ArticlePDF Available

Flight speeds of swifts (Apus apus): Seasonal differences smaller than expected

The Royal Society
Proceedings of the Royal Society B
Authors:
Per Henningsson at Lund University
Per Henningsson
Håkan Karlsson at Lund University
Håkan Karlsson
Johan Bäckman at Lund University
Johan Bäckman
T. Alerstam
T. Alerstam
T. Alerstam
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Show all 5 authorsHide
Download full-text PDF
Read full-text
Download citation

Abstract and Figures

We have studied the nocturnal flight behaviour of the common swift (Apus apus L.), by the use of a tracking radar. Birds were tracked from Lund University in southern Sweden during spring migration, summer roosting flights and autumn migration. Flight speeds were compared with predictions from flight mechanical and optimal migration theories. During spring, flight speeds were predicted to be higher than during both summer and autumn due to time restriction. In such cases, birds fly at a flight speed that maximizes the overall speed of migration. For summer roosting flights, speeds were predicted to be lower than during both spring and autumn since the predicted flight speed is the minimum power speed that involves the lowest energy consumption per unit time. During autumn, we expected flight speeds to be higher than during summer but lower than during spring since the expected flight speed is the maximum range speed, which involves the lowest energy consumption per unit distance. Flight speeds during spring were indeed higher than during both summer and autumn, which indicates time-selected spring migration. Speeds during autumn migration were very similar to those recorded during summer roosting flights. The general result shows that swifts change their flight speed between different flight behaviours to a smaller extent than expected. Furthermore, the difference between flight speeds during migration and roosting among swifts was found to be less pronounced than previously recorded.
Figures - uploaded by Anders Hedenström
Author content
All figure content in this area was uploaded by Anders Hedenström
Content may be subject to copyright.
Content uploaded by Anders Hedenström
Author content
All content in this area was uploaded by Anders Hedenström
Content may be subject to copyright.
A preview of the PDF is not available
A preview of this full-text is provided by The Royal Society.
Learn more
Content available from Proceedings of the Royal Society B 
This content is subject to copyright.
... The authors of these investigations noted that in some cases the behaviour of the treated birds could not be typical of their free migratory flight. In rare cases species identification is possible at night using tracking radar where the major criterion is the wing-beat pattern, such as in Common Swifts (Apus apus) in Scandinavia (Bäckman & Alerstam 2001, Henningsson et al. 2009, Karlsson et al. 2010. Hence, field data on the speed of free flying nocturnal passerine migrants of particular species are far from being numerous yet. ...
... Flight speed measurements of free flying nocturnal migrants of particular species are known only from a few publications (Stark 1996, Bruderer & Boldt 2001, Cochran & Wikelski 2005, Henningsson et al. 2009) mainly because of difficulties of species identification in the dark. In our investiga- Fig. 4 The relationship between theoretical minimal power speed (Vmp), maximum range speed (Vmr) and observed airspeed of the flying Song Thrushes during autumn migration. ...
Airspeed of the Song Thrush in relation to the wind during autumnal nocturnal migration
Article
Full-text available
  • Jul 2019
  • ORNIS FENNICA
Birds possess behavioural and physiological adaptations which permit them to minimize time and energy expenditure during migration in a broad spectrum of winds, for instance, by varying their airspeed. Nocturnally migrating birds were recorded by an optical-electronic matrix system, which permitted recording their images and flight parameters in the dark. Among medium size birds, Song Thrushes (Turdus philomelos) were identified by their silhouette, linear size, wing-beat pattern, and phenology. The equivalent airspeed at sea level (VEq) of the observed thrushes without wind assistance (mean value 14.4 m/s) was close to the maximum range speed (Vmr) predicted from flight mechanical theory. This indicated an energy-selected migration strategy of the thrushes in autumn. The characteristic speed Vmr is wind-dependent: it increases with increasing velocity of head- and sidewinds. The airspeeds of the Song Thrushes showed a similar pattern of wind-dependence.
View
... Swifts (Apodidae) have an eminently aerial life. They spend most of their time aloft (Åkesson et al., 2012;Liechti et al., 2013), using their beaks and feet to collect nest materials suspended in the air while flying at high and low altitudes (Lack, 1956;Henningsson et al., 2009;Hedenström et al., 2016). Thus, swifts are potential candidates to use atmospheric plastic debris and other anthropogenic debris carried by the wind as nest material. ...
