amplitude when compared with control conditions. The ratio between peak forces produced by single drops on a wing and on a solid surface suggests that feathers can absorb associated impact forces by up to approximately 50 per cent. Remarkably, hummingbirds hovered well even under heavy precipitation (i.e. 270 mm h(-1)) with no apparent loss of control, although mechanical power output assuming perfect and zero storage of elastic energy was estimated to be about 9 and 57 per cent higher, respectively, compared with normal hovering.     

Effect of temperature and humidity on evaporative water loss in Anna's hummingbird (Calypte anna)

Summary Evaporative water loss (EWL), oxygen concumption , and body temperature (T_b) of Anna's Hummingbirds (Calypte anna; ca. 4.5g) were measured at combinations of ambient temperature (T_a) and water vapor density (_va) ranging from 20 to 37 °C and 2 to 27 g·m^-3, respectively. The EWL decreased linearly with increasing _va at all temperatures. The slopes of least squares regression lines relating EWL to _va at different temperatures were not significantly different and averaged-0.50 mg H₂O·m^-3·g^-2·h^-1 (range:-0.39 to-0.61). Increased _va restricted EWL in C. anna more than has been reported for other endotherms in dry air. The percent of metabolic heat production dissipated by evaporation ( ) was lower than that of other birds in dry air, but higher than that for other birds at high humidity when T_a 33 °C. When T_a>33 °C the effect of humidity on was similar to that in other birds. Calypte anna might become slightly hyperthermic at T_a>37 °C, which could augment heat transfer by increasing the T_b-T_a gradient. Body temperature for C. anna in this study was 43 °C (intramuscular) at T_as between 25 and 35 °C, which is above average for birds. It is estimated that field EWL is less than 30% of daily water loss in C. anna under mild temperature conditions (<35 °C).Abbreviations BMR basal metabolic rate - EWL evaporative water loss - percent of metabolic heat production dissipated by evaporation - ambient water vapor density - body surface water vapor density - RMR resting metabolic rate - T_a ambient-temperature - T_b body temperature - T_d dew-point temperature - TNZ thermoneutral zone - T_s body surface temperature - carbon dioxide production - oxygen consumption     

Lift production in the hovering hummingbird

Aerodynamic theory and empirical observations of animals flying at similar Reynolds numbers (Re) predict that airflow over hummingbird wings will be dominated by a stable, attached leading edge vortex (LEV). In insects exhibiting similar kinematics, when the translational movement of the wing ceases (as at the end of the downstroke), the LEV is shed and lift production decreases until the energy of the LEV is re-captured in the subsequent half-cycle translation. We here show that while the hummingbird wing is strongly influenced by similar sharp-leading-edge aerodynamics, leading edge vorticity is inconsistent, varying from 0.7 to 26 per cent (mean 16%) of total lift production, is always generated within 3 mm of the dorsal surface of the wing, showing no retrograde (trailing to leading edge) flow, and does not increase from proximal to distal wing as would be expected with a conical vortex (class III LEV) described for hawkmoths. Further, the bound circulation is not shed as a vortex at the end of translation, but instead remains attached and persists after translation has ceased, augmented by the rotation (pronation, supination) of the wing that occurs between the wing-translation half-cycles. The result is a near-continuous lift production through wing turn-around, previously unknown in vertebrates, able to contribute to weight support as well as stability and control during hovering. Selection for a planform suited to creating this unique flow and nearly-uninterrupted lift production throughout the wingbeat cycle may help explain the relatively narrow hummingbird wing.     

Song Learning in the Anna Hummingbird (Calypte anna)

Songs of wild male Anna hummingbirds (Calypte anna) consist of syllables grouped into phrases. Nearest neighbors tend to share similar syllable types, rhythms and syntax. Songs from different localities contain different syllable types, syntax and repetition indices. A male raised by hand in isolation produced a song consisting of highly variable syllable types of a wide frequency range. The song was simple in structure, and syllables were not grouped into phrases. Three males raised by hand as a group sang songs containing two stereotyped syllable types sung in alternating sequence and without phrase structure. These three males shared syllable types and syntax. The data from our study indicate that despite its relatively simple syrinx the Anna hummingbird learns syllable types, frequency, rhythm and syntax (as do oscines with their more complex syringes) during the song development process.     

