The Purple Martin Conservation Association (PMCA) is a non-profit organization “devoted exclusively to the scientific study of Purple Martins (Progne subis), their biology, and habitat requirements”.
One important project of the PMCA is called the Scout Arrival Study. They collect migration reports from all over the United States and then map them according to when and where they arrive. Each year (since 1998) the PMCA collects more and more data.
This map shows the arrival dates and breeding ranges of Purple Martins