Crazy Bird Videos

I am asking this question for my husband. He wants to know how a bird that flys south for the winter knows when to fly north for mating season.

Example: the Red Breasted Robin flys south for the winter months and then returns north in the early springtime to nest.

What instincts, or what in nature tells the birds to return to the north?

We both thank you for any and all answers in advance.

There are both "proximate" and "ultimate" causes to why a bird migrates. It can deal with shortening or lengthing of daylight, food supply, celestial cues (stars), hormones, etc. I added some links that will hopefully help you two understand migration a bit more.