Figure 4.

Regression of R (a measure of angular dispersion around the mean flight heading) against insect flight altitude for the 48 migration events. Higher values of R denote occasions with lower angular dispersion (i.e. ‘tighter’ orientation around the mean). There is a highly significant positive relationship between R and flight altitude (r² = 0.17; p = 0.002), indicating that nocturnal insects orient their flight headings with respect to the downwind direction better at higher altitudes. This provides strong evidence that visual assessment is not the mechanism used for alignment of headings with the downwind direction.