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Harmonic oscillatory orientation relative to the wind in nocturnal roosting flights of the swift Apus apus

Johan Bäckman^* and Thomas Alerstam

Department of Animal Ecology, Ecology Building, Lund University, S-22362 Lund, Sweden

View larger version (29K): [in a new window]   Fig. 1. Three selected tracks of swifts in nocturnal roosting flight. The blue line shows the ground track, and the red lines represent the heading direction every 10s. Numbers denote the time (in s) after the start of tracking. The arrows beside the letters indicate the wind direction. Geographic north is upwards. Track A is a swift flying into a wind with a mean direction of 161° and a speed of 6.9 m s-1. The index of straightness relative to the ground, ISg, is 0.67. In track B, the swift is facing a wind of 8.1 m s-1 at 314°, and ISg is 0.59. The corresponding values for track C are 2.9 m s-1, 190° and 0.22.

View larger version (11K): [in a new window]   Fig. 2. Straightness indices in different wind speeds. (A) The straightness index relative to the ground, ISg. A low value means that the resulting flight path is greatly convoluted (see Fig. 1 track C), while a value close to 1 means that the swift has been flying on a straight course. (B) The straightness index relative to the surrounding air, ISa. Circles indicate that the swift has been circling, i.e. that the bird has crossed its own flight path relative to the ground (ISg) and the air (ISa).

View larger version (19K): [in a new window]   Fig. 3. Analysis of variations in flight direction relative to the wind direction (H-W) for three selected trackings (same tracks as in Fig. 1). The first column presents the deviations from a perfect headwind orientation over the entire tracking. In the second column, there are autocorrelation diagrams for tracks A and B. The last column is an alternative analysis of periodicity, in the form of a DFT power spectrum, showing the relative frequency content of the H-W data series. Track A has an autocorrelation period of approximately 160s and a significant frequency component of 6.74 mHz. Track B shows no distinct period, and the frequency peaks fail to pass the significance level.

View larger version (18K): [in a new window]   Fig. 4. Frequency histogram of the cyclic components of the heading direction relative to the wind direction (H-W) in our 49 trackings of swifts. We found a statistically significant frequency component in 36 out of 49 trackings and in 31 trackings with distinct autocorrelation periods.

View larger version (14K): [in a new window]   Fig. 5. The periodicity of deviations in the heading direction relative to the wind direction in relation to wind speed. (A) Significant frequency components versus wind speed; (B) distinct autocorrelation period versus wind speed. There was no significant correlation in either case.