Here's how the maps are described on the eBird website:
"These maps, which are called STEM (Spatio-Temporal Exploratory Model) maps, use eBird stationary and traveling count checklists that report all species. The location of each checklist is associated with remotely-sensed information on habitat, climate, human population, and demographics generating a suite of approximately 60 variables describing the environment where eBird searches take place. By relating these environmental variables to observed occurrences, STEM is used to make predictions at unsampled locations and times. Models are trained one species at a time. Following model training, the expected occurrence for that species is predicted on each of 52 days, one per week throughout 2009, at some 130,000 locations sampled throughout the conterminous US. This massive volume of information is then summarized on maps, which in many cases reveal novel information about the annual cycles of North American birds. These maps showcase the power of eBird – year-round, continental-scale monitoring of all species.
"Each species map is displayed with a text overview of the broad-scale migration patterns, along with an interesting biological story to consider. "Visit the website, eBird.org, to see these maps, results of Christmas and other national bird counts, and more interesting information on wild birds.