Ever wondered why air tickets for short flights seem disproportionately expensive, compared with long-haul journeys? The reason is that take-off and landing require the most energy, irrespective of the distance between. This principle applies just as much to birds, as Robert L. Nudds of the University of Oxford, and David M. Bryant of the University of Stirling, UK, now demonstrate in the Journal of Experimental Biology1.

Flight is an expensive activity, in terms of the energy it requires. And certain kinds of flight are particularly so. Work on the energetic expenditure of flight has tended to concentrate on birds trained to fly in wind tunnels, simulating horizontal, steady-state, flapping flight, in which the effects of take-off and landing are kept to a minimum.

But casual observation of your back garden tells a different story. Many birds routinely perform short flights, from tree to bird-feeder, for example. These flight plans comprise only take-off, ascent and descent -- the birds never achieve the horizontal, steady-state flight found in textbooks.

With a series of carefully designed experiments and a clutch of willing zebra finches (Taeniopygia guttata), Nudds and Bryant show that, during short hops, the birds expend energy equivalent to almost 30 times their basal, 'resting' metabolic rate (BMR). This is more than three times the energy use predicted by theoretical models (which are derived from the principles of aerodynamics and based on horizontal, flapping flight). The new work should help ecologists and zoologists get a picture of the role of short flight in the lives of birds.

Why should short flights cost so much? As with aircraft, much of the cost is incurred by take-off. Getting airborne from a standing start requires work against the force of gravity. (The researchers trained their finches to take off upwards, rather than cheat by starting with an energy-saving plummet that uses gravity to gain thrust.)

And to make matters more difficult, take-off requires acceleration: in other words, the bird must work to increase airspeed, as well as fighting gravity. Thirdly, and perhaps most significantly, take-off is a low-speed activity by definition, as it starts from a speed of zero.

In general, birds fly most economically at speeds that are neither too high nor too low. At these intermediate speeds, forward thrust generates some of the lift required to keep the bird airborne. Flying faster is difficult as air resistance increases, but birds must work even harder at lower speeds, compensating for the lack of forward thrust and resulting lift.