Energetics of Flight



Our work on the energetics of flight has involved three main lines of investigation.

  1. Quantification of the efficiency of flight
  2. Development of novel techniques for the measurements of flight energy expenditure, in particular the labelled bicarbonate method
  3. Evaluation of the energetic significance of formation flight

The efficiency of vertebrate flight

Work on the energy demands of flight has concerned the issue of quantifying the efficiency of flight by comparing the aerodynamic mechanical power out put with the directly measured metabolic power input. This project has involved a collaboration between our own research group in Aberdeen, with the biomechanics research group of Professor Jeremy Rayner at the University of Leeds and the flight research group headed by Professor Werner Nachtigall at the University of Saarbrucken in Germany.
The work was performed by post doctoral research assistant (Dr Sally Ward - now SNH in Caithness) and two post graduate research assistants (Diane Jackson - now Dr Jackson at the Rowett Research Institute in Aberdeen) and a german student (Udo Moller).

Schematic diagram of the wind tunnel at the University of Saarbrucken in Germany. Werner Nachtigall

The work involved training starlings to fly in the wind tunnel at the University of Saarbrucken (above) while they were wearing respiratory masks. The birds were then flown and their energy expenditures quantified by mask respirometry and also by use of the doubly-labelled water method. Simultaneous to these measurements we also measured the heat output from the animals by measuring their surface temperatures using infra-red thermography and filmed the birds using high speed cine film to generate biomechanical calculations of their mechanical energy output.

Starling (Sturnus vulgaris) flying unconstrained in a wind tunnel at the University of Saarbrucken, Germany

Picture of starling flying in same wind tunnel taken by thermal imaging camera. Emitted radiation is translated into surface temperatures using its wavelength distribution to inform of hot and cool spots. Temperature scale is to right of picture. Flying starlings have hot spots in their bills but also under their armpits and their feet.


Pdfs for most of the following publications are available for free download here.

SPEAKMAN, J.R. and Ward, S. (1998)
Infrared thermography : principles and applications.
Zoology : Analysis of Complex Systems. 101: 224-232

Ward, S.M., Moller, U., Rayner, J.M.V., Jackson, D.M., Nachtigall, W. and SPEAKMAN, J.R. (1998).
Power requirement for Starling flight in a wind tunnel.
Biologia e Conservazione della Fauna 102: 335-339

Ward, S., Rayner, J.M.V., Moller, U., Jackson, D.M., Nachtigall, W. and SPEAKMAN, J.R. (1999)
Heat transfer from starlings Sturnus vulgaris during flight Journal of Experimental Biology 202: 1589-1602.

Rayner, J.M.V., Ward, S., Poaloa, V. and SPEAKMAN, J.R. (2000)
Bounding flight in the Starling American Zoologist 41: 188-204

Ward, S., Rayner, J.M.V., Moller, U., Jackson, D.M., Nachtigall, W. and SPEAKMAN, J.R. (2001)
Efficiency of flight in the starling (Sturnus vulgaris) Journal of Experimental Biology204: 3311-3322.

Ward, S., Moller, U., Rayner, J.M.V., Jackson,D.M., Nachtigall, W., and SPEAKMAN, J.R. (2004)
Metabolic power of European Starlings (Sturnus vulgaris) during flight in a wind tunnel measured by heat transfer modeling, doubly-labelled water, mask respirometry and aerodynamic modeling. Journal of Experimental Biology 207: 4291-4298

Measurement of flight energy demands using labelled bicarbonate

There have been two projects run under this program, both involved PhD studentships. In the first, Susan Thomson on a BBSRC studentship developed a method for measuring energy costs of short duration behaviours using turnover of an injected bolus of C-13 labelled bicarbonate, as part of a wider project on quantifying heat balance during flight in bats. The bicarbonate elimination method was validated against indirect calorimetry in mice and then later in small insectivorous bats (Pipistrellus pygmaeus). The validation equation in bats was then used to evaluate flight energy demands of bats when they were engaged in short duration flights. The estimated flight energy expenditures of this bat were not significantly different from estimates of flight cost made previously in the same species using the doubly-labelled water technique.


SPEAKMAN, J.R. and Thomson, S.C. (1997)
Validation of the labeled bicarbonate technique for measurement of short term energy expenditure in the mouse.
Zeitschrift Fur Ernahrungswissenschaft 36: 273-277

Developing on from the preliminary work in bats a second studentship addressed the energy demands of short duration flights in birds.This project was a CASE studentship funded by BBSRC and The Waltham Centre for Pet Nutrition. The PhD student working on the project was Catherine Hambly.

Fig 1

The 13C labelled bicarbonate technique was developed for use with small birds, namely zebra finches, orange-tufted sunbirds, cockatiels and starlings. The validation of the technique involved comparing the isotope elimination rate with known metabolism measured using indirect calorimetry. Isotope elimination rate is the gradient of the isotope enrichment with time.
A typical washout curve for a zebra finch is shown in Fig 1. This plot shows that the enrichment of 13C in the breath rises to a peak after only a few minutes and thereafter washes out of the system with a half life of around 7-9 minutes.

By fitting a regression to the log converted elimination curve it is possible to evaluate the isotope elimination rate (Kc). This can then be correlated to   oxygen consumption using simultaneous respirometry (Fig 2). The relationship is strong (r2 = 0.73) and shows that elimination of a bolus of b13C labelled bicarbonate can be used to trace animal metabolism over short durations.

