Circular diagrams of the headings of landing mallards (n = 1,040 for each group) during different seasons: (a) spring (b) autumn. The mean vector of each season is indicated by the double-headed arrow. Each triangle in (a) and (b) represents 12 and 16 measurements, respectively. The inner circle indicates the 5% significance level of the Rayleigh test. The same results were obtained when we used the whole sample; balanced subsamples with a sample size of n = 1,040 were chosen for better illustration.