Table 1.
Comparisons of nocturnal migratory flight characteristics of silver Y moths and passerines, based on nightly means. Altitude is measured above the radar. Means and concentrations of directional data were calculated as mean vector direction and length, respectively. Mean vector length r (in parentheses) is a measure of the concentration of circular data (inversely related to angular scatter) ranging from 0 (uniform distribution in all directions) to 1 (all observations in the same direction). Wind direction is the direction from which the wind is blowing. Differences between characteristics of moth and bird migration were analysed on the basis of the nightly mean values by t-tests (equal variances not assumed) for ground speed, wind speed and altitude and by Watson's U2-tests for the remaining circular variables [25]. Two-tailed significance levels from these tests are given. The two right-hand columns show overlap values of the frequency distributions of nightly mean values for moths and birds, respectively (in intervals of 2 m s−1 for speeds, 200 m for altitude and 10° for directions).
| moths |
birds |
difference moths versus birds |
overlap moths versus birds |
|||||
|---|---|---|---|---|---|---|---|---|
| spring | autumn | spring | autumn | spring | autumn | spring | autumn | |
| ground speed (m s−1), mean (s.d.) | 15.7 (4.6) | 13.0 (3.5) | 13.1 (3.2) | 12.1 (3.7) | * | n.s. | 0.83 | 0.92 |
| wind speed (m s−1), mean (s.d.) | 8.7 (4.4) | 8.2 (3.3) | 7.8 (4.0) | 8.3 (4.1) | n.s. | n.s. | 0.81 | 0.88 |
| altitude (m), mean (s.d.) | 550 (107) | 486 (95) | 1094 (319) | 1050 (280) | *** | *** | 0.12 | 0.02 |
| track direction, mean vector (r) | 344° (0.70) | 179° (0.48) | 15° (0.92) | 172° (0.84) | ** | * | 0.65 | 0.71 |
| wind direction, mean vector (r) | 171° (0.56) | 351° (0.42) | 256° (0.09) | 264° (0.59) | *** | *** | 0.44 | 0.46 |
| heading minus wind, mean vector (r) | 178° (0.77) | −164° (0.83) | 26° (0.24) | −37° (0.85) | *** | *** | 0.16 | 0.17 |
| heading direction, mean vector (r) | 347° (0.79) | 200° (0.62) | 22° (0.91) | 209° (0.95) | n.s. | *** | 0.68 | 0.69 |
| track minus heading, mean vector (r) | −6° (0.93) | −15° (0.93) | 6° (0.79) | −55° (0.68) | n.s. | ** | 0.78 | 0.48 |
| number of nights | 31 | 59 | 41 | 25 | ||||
***p < 0.001.
**p < 0.01.
*p < 0.05.
n.s., p > 0.05.