View
... Contrary, if S exp is decreasing along the route, stopovers will be relatively long, fuel loads large and flight steps long at the beginning of migration and will successively decrease with the progress of migration (figure 1). These situations are expected to apply to autumn and spring migration, respectively, where migration speed increases during autumn migration [21][22][23], while spring migration is against a resource gradient, with diminishing food abundance from the wintering to breeding area [14,[24][25][26][27]. (H4) It has been suggested that the overall migration strategy may differ between spring and autumn, as indicated by faster flight speeds in spring than in autumn [28,29], since time-minimizing birds should fly at a faster airspeed than if minimizing cost of transport [9]. A compilation of seasonal migration and flight speeds showed that birds tended to migrate and fly faster in spring than autumn [30], although this is not evidence of seasonally differing strategies since the environment may provide seasonally differing conditions such as winds and food availability [31,32]. ...
Seasonal patterns and processes of migration in a long-distance migratory bird: energy or time minimization?
Article
Full-text available
  • Jun 2024
Optimal migration theory prescribes adaptive strategies of energy, time or mortality minimization. To test alternative hypotheses of energy- and time-minimization migration we used multisensory data loggers that record time-resolved flight activity and light for positioning by geolocation in a long-distance migratory shorebird, the little ringed plover, Charadrius dubius. We could reject the hypothesis of energy minimization based on a relationship between stopover duration and subsequent flight time as predicted for a time minimizer. We found seasonally diverging slopes between stopover and flight durations in relation to the progress (time) of migration, which follows a time-minimizing policy if resource gradients along the migration route increase in autumn and decrease in spring. Total flight duration did not differ significantly between autumn and spring migration, although spring migration was 6% shorter. Overall duration of autumn migration was longer than that in spring, mainly owing to a mid-migration stop in most birds, when they likely initiated moult. Overall migration speed was significantly different between autumn and spring. Migratory flights often occurred as runs of two to seven nocturnal flights on adjacent days, which may be countering a time-minimization strategy. Other factors may influence a preference for nocturnal migration, such as avoiding flight in turbulent conditions, heat stress and diurnal predators.
View
... Weather radars are well positioned to do so, yet so far have shown mixed results: some showing significantly faster airspeed in spring (Henningsson et al., 2009;Horton, van Doren, Stepanian, Farnsworth, & Kelly, 2016a;Karlsson et al., 2012;Nilsson et al., 2014), others similar speeds in both seasons ) and yet others finding slightly faster airspeeds in autumn (Kemp et al., 2010). ...
Favorable winds speed up bird migration in spring but not in autumn
Article
Full-text available
  • Jul 2022
Wind has a significant yet complex effect on bird migration speed. With prevailing south wind, overall migration is generally faster in spring than in autumn. However, studies on the difference in airspeed between seasons have shown contrasting results so far, in part due to their limited geographical or temporal coverage. Using the first full-year weather radar data set of nocturnal bird migration across western Europe together with wind speed from reanalysis data, we investigate variation of airspeed across season. We additionally expand our analysis of ground speed, airspeed, wind speed, and wind profit variation across time (seasonal and daily) and space (geographical and altitudinal). Our result confirms that wind plays a major role in explaining both temporal and spatial variabilities in ground speed. The resulting airspeed remains relatively constant at all scales (daily, seasonal, geographically and altitudinally). We found that spring airspeed is overall 5% faster in Spring than autumn, but we argue that this number is not significant compared to the biases and limitation of weather radar data. The results of the analysis can be used to further investigate birds' migratory strategies across space and time, as well as their energy use.
View
... Swifts have recently been shown to remain aloft most or all of their non-breeding period (Liechti et al. 2013, Hedenström 2016, Hedenström et al. 2019). This continuous flight puts a strong demand on energy requirements with flight behaviour likely being optimized to sustain this high energy expenditure (Bäckman and Alerstam 2001, Henningsson et al. 2009, Hedrick et al. 2018. Research on the common swift Apus apus and other swift species has shown that they feed in a heterogeneous spatio-temporal landscape and leverage swarming insects selectively across space and time (Cucco et al. 1993, Russell 1999, Collins 2015, de Margerie et al. 2018. ...
