Spring Bird Count
The Illinois Spring Bird Count is an important census of all birds in Illinois, taken every May on the Saturday that falls between May 4 and May 10, sponsored by the Illinois Natural History Survey. The count of birds has data from the mid 1970s, and can be used to estimate changes in populations of species throughout the state.
As an example, below is a chart showing the range of crow populations from 1975-2005, with a sharp decline in their population when the West Nile Virus arrived in Illinois. This and similar data is available on their data page.
Chicago Audubon Society is proud to be responsible for organizing and compiling the annual Spring Bird Counts for Cook County, IL.