Once this relationship had been established flight costs were measured by calculating the elimination rate over short flight periods. Metabolic rate was then predicted for the flight period using the validation. This led to a series of experiments to examine the affect that increased wing loading, wing asymmetry and short bursts of flight had on the energetic cost of flight.

Cockatiels used to evaluate the energy cost of flight when carrying artificial loads

This included a period of work in Israel in collaboration with Professor Berry Pinshow under the Lord Goodman fellowship scheme, applying the method to evaluate the energy demands of flight in Orange tufted sunbirds.

Orange tufted Sunbird female


Hambly, C.S., Harper, E.J. and SPEAKMAN, J.R. (2002)
The cost of flight in the zebra finch (Taenyopygia guttata) : a novel approach based on elimination of carbon-13 labelled bicarbonate.
Journal of  Comparative physiology. 172: 529-539

Hambly, C., Pinshow, B., Wiersma, P., Verhulst, S., Piertney, S.B., Harper,E.J. & SPEAKMAN, J.R. (2004)
Comparison of the cost of short flights in a nectarivorous and non-nectarivorous bird.
Journal of Experimental Biology 207: 3959-3968

Hambly, C., Harper, E.J., and SPEAKMAN, J.R. (2004)
The energy cost of loaded flight is substantially lower than expected
due to alterations in flight kinematics.
Journal of Experimental Biology 207: 3969-3976

Hambly, C., Harper, E.J., and SPEAKMAN, J.R.(2004)
The energetic cost of variations in wing span and wing asymmetry
in the zebra finch (Taeniopygia guttata).
Journal of Experimental Biology 207: 3977-3984

Formation Flight

Many birds fly in formations (Figs 1 and 2). These often involve V shaped formations, but also lines, and echelons. The phenomenon is most often noted in larger birds, and smaller birds tend to fly in much less structured and loose aggregations (Fig 3). The function of formation flying has been a matter of debate for over 100 years.

Two dominant hypotheses have been developed. The first is that birds fly in formations as a method of orienting themselves. By flying closely adjacent to another individual this allows the following individual to orient to a feeding or roosting site more easily. Alternatively it is suggested that because wings shed vorticies by strategically locating themselves behind and to the side of another bird, a trailing bird can gain lift and thus reduce its own energy demands for flying.

Fig 1

Fig 2

Fig 3

The energy saving hypothesis makes very specific predictions about the optimal location in which birds should position themselves to make maximum advantage of the trailing vortex. This involves a slight overlap of wing tips (Fig 4).

The orientation model does not make such specific predictions and is thus harder to refute. Indeed the function of the behaviour may in any case be dual - involving benefits in both energy saving and orientation. Demonstrating that the behaviour has energetic benefits does not therefore preclude the possibility that orientation benefits occur. Moreover demonstrating no energetic benefits for the behaviour does not by default provide support for the orientation hypothesis.

Figure 4 – shed vortex from the wing of a bird (blue) and the cross sectional pattern of variation in lift (red). By positioning its wing inside the area of uplift a following bird can gain lift from the trailing vortex and in theory reduce its energy costs for flight.

Our work has involved photographing skeins of geese from below and reconstructing the positions of the animals relative to each other and then asking how close the positioning of the birds equates to the position which maximises energy saving.

By using a model of the impact of position on the assisted lift that can be achieved we have also attempted to quantify the energetic benefit that accrues to a bird when it is flying in formation. We have studied two types of geese, the small pink footed goose (Anser brachyurus) and the much larger grey lag goose (Anser anser) (Fig 5).

Fig 5 – Pink footed geese (a) and greylag (b) geese flying in formation

We have found that the birds roughly locate themselves around the optimal position for energy saving (Fig 6), but their ability to sustain this position is limited.


Fig 6 –frequency of wing tip spacings (see fig 4 for explanation of this term) of birds relative to the birds in front for pink-footed geese (n = 393). The theoretical optimal position to maximise energy saving is at –6cm (indicated by the arrow) (from Cutts and Speakman 1994).

Consequently the energy savings from formation flying are relatively trivial in the small pink footed goose at around 2.5% of total flight costs. In contrast the savings for grey lag geese were substantially higher. This effect of size on the realised savings in formation flight may suggest that larger animals find it easier to fly in the most favourable position.

Several factors may influence this effect but an important factor is probably wing beat frequency. Smaller birds beat their wings faster. Accordingly the exact location of the vortex may be difficult to track accurately (The predicted locations assume no flapping and positional stability of the vortex). In addition larger birds may be less affected by small gusts of wind.

In combination then these effects mean the realised savings from formation flight get lower and lower as the birds get smaller, until at some point the benefits of formation flying become minute. At this point there would be no evolutionary pressure for the trait to evolve. This probably explains why small birds do not fly in formations unlike larger birds.


Cutts, C.J. and SPEAKMAN, J.R. (1994)
Energy savings in formation flight of pink-footed geese.
Journal of Experimental Biology 189: 251-261.

SPEAKMAN, J.R. and Banks, D. (1998)
The function of flight formations in Greylag Geese Anser anser; energy saving or orientation?
Ibis 140 Iss 2: 280-287.

See also independent reviews of this paper in
BBC Wildlife magazine March 1999
Nature Australia November 1999