It remains unclear whether coordination of wing and respiratory movements is universal among birds during flight (Tomlinson, 1963). However, it is certain that many species demonstrate some form of coordination between wing and respiratory movements (Marey, 1890; Groebbels, 1932; Zimmer, 1935; all cited in Hart and Roy, 1966; Tomlinson, 1957, 1963; Lord et al. 1962; Hart and Roy, 1966; Berger et al. 1970b; Butler et al. 1977; Butler and Woakes, 1980). Ratios of wing beat frequency to respiratory frequency (fw/fv) ranging from 1:1 to 5:1 have been described, some species showing variation in fw/fv (Lord et al. 1962; Berger et al. 1970b; Butler and Woakes, 1980).

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