The aeroecology of atmospheric convergence zones: the case of pallid swifts
Article
Full-text available
  • Apr 2022
Trans‐Saharan migratory bird species encounter large scale seasonal atmospheric convergence zones, where opposing monsoon and continental air masses meet. These macro‐scale atmospheric conditions determine local weather, influence migratory and foraging behaviour and seasonal bird survival rates. Here, we investigate the flight behaviour of pallid swifts Apus pallidus, a small aerial insectivore, in relation to non‐breeding season atmospheric conditions using state‐of‐the‐art GPS logged data. Our analysis shows two novel diurnal flight patterns which suggest that pallid swift prey on insects concentrated along frontal convergence zones, in particular the continental Inter‐Tropical Convergence Zone (ITCZ) and a coastal sea‐breeze front. Resource use seems not only contingent on the abundance of insects, but also favourable atmospheric conditions. Persistence of swifts in wintering feeding grounds might therefore depend on the prevailing atmospheric conditions and their concentrating effects on insects rather than solely the vegetation state and co‐dependent insect populations. Migration events within, to and from, the non‐breeding season foraging locations might not only be guided by a decline in vegetation as common metric for prey availability, but also by shifting wind directions and their concentrating effects.
View
... This has been confirmed among other species, e.g. waders (Alves et al. 2016), gulls (Bustnes et al. 2013), swifts (Henningsson et al. 2009) and songbirds (DeLuca et al. 2015) as well as in Bar-headed Geese (Anser indicus) (Hawkes et al. 2013). ...
Seasonal and regional differences in migration patterns and conservation status of Swan Geese (Anser cygnoides) in the East Asian Flyway
Article
Full-text available
  • Dec 2021
Background The Swan Goose ( Anser cygnoides ) breeds across Mongolia and adjacent China and Russia and winters exclusively in China. It is globally threatened, showing long-term major range contractions and declining abundance, linked to habitat loss and degradation. We remain ignorant about the biogeographical subpopulation structure of the species and potential differences in their migration timing, stopovers and schedules, information that could be vital to effective conservation of different elements of the species population, which we address here with results from a telemetry study. Methods In 2017–2018, we attached GPS/GSM telemetry devices to 238 Swan Geese on moulting sites in three discrete parts of their summering area (Dauria International Protected Area, Central Mongolia and Western Mongolia), generating 104 complete spring and autumn migration episodes to compare migration speed and nature between birds of different summer provenances. Results Birds from all three breeding areas used almost completely separate migration routes to winter sympatrically in the Yangtze River floodplain. Although many features of the spring and autumn migrations of the three groups were similar, despite the significantly longer migration routes taken by Western Mongolian tagged birds, birds from Dauria Region arrived significantly later in winter due to prolonged staging in coastal areas and took longer to reach their breeding areas in spring. Among birds of all breeding provenances, spring migration was approximately twice as fast as autumn migration. Areas used by staging Swan Geese (mainly wetlands) in autumn and spring almost never fell within national level protected areas, suggesting major site safeguard is necessary to protect these critical areas. Conclusions This study showed the discreteness of migration routes taken by birds of different summer provenances and differences in their migratory patterns, highlighting key staging areas (Yalu River Estuary in China/North Korea for Dauria Region breeding birds, Daihai Lake for Central Mongolian and Ordos Basin for Western Mongolian birds). Based on this new knowledge of the biogeographical subpopulation structure of the Swan Goose, we need to combine data on subpopulation size, their distribution throughout the annual life cycle and conservation status, to develop more effective conservation strategies and measures to reverse population decline throughout the range.
View
Integrating flight mechanics, energetics and migration ecology in vertebrates
Article
  • Feb 2025
  • J EXP BIOL
Animal locomotion is constrained by Newtonian laws of motion and therefore biomechanics is a useful approach for quantitative analysis of force and power requirements. Aerial locomotion in vertebrates is no exception, and arguably the most significant developments are to be found in this journal. Evolutionary birds and bats are very successful groups, doubtless largely because of their ability to shift location in a short time. This has enabled birds and to a lesser extent bats to perform seasonal long-distance migrations between habitats suitable for reproduction and survival. Power required to fly and potential flight range in relation to fuel load are two fundamental relationships derived from flight mechanics, which both serve as a foundation for the development of optimal migration theory. From this framework where biomechanics, energetics and ecology combine, we can analyse which of the alternative strategies migrants adopt. Such adaptive behaviours include the selection of optimal flight speed and the migratory travel itinerary. However, despite decades of research efforts, there are still many unsolved problems concerning flight mechanics and energetics of vertebrate flight. One such is how the power–speed relationship maps onto metabolic rate during flight, the so-called energy conversion efficiency. There is conflicting empirical evidence concerning how energy conversion possibly varies with flight speed, body mass and body size. As ultimately it is the metabolic energy consumption that is under selection pressure, this is an urgent question for the utility of flight mechanical principles in ecology. In this Review, I discuss this and other knowledge gaps in vertebrate flight and migration.
