Table 1.
Slopes and intercepts for the regression of forward rates of movement (km d−1) in relation to forward wind components (m s−1) and of perpendicular rates of movement (km d−1) in relation to perpendicular wind components (m s−1), for ospreys and marsh harriers during different seasons and in different regions. 95 per cent confidence limits are provided for regression slopes. Numbers are printed in bold if there was a significant difference between Europe and Africa in the slope of the perpendicular regression. We also tested whether the slopes of the forward and perpendicular regressions differ from each other (test between slopes; *, p < 0.05, n.s., not significant). The last column provides the estimated drift effect, which is the slope of the perpendicular regression divided by the slope of the forward regression. A ratio equal to 1 means that the perpendicular drift is equal to the effect of wind on forward speed (full drift), a ratio equal to 0 means that there is no perpendicular drift (full compensation), and a ratio in between 0 and 1 implies partial drift.
| n | regression forward movement versus wind |
regression perpendicular movement versus wind |
test between slopes | ratio between perpendicular and forward slope | |||||
|---|---|---|---|---|---|---|---|---|---|
| slope (95% conf. lim.) | intercept (km d−1) | slope (95% conf. lim.) | intercept (km d−1) | ||||||
| osprey | autumn | Europe | 135 | 10.9 (5.7–16.1) | 195.4 | 6.1 (3.7–8.4) | −8.5 | n.s. | 0.56 |
| Africa | 157 | 21.7 (16.6–26.7) | 208.8 | 3.7 (0.27–7.0) | −8.4 | * | 0.17 | ||
| total | 292 | 16.0 (12.6–19.4) | 208.7 | 5.2 (3.2–7.1) | −7.6 | * | 0.33 | ||
| spring | Europe | 58 | 13.4 (4.4–22.5) | 203.5 | 6.3 (1.6–11.0) | 1.7 | n.s. | 0.47 | |
| Africa | 81 | 20.6 (15.6–25.6) | 298.1 | 15.8 (9.1–22.6) | 4.1 | n.s. | 0.77 | ||
| total | 139 | 16.2 (11.4–20.9) | 253.9 | 10.5 (6.6–14.5) | 8.3 | n.s. | 0.65 | ||
| seasons combined | 431 | 15.3 (12.5–18.1) | 222.9 | 6.7 (4.9–8.6) | −3.6 | * | 0.44 | ||
| marsh | autumn | Europe | 136 | 16.4 (12.7–20.2) | 182.7 | 5.9 (4.1–7.6) | −9.5 | * | 0.36 |
| harrier | Africa | 130 | 14.6 (9.4–19.7) | 180.8 | 12.1 (8.0–16.2) | 12.9 | n.s. | 0.83 | |
| total | 266 | 15.7 (12.8–18.6) | 180.9 | 7.2 (5.3–9.0) | −1.7 | * | 0.46 | ||
| spring | Europe | 70 | 8.9 (2.3–15.4) | 192.9 | 1.6 (−0.94–4.0) | 0.1 | * | 0.18 | |
| Africa | 90 | 16.9 (13.1–20.6) | 223.5 | 11.4 (8.4–14.3) | −23.6 | * | 0.67 | ||
| total | 160 | 12.1 (8.7–15.5) | 203.4 | 6.0 (4.1–8.0) | −1.4 | * | 0.50 | ||
| seasons combined | 426 | 13.8 (11.6–16.0) | 190.4 | 6.5 (5.2–7.9) | −1.6 | * | 0.47 | ||
| overall (species and seasons combined) | 857 | 14.2 (12.4–16.0) | 206.5 | 6.6 (5.5–7.2) | −2.6 | * | 0.47 | ||