Thousands of species of birds partake in their yearly migration, be it for weather, food, or mating. Birds may migrate short or long distances; some birds only change altitude. The path along which birds migrate is called a flyway.
In the US, there are four flyways - Atlantic (along the east coast), Mississippi (following the longest river in the US), Central (east of the Rockies), and Pacific (along the west coast). "As a matter of fact, in the region of Panama, parts of all four flyways merge into one." 
Flyways over the US (Source: birdnature.com)
Their journey is remarkable. For example, the bird with the longest non-stop migration flight of any species is the Bar-tailed Godwit. This bird has been known to fly 11,000 km from Alaska to New Zealand. Their uninterrupted journey is fueled by stored body fat. 
While migration is a natural instinct in birds, many species have been threatened during their migrations by humans by hunting and building structures that interfere with their breeding grounds and flyways.
To better understand our feathered friends, a website called eBird.org has been busy mapping bird migratory patterns. You can even view flyways by particular species viewing what they call STEM (Spatio-Temporal Exploratory Model) maps.