Aerodynamic performance of a hovering hawkmoth with flexible wings: a computational approach

Insect wings are deformable structures that change shape passively and dynamically owing to inertial and aerodynamic forces during flight. It is still unclear how the three-dimensional and passive change of wing kinematics owing to inherent wing flexibility contributes to unsteady aerodynamics and energetics in insect flapping flight. Here, we perform a systematic fluid-structure interaction based analysis on the aerodynamic performance of a hovering hawkmoth, Manduca, with an integrated computational model of a hovering insect with rigid and flexible wings. Aerodynamic performance of flapping wings with passive deformation or prescribed deformation is evaluated in terms of aerodynamic force, power and efficiency. Our results reveal that wing flexibility can increase downwash in wake and hence aerodynamic force: first, a dynamic wing bending is observed, which delays the breakdown of leading edge vortex near the wing tip, responsible for augmenting the aerodynamic force-production; second, a combination of the dynamic change of wing bending and twist favourably modifies the wing kinematics in the distal area, which leads to the aerodynamic force enhancement immediately before stroke reversal. Moreover, an increase in hovering efficiency of the flexible wing is achieved as a result of the wing twist. An extensive study of wing stiffness effect on aerodynamic performance is further conducted through a tuning of Young's modulus and thickness, indicating that insect wing structures may be optimized not only in terms of aerodynamic performance but also dependent on many factors, such as the wing strength, the circulation capability of wing veins and the control of wing movements.     

Numerical investigation of the aerodynamic characteristics of a hovering Coleopteran insect

The aerodynamic characteristics of the Coleopteran beetle species Epilachna quadricollis, a species with flexible hind wings and stiff elytra (fore wings), are investigated in terms of hovering flight. The flapping wing kinematics of the Coleopteran insect are modeled through experimental observations with a digital high-speed camera and curve fitting from an ideal harmonic kinematics model. This model numerically simulates flight by estimating a cross section of the wing as a two-dimensional elliptical plane. There is currently no detailed study on the role of the elytron or how the elytron-hind wing interaction affects aerodynamic performance. In the case of hovering flight, the relatively small vertical or horizontal forces generated by the elytron suggest that the elytron makes no significant contribution to aerodynamic force.     

Leading edge vortex in a slow-flying passerine

Most hovering animals, such as insects and hummingbirds, enhance lift by producing leading edge vortices (LEVs) and by using both the downstroke and upstroke for lift production. By contrast, most hovering passerine birds primarily use the downstroke to generate lift. To compensate for the nearly inactive upstroke, weight support during the downstroke needs to be relatively higher in passerines when compared with, e.g. hummingbirds. Here we show, by capturing the airflow around the wing of a freely flying pied flycatcher, that passerines may use LEVs during the downstroke to increase lift. The LEV contributes up to 49 per cent to weight support, which is three times higher than in hummingbirds, suggesting that avian hoverers compensate for the nearly inactive upstroke by generating stronger LEVs. Contrary to other animals, the LEV strength in the flycatcher is lowest near the wing tip, instead of highest. This is correlated with a spanwise reduction of the wing's angle-of-attack, partly owing to upward bending of primary feathers. We suggest that this helps to delay bursting and shedding of the particularly strong LEV in passerines.     

Flight costs of long,sexually selected tails in hummingbirds

The elongated tails adorning many male birds have traditionally been thought to degrade flight performance by increasing body drag. However, aerodynamic interactions between the body and tail can be substantial in some contexts, and a short tail may actually reduce rather than increase overall drag. To test how tail length affects flight performance, we manipulated the tails of Anna''s hummingbirds (Calypte anna) by increasing their length with the greatly elongated tail streamers of the red-billed streamertail (Trochilus polytmus) and reducing their length by removing first the rectrices and then the entire tail (i.e. all rectrices and tail covert feathers). Flight performance was measured in a wind tunnel by measuring (i) the maximum forward speed at which the birds could fly and (ii) the metabolic cost of flight while flying at airspeeds from 0 to 14 m s⁻¹. We found a significant interaction effect between tail treatment and airspeed: an elongated tail increased the metabolic cost of flight by up to 11 per cent, and this effect was strongest at higher flight speeds. Maximum flight speed was concomitantly reduced by 3.4 per cent. Also, removing the entire tail decreased maximum flight speed by 2 per cent, suggesting beneficial aerodynamic effects for tails of normal length. The effects of elongation are thus subtle and airspeed-specific, suggesting that diversity in avian tail morphology is associated with only modest flight costs.     