View
Drink safely: common swifts ( Apus apus ) dissipate mechanical energy to decrease flight speed before touch-and-go drinking
Article
  • Feb 2023
Flight is an efficient way of transport over a unit of distance, but it can be very costly over each unit of time, and reducing flight energy expenditures is a major selective pressure in birds. The common swift (Apus apus) is one of the most aerial bird species, performing most behaviours in flight: foraging, sleeping, and also drinking by regularly descending to various waterbodies and skimming over the surface. An energy-saving way to perform such touch-and-go drinking would be to strive to conserve mechanical energy, by transforming potential energy to kinetic energy during the gliding descent, touching water at high speed, and regaining height with minimal muscular work. Using 3D optical tracking, we recorded 163 swift drinking trajectories, over three waterbodies near Rennes, France. Contrarily to the energy conservation hypothesis, we show that swifts approaching a waterbody with a higher mechanical energy (higher height and/or speed 5 s before contact) do not reach water at higher speeds, but do brake, i.e. dissipate mechanical energy to lose both height and speed. Braking seemed to be linked with sharp turns and the use of headwind to some extent, but finer turns and postural adjustments, beyond the resolving power of our tracking data, could also be involved. We hypothesize that this surprisingly costly behaviour results from a trade-off between energy expenditure and safety, because approaching water surface requires fine motor control, and high speed increases the risk of falling into water, which would have serious energetic and survival costs for a swift.
View
Feeding ecology of a highly aerial bird during its long breeding season
Article
Full-text available
  • Dec 2022
Pallid Swifts, as other swifts, are birds extremely adapted to an aerial lifestyle, showing unique adaptations that allow them to fly almost continuously. The diet of these non-stopping high-altitudinal aerial birds has been mostly studied through techniques that fail to produce highly resolved prey identifications, and for that have been replaced by molecular techniques, as DNA metabarcoding. Faecal samples of Pallid Swifts (Apus pallidus) were monthly collected from a colony in the north of Portugal during the breeding season. DNA from the faecal samples was used to sex the birds and to identify the arthropods present in the diet through DNA metabarcoding. From the detected prey items, 74 families were identified belonging to 16 orders, with Hymenoptera and Hemiptera being the most frequently consumed. There were seasonal variations in diet richness, composition and prey size. Regarding the diet of males and females, although no differences were found between the diet of males and females in terms of composition and richness, there were differences in the size of arthropods preyed by the different sexes, with males feeding on larger arthropods. The large seasonal variation in Pallid Swifts' diet during the breeding season is probably a result of spatiotemporal variation in aerial prey, of which swifts likely predate opportunistically. Although no significant differences were detected in diet richness and composition between sexes, the fact that males consumed larger prey may suggest the existence of sexual dietary segregation in this group of birds. At last, several pest species were found in these swifts’ diet, which, if studied through DNA metabarcoding, can be used to monitor small arthropods, including airborne pests.
View
Flight speed of the song thrush (Turdus philomelos) during autumn nocturnal migration
Article
  • Mar 2014
Nocturnally migrating birds were observed with Electronic-optical system for registration. The Song Thrushes have been distinguished among the nocturnal migrants by the linear size, wing-beat pattern and phonological data. The airspeed of the thrushes varies depending on wind direction and velocity. It increases with increasing headwind component relative to its value in calm air and decreases with increasing tail-wind component. The air speed of the migratory flight in thrushes is proportional to an effective wing-beat frequency calculated for the flapping phases and pauses between them. Under different wind conditions the birds maintain optimal physiological wing beat frequency within rather narrow range but vary their airspeed by duration of inertial phases of flight. The observed airspeeds of the Song Thrushes were close to the theoretically predicted maximum range speed indicating energy-selected migration strategy in autumn.
View
Confronting the winds: orientation and flight behaviour of roosting swifts, Apus apus
Article
  • May 2001
Swifts, Apus apus, spend the night aloft and this offers an opportunity to test the degree of adaptability of bird orientation and flight to different ecological situations. We predicted the swifts' behaviour by assuming that they are adapted to minimize energy expenditure during the nocturnal flight and during a compensatory homing flight if they become displaced by wind. We tested the predictions by recording the swifts' altitudes, speeds and directions under different wind conditions with tracking radar; we found an agreement between predictions and observations for orientation behaviour, but not for altitude and speed regulation. The swifts orientated consistently into the head wind, with angular concentration increasing with increasing wind speed. However, contrary to our predictions, they did not select altitudes with slow or moderate winds, nor did they increase their airspeed distinctly when flying into strong head winds. A possible explanation is that their head-wind orientation is sufficient to keep nocturnal displacement from their home area within tolerable limits, leaving flight altitude to be determined by other factors (correlated with temperature), and airspeed to show only a marginal increase in strong winds. The swifts were often moving 'backwards: heading straight into the wind but being overpowered by wind speeds exceeding their airspeed. The regular occurrence of such flights is probably uniquely associated with the swifts' remarkable habit of roosting on the wing.
