Mine start heading into the coop about 15 minutes before legal sunset regardless of the weather. By legal sunset, they're all inside and the last few stragglers need help getting on the roosts. (Maybe they don't need help, but it is a routine with us now. I feel like the usher at a movie theater using a flashlight to illuminate each roost so the last few stragglers can jump up.)
Why?
Chickens don't see like we see. Chickens have tetra-chromatic vision, while we have tri-chromatic. That means they see an additional sector of the light spectrum that human eyes can't see. (In English, chickens have four wavelengths they are sensitive to, while we have three. Humans and chickens see red, green and blue, but chickens are also sensitive to ultraviolet light.) Even if there is artificially low light, such as with heavy storm cloud cover, which to our eyes it seems dark out, chickens can see just fine.
They have a disadvantage when the sun goes down, however, and they lose the benefit of that extra light spectrum. The retina in humans and other mammals is made up of rods and cones -- rods to see at night and cones to see color. Chickens have very few cones, and their rods are not very sensitive. Once the sun drops below the horizon, our eyes still have enough light to see and even enough light to still see color for a while during twilight, but chickens are pretty much blind. Before they lose the light and the ability to see, they want to be in a safe place, which is why most times chickens head in around legal sunset, which is a minute or two different each day.
Why?
Chickens don't see like we see. Chickens have tetra-chromatic vision, while we have tri-chromatic. That means they see an additional sector of the light spectrum that human eyes can't see. (In English, chickens have four wavelengths they are sensitive to, while we have three. Humans and chickens see red, green and blue, but chickens are also sensitive to ultraviolet light.) Even if there is artificially low light, such as with heavy storm cloud cover, which to our eyes it seems dark out, chickens can see just fine.
They have a disadvantage when the sun goes down, however, and they lose the benefit of that extra light spectrum. The retina in humans and other mammals is made up of rods and cones -- rods to see at night and cones to see color. Chickens have very few cones, and their rods are not very sensitive. Once the sun drops below the horizon, our eyes still have enough light to see and even enough light to still see color for a while during twilight, but chickens are pretty much blind. Before they lose the light and the ability to see, they want to be in a safe place, which is why most times chickens head in around legal sunset, which is a minute or two different each day.