Can anyone explain circannual rhythms in relation to migration?

Question

I understand the circadian rhythm quite well, but I'm confused about circannual rhythms. I know the circadian rhythm involves animals being light sensitive at certain times of day.
Animals who live in areas with little seasonal variation in day length (eg. the equator) use the circannual rhythm to tell when to migrate. But what exactly is the circannual rhythm? How does it work?

Answer 1

Its where birds get triggered by the changing seasons to fly great distances for food, bredding etc. Don'y know how it works though. Probably a hormone.