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

Felix Liechti^* and Lukas Bruderer

Swiss Ornithological Institute, CH-6204 Sempach, Switzerland

View larger version (59K): [in a new window]   Fig. 1. Characteristic unfiltered echo signatures of barn swallows (BS) and house martins (HM) during climbing (BS1, HM1), horizontal (BS2, HM2) and descending (BS3, HM3) flight recorded by tracking radar. The examples show wingbeat patterns for 5 s intervals. Flight angle and mean air speed are given in the upper right corner. Large ticks above each signature indicate time in seconds. For each example, five consecutive flapping cycles are marked (three shaded + two in between).

View larger version (24K): [in a new window]   Fig. 2. Relative frequency distribution of the duration of flapping cycles for barn swallows (A) (BS) and house martins (B) (HM) in free flight. Only one 20 s interval per bird was included per flight situation. Climbing (flight angles 3.5±1.5°), NBS individuals=4, NBS flapping cycles=372, NHM individuals=6, NHM flapping cycles=779; horizontal flight (flight angles 0±1.5°), NBS individuals=13, NBS flapping cycles=1387, NHM individuals=8, NHM flapping cycles=890; descent (flight angles -3.5±1.5°), NBS individuals=18, NBS flapping cycles=1691, NHM individuals=9, NHM flapping cycles=952. True air speed was restricted to 10-16 m s-1.

View larger version (16K): [in a new window]   Fig. 3. Effective wingbeat frequency averaged over 20 s with respect to true air speed (A) and flight angle (B). Barn swallows, open circles, N=65; house martins, filled circles, N=51.

View larger version (18K): [in a new window]   Fig. 4. Flight angle with respect to true air speed for barn swallows (open symbols, N=65) and house martins (filled symbols, N=51). The geometric mean regressions for barn swallows (thin line) and house martins (bold line) are shown. The correlation between air speed and flight angle is much higher for barn swallows (r=-0.71) than for house martins (r=-0.44) even when the two highest values of flight angle for barn swallows are excluded.

View larger version (16K): [in a new window]   Fig. 5. Effective wingbeat frequency with respect to flight angle for two barn swallows (open symbols), two house martins (filled symbols) and one unidentified hirundine (black crosses) for continuous flight intervals of 20 s. Lines connect consecutive 20 s intervals of an individual flight path.

View larger version (23K): [in a new window]   Fig. 6. Comparison of the effective wingbeat frequency for radar-tracked hirundines with wind tunnel observations. (A) Barn swallows; (B) house martins. Medians (horizontal bars), 25-75% ranges (boxes) and range (whiskers) are given. Shaded boxes represent wind tunnel data, open boxes represent field data. Flight angles of radar-tracked hirundines were restricted to climbing (5±3°), horizontal flight (0±1.5°), descent at -5±1.5° and descent at -10±3° (means ± S.D.). Only one measurement per bird was included per flight angle. Numbers of individuals are given on the graph.