Poultry have four types of cone in the retina of the eye, and this means th
at they probably see colour differently from trichromatic humans. Notwithst
anding the fact that humans and birds have maximum sensitivity in a similar
part of the spectrum (545-575 nm), poultry are likely to perceive light fr
om various types of lamp at a different intensity from humans because they
are more sensitive to the blue and red parts of the spectrum. Although colo
ur has been confounded with illuminance in many trials, wavelength has an u
nquestionable effect on poultry production and behaviour. Growth and behavi
our responses depend principally on retinal photoreception, whereas photose
xual responses are mainly influenced by hypothalamic light reception. In tu
rkeys and chickens growth under red illumination is inferior to that under
blue or green light, and this may be a result of birds exposed to red light
being more active and showing more aggression than birds exposed to shorte
r wavelength radiation. In contrast, the easier penetration of longer wavel
ength radiation to the hypothalamus makes red light more sexually stimulato
ry than blue or green, although the hypothalamic photoreceptors are more se
nsitive to blue/green light when illuminated directly. Egg production trait
s, however, appear to be minimally affected by wavelength.