Heliotropic plants like sunflowers can turn toward the sun, but they respond to the sun as a stimulus, not to the sun as a sign. Responding to the sun as a sign requires at least some intelligence. Certain migratory fish, birds, and animals respond to the sun as a sign. They alternate between their breeding and feeding areas according to the seasons. The proof that northern-hemisphere birds, for example, do not respond to the sun as a mere stimulus is that in autumn they migrate toward the noontime sun (when it is lower and lower in the sky) and in spring away from the noontime sun (when it is higher and higher in the sky). In the northern hemisphere the sun is always in the south, but in autumn the birds fly toward it, and in spring away from it. There is nothing about the position of the sun to tell them where to seek comfortable temperatures. They combine the position of the sun in the sky with instinct to determine when to migrate.