Although each one of us sees birds outside each day, it is difficult for one person to see what a population of birds is doing. By gathering observations from many observers in many locations over long time periods, we can map where the birds were sighted at any time. Maps of different time periods can be used to understand whether a population is changing.
The location of a population can remain static, expand, contract, or shift. By looking at the spatial extent and the time scale of these changes, we can see that the changes represent different biological activities. The figure above shows several commonly recognized movements of birds separated along time and space axes. These classes of movements are the theme for the maps of bird population movements shown on this site.
These maps, definitions, and discussions can be seen by clicking on the species list on the left of your screen. The animated maps can be halted by pressing your browser's stop button and re-started by pressing your browser's re-fresh button.