You can follow the invasion on the maps at http://www.hummingbirds.net/map.html. I have always assumed that the appearance is random but one scientific source says otherwise. It is even possible that the hummingbird that left last fall (was it just a few months ago?) is the same one that you will see this year.
"Banding studies suggest that individual birds may follow a set route year after year, often arriving at the same feeder on the same day. We do not know if any individual bird follows the same route in both directions, and there are some indications that they do not."These birds originated in the tropics but with the ending of the last Ice Age, they began to expand their summer range, now making the long trip north each spring. They begin in Panama and Southern Mexico and migrate as individuals rather than flocks. They fuel up on insects and spiders for high energy stores of fat - can you picture a fat hummingbird?
"Some will skirt the Gulf of Mexico and follow the Texas coast north, while most apparently cross the Gulf, typically leaving at dusk for a nonstop flight of up to 500 miles, which takes 18-22 hours depending on the weather. Although hummingbirds may fly over water in company of mixed flocks of other bird species, they do not "hitchhike" on other birds. Some hummingbirds land on offshore oil rigs or fishing boats to rest."There is more detailed information on the Ruby-Throated Hummingbird migration at http://www.hummingbirds.net/migration.html.