I had absolutely no idea about this. That humming birds migrate. Their normal range is in Central America but as a result of the last ice age (more specifically, the end of it) they've come to migrate up into the Lower 48 in an expansion of their range. There's not the food supplies to support them year round though, so they migrate back and forth each year.
Simply something I didn't know and had never really thought about.
However, this does lead to a further thought. As they migrate rather than stay put it should be easy to attract them to your garden by feeding them. Have a look at these best hummingbird feeders for some ideas. They sound like a great gift actually: who wouldn't like to have humming birds in the garden in spring and summer?