Color: From Hexcodes to Eyeballs
Dan Burzo

Other animals have eyes that are sensitive to a different range of wavelengths, and therefore different luminosity functions. Birds can see ultraviolet radiation in the range between $$\lambda=300\text{nm}$$ to $$\lambda=400\text{nm}$$, so if scholarly birds had defined the electromagnetic spectrum, that would've been part of the "visible light" range for them!