eBird enables professional and recreational birders to submit their bird sightings to an online database. Launched in 2002, it is a joint project of the Cornell Lab of Ornithology and National Audubon Society and has become a significant source of real-time data about current bird locations and numbers. As an example, in March 2012 alone participants submitted more than 3.1 million observations.
Now eBird has taken this trove of information to create bird occurrence maps, which display the locations and density of different bird species depending on the time of year.
The modeling and data used to create these maps is complex, as is indicated by their technical name: Spatio-Temporal Exploratory Model (STEM) maps. To create them, more than 60 variables are built into each submitted eBird search (habitat, climate, human population, etc.) and then, based on the submitted data, the model predicts expected species occurrence at approximately 130,000 locations throughout the lower 48 states.
All of that translates into elegant and easy-to-understand maps. Here's one example—the movements of the scarlet tanager over the course of the year: