The migration routes of migrant birds in all the world.
There are eight migratory routes for birds in the world. Three routes of them pass by China: purple line, orange line and blue line. Light green region represents China and Red star represents the location of H7N9 outbreak, which is the Yangtze river delta area. World map is drawn by Matlab R2009a. China map is drawn by ArcGIS 10. Lines and red star are added by Microsoft Office Word 2003.