Pennycuick (Pennycuick, 1990; Pennycuick, 1996) proposed that, like a pendulum, a bird in level flapping flight has a natural wingbeat frequency that is to a large extent determined by its body mass, wing morphology and the ultrastructure of its muscles, but can vary slightly depending on air density and gravity. On the basis of this ‘concept of a natural wingbeat frequency’, models were developed to predict wingbeat frequencies for a species whose mass, wingspan and wing area were known. If such a natural frequency exists, the general expectation would be for a bird to select a wingbeat frequency close to this value as often as possible (Pennycuick, 1996). In what has become known as the ‘muscle efficiency hypothesis’, Rayner (Rayner, 1977; Rayner, 1985) and Goldspink (Goldspink, 1977) pointed out that vertebrate muscle fibres contract most efficiently only over a narrow range of contraction speeds. Rayner argued that birds are subject to strong constraints to minimise body mass, and that they may therefore be unable to transport muscles comprising a sufficient variety of fibres for various flapping speeds. This homogeneity in the ultrastructure of flight muscles and the need to maintain efficiency were suggested to constrain small birds to flap their wings at fixed speeds (Rayner, 1985). To vary mean power output, they should use the optimal wingbeat frequency intermittently and control the relative duration of flapping phases. The different power requirements of climbing, horizontal flight and descent, etc., could then be met without major changes in wingbeat frequency (Rayner, 1985). On long-distance flights, e.g. migration, the majority of small birds use intermittent flight styles, characterised by regularly alternating phases of flapping and resting (Bruderer and Steidinger, 1972; Emlen, 1974; Bloch et al., 1981). In contrast, flapping flight in hirundines is characterised by a high degree of flexibility, with no obvious regularity in the timing of wingbeats. These aerial insectivores spend a large part of their lives in flight, hunting or on migration. Barn swallows and house martins thus share a common need for economic flight performance and maintenance of sufficient manoeuvrability for hunting. Indirect measurements of flight costs in free-living birds using the doubly labelled water technique (Bryant and Westerterp, 1980; Bryant and Westerterp, 1983; Hails, 1979) showed that the flight costs of hirundines are 50–70 % lower than those of other birds of similar size (Hails, 1979). Thus, this wind tunnel study was designed to investigate how hirundines apply intermittent flight as a flexible means to adjust mechanical power output to different flight situations. The consequences of this flexibility 1473 The Journal of Experimental Biology 204, 1473–1484 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 JEB2995 The flight behaviour of barn swallows (Hirundo rustica) and house martins (Delichon urbica) was tested in a wind tunnel at 15 combinations of flight angles and speeds. In contrast to that of most other small passerines, the intermittent flight of hirundines rarely consists of regular patterns of flapping and rest phases. To vary mechanical power output, both species used intermittent flight, controlling the number of single, pulse-like wingbeats per unit time. House martins in descent tended to concentrate their wingbeats into bursts and performed true gliding flight during rest phases. Barn swallows mainly performed partial bounds during brief interruptions of upstrokes, which they progressively prolonged with decreasing flight angle. Thus, identification of distinct flapping phases to calculate wingbeat frequencies was not feasible. Instead, an effective wingbeat frequency for flight intervals of 20 s, including partial bounds, was introduced. The effective wingbeat frequencies of house martins (N=3) ranged from 2 to 10.5 s -1 , those of barn swallows (N=4) from 2.5 to 8.5 s -1 . In both hirundine species, effective wingbeat frequency was found to decrease almost linearly with decreasing flight angle. With changes in air speed, wingbeat frequency varied according to a U-shaped curve, suggesting a minimum power speed of roughly 9 m s -1 . The duration of the down- and upstrokes varied systematically depending on flight angle and air speed. Key words: barn swallow, Hirundo rustica, house martin, Delichon urbica, intermittent flight, partial bounding, wind tunnel, flight energetics, minimum power speed. Summary Introduction FLEXIBILITY IN FLIGHT BEHAVIOUR OF BARN SWALLOWS (HIRUNDO RUSTICA) AND HOUSE MARTINS (DELICHON URBICA) TESTED IN A WIND TUNNEL LUKAS BRUDERER, FELIX LIECHTI* AND DIETRICH BILO Swiss Ornithological Institute, CH-6204 Sempach, Switzerland and University of the Saarland, D-66123 Saarbrücken, Germany *Author for correspondence (e-mail: felix.liechti@vogelwarte.ch) Accepted 30 January; published on WWW 28 March 2001