Projected changes in elevational distribution and flight performance of montane Neotropical hummingbirds in response to climate change

The hovering flight of hummingbirds is one of the most energetically demanding forms of animal locomotion and is influenced by both atmospheric oxygen availability and air density. Montane Neotropical hummingbirds are expected to shift altitudinally upwards in response to climate change to track their ancestral climatic regime, which is predicted to influence their flight performance. In this study, we use the climate envelope approach to estimate upward elevational shifts for five Andean hummingbird species under two climate change scenarios. We then use field‐based data on hummingbird flight mechanics to estimate the resulting impact of climate change on aerodynamic performance in hovering flight. Our results show that in addition to significant habitat loss and fragmentation, projected upwards elevational shifts vary between 300 and 700 m, depending on climate change scenario and original mean elevation of the target species. Biomechanical analysis indicates that such upwards elevational shifts would yield a～2–5° increase in wing stroke amplitude with no substantial effect on wingbeat frequency. Overall, the physiological impact of elevational shifts of <1000 m in response to climate change is likely to be small relative to other factors such as habitat loss, changes in floristic composition, and increased interspecific competition.     

How to perform measurements in a hovering animal's wake: physical modelling of the vortex wake of the hawkmoth, Manduca sexta

The vortex wake structure of the hawkmoth, Manduca sexta, was investigated using a vortex ring generator. Based on existing kinematic and morphological data, a piston and tube apparatus was constructed to produce circular vortex rings with the same size and disc loading as a hovering hawkmoth. Results show that the artificial rings were initially laminar, but developed turbulence owing to azimuthal wave instability. The initial impulse and circulation were accurately estimated for laminar rings using particle image velocimetry; after the transition to turbulence, initial circulation was generally underestimated. The underestimate for turbulent rings can be corrected if the transition time and velocity profile are accurately known, but this correction will not be feasible for experiments on real animals. It is therefore crucial that the circulation and impulse be estimated while the wake vortices are still laminar. The scaling of the ring Reynolds number suggests that flying animals of about the size of hawkmoths may be the largest animals whose wakes stay laminar for long enough to perform such measurements during hovering. Thus, at low advance ratios, they may be the largest animals for which wake circulation and impulse can be accurately measured.     

Evolution of sexual dimorphism in bill size and shape of hermit hummingbirds (Phaethornithinae): a role for ecological causation

Unambiguous examples of ecological causation of sexual dimorphism are rare, and the best evidence involves sexual differences in trophic morphology. We show that moderate female-biased sexual dimorphism in bill curvature is the ancestral condition in hermit hummingbirds (Phaethornithinae), and that it is greatly amplified in species such as Glaucis hirsutus and Phaethornis guy, where bills of females are 60 per cent more curved than bills of males. In contrast, bill curvature dimorphism is lost or reduced in a lineage of short-billed hermit species and in specialist Eutoxeres sicklebill hermits. In the hermits, males tend to be larger than females in the majority of species, although size dimorphism is typically small. Consistent with earlier studies of hummingbird feeding performance, both raw regressions of traits and phylogenetic independent contrasts supported the prediction that dimorphism in bill curvature of hermits is associated with longer bills. Some evidence indicates that differences between sexes of hermit hummingbirds are associated with differences in the use of food plants. We suggest that some hermit hummingbirds provide model organisms for studies of ecological causation of sexual dimorphism because their sexual dimorphism in bill curvature provides a diagnostic clue for the food plants that need to be monitored for studies of sexual differences in resource use.     

Pollination ofPlectranthus vestitus (Lamiaceae) by trap-lining hovering bees in Madagascar

In a primary forest on the Central Plateau of MadagascarPlectranthus vestitus (Lamiaceae) was principally pollinated by the beePachymelus limbatus (Hymenoptera, Anthophoridae). A species ofStylogaster (Diptera, Conopidae) acted as a co-pollinator. Flower — pollinator interactions are described. The bee performed pollination while hovering, a flower-visit lasting onlyc. 0.3 seconds. Floral features such as shape and size of the corolla tube, and the lack of a landing place suggest specialization to hovering anthophorid beepollinators. Floral biology and pollination in the genusPlectranthus are discussed.     

