Fig. 5. Four possible cone opponency mechanisms plotted against total quantum catch for the receptors involved. (A) LW–MW, (B) MW–SW, (C) LW–SW, (D) LW–(MW+SW). The broken lines show the irradiance spectra during sunset. The black circles denote the conditions when the sun is at the horizon; open circles, solar elevations that are not sunset. The other points are for conditions separated by approximately 1° of solar elevation. Filled symbols, European robin; open symbols, other birds (see Table 1).