The spectral reflectance of feathers was measured in the range between 310 and 730 nm by means of a diode array spectrometer. In many feathers an ultraviolet (UV) reflection adds to the reflection in the visible range which causes their coloration as seen by man. The UV reflectance is related to the presence of pigments in feathers and to the arrangement of structures which influence the light reflecting properties. Feathers giving strong UV reflection are called type A, without UV reflection type B, and giving weak to medium UV reflection type A/B. On the assumption that birds are tetrachromates, colour vision in birds and their possible chromaticity diagrams are discussed. If red, green, blue and UV are primary colours, three secondary colours are present in the daylight spectrum: yellow, blue-green, and violet-ultraviolet. Three more secondary hues may originate from mixing spectral lights: purple (red and violet), bird's purple (red and UV), and green purple (green and UV). Some feathers with double-banded reflectance curves will produce hues which are not present in the daylight spectrum.