Circular diagrams of the headings of landing mallards (n = 554 for each group) under different weather conditions: (a) clear sky (b) overcast. The mean vector in each condition is indicated by the double-headed arrow. Each triangle in (a) and (b) represents 7 and 9 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; the balanced subsamples with a sample size of n = 554 were chosen for better illustration.