Patagial complex evolution in hummingbirds and swifts (Apodiformes): a molecular phylogenetic perspective

Studies of the role of flight in vertebrate evolution often have focused on the propatagial muscle complex because this structure forms the wing's leading edge. However, historical narratives about the evolution of flight anatomy are compromised because traditional higher-level taxonomies typically are based in part on the propatagium itself. To avoid this circularity, I used a consensus molecular phylogeny to examine propatagial evolution in the highly aerial sister groups, hummingbirds and swifts (Apodiformes). Mapping of anatomy on molecular-based phylogeny indicates that structural variation in M .  tensor propatagialis pars brevis (TPB) is congruent with the major subclades of both hummingbirds and swifts. However, the humeral tendon and broad attachment of the fleshy belly of TPB with M .  extensor metacarpi radialis (EMR) most likely underwent parallel change in hummingbirds and swifts, while the distal tendon present only in hummingbirds changed from a thin sheet to a strong tendon. The combination of divergent (within hummingbirds or swifts) and parallel (between hummingbirds and swifts) evolutionary patterns implies that the taxonomic value of the propatagial complex in apodiformes depends on anatomical component and level of divergence. The congruence of anatomy with molecular phylogeny provides independent criteria for designating relatively ancestral versus derived clades of apodiformes. Based on these polarities, living hummingbirds and swifts express additional parallel trends from arboreal to more aerial foraging styles, and from depauperate to species-rich clades. Within apodiformes, the link of flight anatomy with taxonomic and ecologic diversity suggests that elaboration of locomotor modes was important for apodiform diversification, echoing a similar pattern for birds relative to their reptilian ancestors.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 211–219.     

Pollen transfer by hummingbirds and bumblebees, and the divergence of pollination modes in Penstemon

We compared pollen removal and deposition by hummingbirds and bumblebees visiting bird-syndrome Penstemon barbatus and bee-syndrome P. strictus flowers. One model for evolutionary shifts from bee pollination to bird pollination has assumed that, mostly due to grooming, pollen on bee bodies quickly becomes unavailable for transfer to stigmas, whereas pollen on hummingbirds has greater carryover. Comparing bumblebees and hummingbirds seeking nectar in P. strictus, we confirmed that bees had a steeper pollen carryover curve than birds but, surprisingly, bees and birds removed similar amounts of pollen and had similar per-visit pollen transfer efficiencies. Comparing P. barbatus and P. strictus visited by hummingbirds, the bird-syndrome flowers had more pollen removed, more pollen deposited, and a higher transfer efficiency than the bee-syndrome flowers. In addition, P. barbatus flowers have evolved such that their anthers and stigmas would not easily come into contact with bumblebees if they were to forage on them. We discuss the role that differences in pollination efficiency between bees and hummingbirds may have played in the repeated evolution of hummingbird pollination in Penstemon.     

Energetic cost of hovering flight in nectar-feeding bats (Phyllostomidae: Glossophaginae) and its scaling in moths, birds and bats