View
Strategies for the transition to breeding in time-selected bird migration
Article
  • Jan 2006
In time-selected migration birds adapt their fuel deposition and flight behaviour to maximise sustained migration speed. What are the expectations for the final part of spring migration when the transition to breeding takes place and the criterion of a maximum total migration speed is no longer relevant? Two possible strategies representing different ends of the capital - income spectrum are evaluated. In the first strategy the birds gain an advancement in the breeding cycle by depositing breeding resources while still on migration, as long as the marginal resource deposition rate at the final stopover site, devaluated for the flight transport costs, exceeds that at the breeding destination. In the second strategy an early arrival at the breeding site, before competitors, is of overriding importance, and sprint migration is predicted. In this case migration towards the breeding grounds would to a large degree be a race between competitors, where the birds are expected to change from a maximum sustained speed during much of migration to a final sprint. In such sprint migration the birds exhaust their resources and expose themselves to increased risks in order to obtain the critical priority benefits associated with an arrival before competitors. If and to what degree these strategies exist among migratory birds is unknown. Predictions are given for testing if capital breeding is driven by differential resource gain rates at stopover versus breeding sites. For testing the strategy of sprint migration, investigations of the migrants' flight behaviour on their final approach to the breeding destinations will be decisive. Inspection of satellite tracking data for two Ospreys Pandion haliaetus revealed an accelerated final approach to the breeding site including nocturnal flights in addition to the regular diurnal thermal soaring migration in one but not the other individual.
View
Optimal Bird Migration: The Relative Importance of Time, Energy, and Safety
Chapter
  • Jan 1990
“Optimization is the process of minimizing costs or maximizing benefits, or obtaining the best possible compromise between the two. Evolution by natural selection is a process of optimization” (R. McNeill Alexander 1982).
View
Bird Migration Speed
Chapter
  • Jan 2003
Migrating birds alternate between two main phases of (1) flight when distance is covered and energy (fuel) is consumed, and (2) fuel deposition when energy is accumulated by intensive foraging. Fuel deposition takes place during stopover periods between flights as well as during a premigratory fuelling period before the initial flight. Thus, the total duration of migration T is the sum of flight time T flight and fuel deposition time T dep: T=Tflight+Tdep.T = {T_{flight}} + {T_{dep}}. (1)
View
How Fast Can Birds Migrate?
Article
  • Dec 1998
Bird migration is typically characterized by periods of flight, when fuel is consumed, and intervening stopover periods when fuel is deposited. The resulting overall migration speed can be calculated on the basis of flight speed, rate of fuel deposition and power consumption during flight. Energy deposition rate (Pdep)(P_{{\rm dep}}) can be estimated as the difference between metabolic scope and field metabolic rate during stopover. Evaluating how migration speed scales with body mass yields a declining speed with increasing mass for flapping flight (∝m−0.19)(\propto m^{-0.19}) , while migration speed increases with increasing mass for soaring flight. For flapping flight, migration speed and power can be calculated according to aerodynamic theory, and in soaring flight cross-country speed can be estimated from rate of climb (when circling in thermals) and the glide polar. Power in soaring/gliding flight is assumed to be a constant multiple of the basal metabolic rate (BMR). We used this approach to calculate expected speeds of migration for 15 species. For a small bird with a typical energy deposition rate (Pdep=1.0⋅BMR)(P_{{\rm dep}}=1.0\cdot {\rm BMR}) and using flapping flight, predicted migration speed is about 200 km/day, while a maximum energy deposition rate (Pdep=2.5⋅BMR)(P_{{\rm dep}}=2.5\cdot {\rm BMR}) yields migration speeds of 300-400 km/day. In larger birds, migration speeds will be lower, ranging from 70 to 100 km/day for typical energy deposition rates to 150 to 200 km/day for maximum energy deposition rates. In large species, soaring flight gives higher migration speed than flapping flight. For large species using flapping flight, migration speed per se may restrict the potential migration distance that can be accommodated within an annual breeding-migration programme.
