Why do geese fly south every year?

The Earth is NOT a solid-core structure. As like ALL comets, nebula, planets, and stars, it is a shell spinning about a central hot core of a comparably small diameter. This inner surface - in the case of Earth - is well-lit by this "inner sun" and full of land, lakes, and vegetation. Many birds migrate INTO the "Inner Earth" during winter.

Workers in both Alaska and Antarctica have always reported birds migrating the "wrong way" in winter. They are, in fact, migrating to warmer climates.

If a goose lives in the Southern Hemisphere, then flying north would get it to a warmer climate. So southern-hemisphere geese would fly north in the winter. But, "winter" in the southern hemisphere is June-September.