Q: Why do birds fly south for the winter?
A: Because it is too far to walk!

Explanation: Where I grew up, in Minnesota, it was very common to see birds fly south for the winter. When all of the lakes freeze, the trees lose their leaves, and ground is covered with snow, most birds cannot find food.  It’s also just too cold for them to survive.  So, they migrate to an area where they will be able to survive.

While birds can walk a bit, their main way of traveling is by flying. 

This joke is funny because it sets up an expectation for an explanation about why birds have to migrate to a different area.  What you get, though, is a silly answer about birds not being able to walk so far.

Check out The Great Migration from KQED-