Wingbeat frequency of barn swallows and house martins: a comparison between free flight and wind tunnel experiments

The flight paths and wingbeat patterns of 39 barn swallows (Hirundo rustica) and 26 house martins (Delichon urbica) were recorded by tracking radar during the spring migration. Depending mostly on flight angle, hirundines performed anything from continuous flapping flight during climbing to single pulse-like wing beats during descent. Unlike most other passerines, hirundines rarely showed regular flapping and rest phases, allowing them to be distinguished from other bird migrants by radar echo signatures. Effective wingbeat frequency (F(eff)) was calculated as the mean number of wing beats per second, including non-flapping phases. Under comparable flight conditions, F(eff) was higher in house martins than in barn swallows. Within species, F(eff) values were higher during climbing and slow flying than during descent. Of the variance in F(eff), 71% could be explained by climb rate, air speed and species; similar results were obtained in the wind tunnel. Under comparable flight conditions, barn swallows and house martins in free flight had significantly lower values of F(eff) than individuals in wind tunnel experiments (by 40% and 32%, respectively). This difference may at least partly be due to the shorter wings of the juveniles tested in the wind tunnel during autumn. However, it seems unlikely that this can account for all of the large difference. It is suggested that wind tunnel experiments might overestimate birds' flight costs compared with free flight.