Fig. 1. Orientation behaviour of European robins under red light in spring 1999. The test conditions are indicated within the circular diagrams: C, control tests under `white' light; R, red light with a low intensity of 7x10¹⁵ quanta s^–1 m^–2; RX, red light with a higher intensity of 43x10¹⁵ quanta s^–1 m^–2; RpeR, tests under low-intensity red light after the test birds had been pre-exposed to red light; RpeRX, corresponding tests under higher intensity red light after pre-exposure to red light. The triangles at the periphery of the circles mark the mean headings of individual birds; the arrow represents the grand mean vector, with its length proportional to the radius of the circle=1. The two inner circles are the 5% (broken) and 1% significance borders of the Rayleigh test.