Circular diagrams of the mean headings of landing water birds of all species investigated demonstrating the preference for landings along the north-south axis. (a) Raw data plot. Each pair of dots (located on opposite sites within the unit circle) represents the direction of the second order bimodal mean vector calculated from the animals’ directional headings of one species (see Methods). The double-headed arrow indicates the grand mean axial vector calculated over all species. The inner circle marks the 5% significance level of the Rayleigh test. (b) Scatter plot summarizing statistics weighted by the length of the mean vectors of different species. The position of each pair of dots within the circle represents both the direction and the length of the bimodal mean vector of one species. The double-headed arrow indicates the weighted grand mean axial vector calculated over all species means.