View
Fifteen Testable Predictions about Bird Flight
Article
  • Jan 1978
On the basis of previously published theory, numerical predictions are made about effects which could be observed by methods in current use. The first few predictions relate to changes in speed, height and flapping fraction in migrating birds, due to wind and the consumption of fuel, and are designed to be tested by radar observation of migrants. Further predictions are morphological in character, and a final group relates to birds migrating by thermal soaring. Comparison of these predictions with observation should enable the theoretical understanding of bird flight to be considerably improved. /// На основании ранее опубликованной теории проведена количественная оценка ожидаемых резулътатов при исполъзовании применяемых ныне методов. Первая группа предсказаний касается изменений скорости, высоты и времени махового полета в зависимости от ветра, расхода энергии и предназначены для проверки радарных наблюдений за полетом мнгрантов. Лругая группа расчетов касаласъ морфологических особенностей, и последняя - относиласъ к птицам, летяшим по температурньм ориентирам. Сравнения этих расчетов с резулътатами наблюдений дают возможностъ обосноватъ теоретические представления о перелете птиц.
View
The Optimum Flight Speeds of Flying Animals
Article
  • Dec 1998
Conventional models of bird flight combine metabolic, mechanical, and aerodynamic components to calculate the rate of fuel consumption and power required for flight, from which we may derive the optimal speeds flying animals should use in different situations. These models contain the implicit assumption that the metabolic and mechanical components of power output are constant and show no systematic variation with speed. This assumption underlies the optimum flight speeds, optimum climb protocol, maximum endurance and maximum flight range predicted by these models. Here we consider alternatives to the assumption that power is independent of speed and show that if the aerodynamic power output from a constant rate of fuel consumption varies with flight speed, then the optima derived from current models of animal flight need to be revised. In some cases (e.g. optimum flight speeds) the current models give answers that are only quantitatively wrong, but in some cases (e.g. optimal cruising altitude) current models give answers that are qualitatively wrong.
View
The Development of Bird Migration Theory
Article
  • Dec 1998
The full magnitude and complexity of bird migration have not been possible to grasp until after the revolutionary discoveries made with the aid of, e.g., ringing, systematic field observations and radar. These have opened up opportunities for a more formalized theoretical construction work during the recent decades. The introduction of theoretical concepts and tools from flight mechanics into the field of bird flight and migration took place during the 1960s and 1970s and paved the way for the use of optimization analysis to evaluate adaptive aspects of flight behaviour, fuel deposition and responses to wind drift by migrating birds. The approach has been expanded, i.e. through the use of stochastic dynamic programming, to analyse the expected disposition of the migratory journey (with respect to Eight and stopover arrangement) under different ecological conditions and the adaptive temporal structure of the annual cycle of a migratory bird. Theoretical considerations also play an important role in analysing the orientation of migratory birds, their population ecology, and patterns of differential migration. Theoretical developments in these areas are reviewed. In the long run evolutionary and mechanistic theories must meet and join to provide a full understanding. While optimization models standing up to critical tests may help to identify primary forces of balancing selection and constraints, mechanistic theories are needed to tell us how the inherent biological algorithms operate within the sensory, neural and physiological systems to control behaviour and design in an adaptive way.
View
View
Effects of Sidewinds on Optimal Flight Speed of Birds
Article
  • Sep 1994
  • J THEOR BIOL
The relationship between flight power and airspeed of birds, as derived from flight mechanical theory, can be used to predict the maximum range speed Vmr which is the optimal speed for minimizing the cost of transport. This speed is not constant but depends on the winds. Earlier predictions about the optimal adjustment of flight speed by birds in relation to winds are only valid in due head- and tailwind situations. We derive Vmr for birds flying along a constant track direction with an arbitrary wind. It emerges that the effect of sidewinds on Vmr depends on the marginal rate of return in groundspeed with an increasing airspeed. This marginal rate increases with the angle between the bird's track and heading direction. Hence, Vmr not only depends on the effect of wind on the bird's actual groundspeed but also on the bird's angle of compensation for the sidewinds. As a result, optimal flight speeds are faster in sidewinds than in tail- or headwinds with a corresponding speed increment caused by wind. The adjustments of optimal airspeed in relation to sidewinds will be analogous also for other characteristic flight speeds besides Vmr, such as the optimal flight speed associated with time-saving rather than energy-saving migration, and with food searching, flight between foraging patches and food delivery flights. We also discuss requirements for empirical tests of the predictions proposed in this paper.
View