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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).