Spectral sensitivity functions of a passeriform bird, the Red-billed Leiothrix Leiothrix lutea (Timalidae) were determined in a behavioural test under different background illuminations.
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With photopic illumination the spectral sensitivity of Leiothrix lutea covered the measured range from 320 nm to 680 nm. Four peaks of spectral sensitivity were found: a UV (370 nm), a blue (460 nm), a green (530 nm) and a red (620 nm) sensitivity peak. The spectral sensitivity was highest in the UV and decreased (over the blue and the green peak) towards the red sensitivity peak. The 4 peaks of spectral sensitivity point to 4 underlying cone mechanisms under photopic illumination and thus to a probably tetrachromatic colour vision of Leiothrix lutea.
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With mesopic illumination the bird's spectral sensitivity covered the measured range from 320 nm to 680 nm. Neural interactions between cone and rod sensitivities are likely to determine this function. The increased overall sensitivity and a dominant sensitivity peak at 500 nm point to a typical rhodopsin as the likely rod photopigment.
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Different aspects of the biological significance of the high UV sensitivity are discussed.