Let’s say you’re a bird. Your breeding area this spring and summer abounded with budding plants, fresh fruit, delicious insects—plenty for you and the kids you raised. But what about the coming fall and winter? Should you stick around or should you head for an area that might offer a better menu this time of year? It all depends on what your environment has in store for you. Birds migrate for 2 primary reasons. In early spring, the press is to find good breeding areas. But as summer draws to a close, the…