Fig. 3. Orientation behaviour of European robins tested under monochromatic light of different wavelengths and intensities (see Table 1 for details of wavelengths and intensities). Each triangle represents the heading of an individual bird. Filled triangles refer to unimodally directed individuals, and two open triangles denote axially distributed individuals (bisected triangles indicate the direction taken into account in the statistical analyses; unmarked, open triangles represent the other end of the axis). The arrows represent the mean orientation of the group and are drawn relative to the mean vector length (r). The inner broken circle gives the 5% significance level, and the outer broken circle gives the 1% significance level according to the Rayleigh test (Batschelet, 1981). See Table 2 for detailed statistical information. G, green light; GY, green-yellow light; R, red light; mN, magnetic north.