Birds have 4 cones in their eyes and see the 3 primary sight colors and a 4th element-UV.
Birds see 4 primary colors. All color visible to birds are blends and combinations of these 4 colors. Humans cannot see this 4th color (UV) as we do not have the 4th cone in our eyes. Birds see the same 3 colors as humans see, PLUS an additional color in the UV spectrum.