What's on Gerda's Mind?

Today Gerda wanted to know  How do migrating birds know when is time to take on the highways in the skies? Apparently or swallows left the southern hemisphere 3 weeks later than usual this year while the northern hemisphere hag one of its coldest springs. How did the birds know that?  We spoke to Les Underhill from the Animal Demography Unit at the University of Cape Town.

 For more info on migrating birds check out www.birdlife.org.za