S31.Summary: Ecology of bird flight: Behavioural and morphological adaptations
Anders Hedenström1 & Adrian L. R. Thomas2
1Department of Animal Ecology, Ecology Building, S-223 62 Lund, Sweden, e-mail
Anders.Hedenstrom@zooekol.lu.se ; 2E. G. I. Oxford University, Department of Zoology, South Parks Road, Oxford OX1 3PS, UK Hedenström, A. & Thomas, A.L.R. 1999. Ecology of bird flight: Behavioural and morphological adaptations. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 1784-1785. Johannesburg: BirdLife South Africa.Bird flight research has entered a very dynamic era where theoretical developments go hand in hand with novel observational and experimental approaches as succinctly demonstrated by the papers of this symposium. During the last decade or so flight mechanics have become an important ingredient in theories on how migratory birds best should manage their journeys and how much fuel to take on board before departures, etc. Such theories rely heavily on aerodynamic theory, but we still have some work to do before we have such a theory that is generally valid. Rayner & Ward (1999. S31.1) review the fundamental relationship between power and speed (usually called the ‘power curve’). Theoretically, this relationship should follow a U-shaped function, at least when considering the mechanical power. But many studies where the flight metabolism has been measured (usually by means of respirometry) show rather flat power curves. The reason for this could be manifold, but some researchers hold the view that the flat curves represent the truth. However, Thomas & Hedenström (1999. S31.5) argue that if anything measured power curves should show more steeply U-shapes than mechanical power curves. This is also the case in some new experiments where rate of mass loss has been measured in relation to speed. A very important factor with respect to flight energetics is the conversion efficiency of metabolic energy rate into useful aerodynamic rate of work. In the classic Pennycuick (1975. Mechanics of flight. In: Farner, D.S. & King, J.R. (eds), Avian Biology Vol. 5; New York: Academic Press: 1-75) model the efficiency is treated as a constant (due to the lack of data in support of otherwise). We may expect that flight muscles should have an optimum (maximum) efficiency at some level of effort to which the bird is adapted, and then the efficiency should decline away from this optimum (Thomas & Hedenström 1998. J. Avian Biol. 29: 00-00).
To estimate the conversion efficiency is notoriously difficult since it requires the simultaneous measurement of power input (metabolism) and power output. Power output has so far only been measured once by Dial et al. (1997. Nature 390: 67-70), and these experiments measure only the mechanical power delivered to the muscle insertions on the wings. However, there is an ingenious approach developed by Tucker (1972. Amer. J. Physiol. 222: 237-245) and Bernstein et al. (1973. J. Exp. Biol. 58: 401-410), where the oxygen consumption of a bird flying horizontally at speed V is first determined. Then, the tunnel is tilted through a small angle to yield a small rate of climb and an associated increase in mechanical power required to fly. The idea is that this small tilt will not affect the kinematics to any significant degree, and so the conversion efficiency will be given by the quotient between climb power and the increase in measured oxygen consumption. This method gave conversion efficiencies around 0.23 which is the value used in the Pennycuick model, but new studies are certainly needed to further investigate this issue. Rayner & Ward (1999. S31.1) also reported on new data from a novel method of digital video thermography to quantify the heat released by a bird flying in a wind-tunnel. Wind-tunnels have been a very important research tool for controlled experiments on bird flight. Their importance to bird flight research is likely to increase, since a new generation of bird wind-tunnels has recently been designed and used (Pennycuick et al. 1997. J. Exp. Biol. 200: 1441-1449).
Chai (1999. S31.2) reports on new findings on limits to flight performance in hummingbirds, where an ingenious method of using a gas mixture of heliox has been developed. By this approach, flight costs can be manipulated experimentally by changing the density of the air and simultaneously the oxygen consumption of a hovering hummingbird is recorded. This approach gave a conversion efficiency of about 10%, i.e. only half the value found for birds in forward flight. The conversion efficiency remained rather constant over a range of work load. Chai (1999. S31.2) also show how this method could be used to investigate the effect of moult gaps on flight performance in hummingbirds.
Aerodynamic theory developed for delta-winged aircraft has proven useful to analyse the effects of tail shape on flight performance (Thomas 1993. Phil. Trans. Roy. Soc. Lond. B 340: 361-380). This approach was applied to the case of the barn swallow Hirundo rustica by Evans (1999. S31.3), a species which has become the Drosophila of sexual selection studies on birds (Møller 1994. Sexual Selection and the Barn Swallow; Oxford University Press, Oxford: 365pp). Evans (1999. S31.3) argued that the barn swallow tail is mainly the outcome of natural selection processes (i.e. selection on flight performance), while sexual selection is only partly responsible for the sexual dimorphism in tail streamer length in this species.
Spaar (1999. S31.4) gives an overview of raptor flight performance during migration measured by means of tracking radar in Israel. This is a very nice demonstration of how accurate field data on flight behaviour can be used to test predictions from flight theory. For instance, large raptors seem to be able to estimate their rate of climb in thermals and adjust their inter-thermal gliding speed (and associated rate of sink) accordingly. Smaller species also migrate by flapping flight outside available thermal time, an observation that indicates that these birds are minimising the overall time of migration (cf. Hedenström 1993. Phil. Trans. Roy. Soc. Lond. B 342: 353-361).
Finally, Thomas & Hedenström (1999. S31.5) review classical flight theory applied to birds. The application of this theory to optimal flight speeds is reviewed in relation to measurements of flight speeds. To conclude, Thomas & Hedenström (1999. S31.5) briefly look beyond the U-shaped power curve and what new developments are likely in the filed of animal aerodynamics.
Bird flight research ranges from mechanics, physiology, morphology to behavioural adjustments. The papers presented at this symposium clearly demonstrate the multi-faceted research approaches currently adopted at different levels. We may see fascinating new developments in the near future and the papers presented here will certainly provide the basis for further research in this field.