The reproduction system for birds is sensitive for light, as it simulates spring time. If the intensity of light and/or the light duration increases, and at the same time birds are old and developed enough to respond to it, the hormones that activate and regulate their reproductive system will be stimulated, and birds will get in reproduction.

Birds see light more or less like humans do... through the eye. Like humans, their eyes are more sensitive for short wavelengths, which means that light with a high color temperature (day light, high levels of green, blue and violet wavelengths) are seen as more intense than light with a low color temperature (red and yellow wavelengths).

However, the reproductive system of birds is not stimulated by the light that is observed by the eye, but by light that stimulates the so-called pituitary glands. These glands are located in the brain and receive light that passes through the skull. This means that the reproductive system is stimulated by the light that is able to pass through the skull.

Light with long wavelengths can pass through the skull more easily than light of short wavelengths. This is comparable for instance with sound waves, as bass sounds (long wavelengths) will be heard outside a building or a car more easily than the high sounds (short wavelengths).

This means that the reproductive system of birds is stimulated by the red fraction of the light (long wavelengths) much more than the green or blue fraction (short wavelengths).

A lux meter measures the total amount of light radiation, regardless of the wavelength. In practice this can mean that a relative high intensity of very bright light (short wavelength) can have less stimulating effect on reproduction than a lower intensity of red/yellow light (long wavelengths).

To avoid problems, light of breeders must always be seen in relation to its colour (wavelength), realising that only the red fraction of the light stimulates reproduction.