Three groups of specialist nectar-feeders covering a continuous size range from insects, birds and bats have evolved the ability for hovering flight. Among birds and bats these groups generally comprise small species, suggesting a relationship between hovering ability and size. In this study we established the scaling relationship of hovering power with body mass for nectar-feeding glossophagine bats (Phyllostomidae). Employing both standard and fast-response respirometry, we determined rates of gas exchange in Hylonycteris underwoodi (7 g) and Choeronycteris mexicana (13–18 g) during hover-feeding flights at an artificial flower that served as a respirometric mask to estimate metabolic power input. The O₂ uptake rate (V˙ _o2) in ml g⁻¹ h⁻¹ (and derived power input) was 27.3 (1.12 W or 160 W kg⁻¹) in 7-g Hylonycteris and 27.3 (2.63 W or 160 W kg⁻¹) in 16.5-g Choeronycteris and thus consistent with measurements in 11.9-g Glossophagasoricina (158 W kg⁻¹, Winter 1998). V˙ _o2 at the onset of hovering was also used to estimate power during forward flight, because after a transition from level forward to hovering flight gas exchange rates initially still reflect forward flight rates. V˙ _o2 during short hovering events (<1.5 s) was 19.0 ml g⁻¹ h⁻¹ (1.8 W) in 16-g Choeronycteris, which was not significantly different from a previous, indirect estimate of the cost of level forward flight (2.1 W, Winter and von Helversen 1998). Our estimates suggest that power input during hovering flight P _h (W) increased with body mass M (kg) within 13–18-g Choeronycteris (n = 4) as P _h  = 3544 (±2057 SE) M ^{1.76 (±0.21 SE)} and between different glossophagine bat species (n = 3) as P _h  = 128 (±2.4 SE) M ^{0.95 (±0.034 SE)}. The slopes of three scaling functions for flight power (hovering, level forward flight at intermediate speed and submaximal flight power) indicate that: 1. The relationship between flight power to flight speed may change with body mass in the 6–30-g bats from a J- towards a U-shaped curve. 2. A metabolic constraint (hovering flight power equal maximal flight power) may influence the upper size limit of 30–35 g for this group of flower specialists. Mass-specific power input (W kg⁻¹) during hovering flight appeared constant with regard to body size (for the mass ranges considered), but differed significantly (P < 0.001) between groups. Group means were 393 W kg⁻¹ (sphingid moths), 261 W kg⁻¹ (hummingbirds) and 159 W kg⁻¹ (glossophagine bats). Thus, glossophagine bats expend the least metabolic power per unit of body mass supported during hovering flight. At a metabolic power input of 1.1 W a glossophagine bat can generate the lift forces necessary for balancing 7 g against gravitation, whereas a hummingbird can support 4 g and a sphingid moth only 3 g of body mass with the same amount of metabolic energy. These differences in power input were not fully explained by differences in induced power output estimated from Rankine-Froude momentum-jet theory. Accepted: 10 November 1998     

Flight in slow motion: aerodynamics of the pterosaur wing

The flight of pterosaurs and the extreme sizes of some taxa have long perplexed evolutionary biologists. Past reconstructions of flight capability were handicapped by the available aerodynamic data, which was unrepresentative of possible pterosaur wing profiles. I report wind tunnel tests on a range of possible pterosaur wing sections and quantify the likely performance for the first time. These sections have substantially higher profile drag and maximum lift coefficients than those assumed before, suggesting that large pterosaurs were aerodynamically less efficient and could fly more slowly than previously estimated. In order to achieve higher efficiency, the wing bones must be faired, which implies extensive regions of pneumatized tissue. Whether faired or not, the pterosaur wings were adapted to low-speed flight, unsuited to marine style dynamic soaring but adapted for thermal/slope soaring and controlled, low-speed landing. Because their thin-walled bones were susceptible to impact damage, slow flight would have helped to avoid injury and may have contributed to their attaining much larger sizes than fossil or extant birds. The trade-off would have been an extreme vulnerability to strong or turbulent winds both in flight and on the ground, akin to modern-day paragliders.     

Specializations in optic flow encoding in the pretectum of hummingbirds and zebra finches

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Swimming kinematics and wake elements in a worm-like insect: the larva of the midge Chironomus plumosus (Diptera)

The kinematics and hydrodynamics of swimming chironomid larvae were investigated with the aid of videography and dye streamers used to visualize near-body flow. Chironomids employ a characteristic 'figure-of-eight' swimming technique based on high-amplitude side-to-side bending of the body. These scissor-like movements produce relatively slow (two body lengths (BL) s⁻¹) forward motion but also serve to support the weight of the insect against its own negative buoyancy. The main wake element identified by the present technique consisted of a discrete ring vortex with an external diameter of c. 0.3 BL which was shed to the rear of the body towards the end of each half-stroke. During level swimming, the jet of the vortex was directed 10° below the horizontal plane indicating that it was mainly providing thrust. An additional, but poorly defined, flow was associated with the rapid downwards motion of the head at the start of each half-stroke and it is proposed that this contributes to the vertical force needed to support the weight of the body during swimming.