In March of 2006, Sciencentral put together a film clip entitled "Bird Fight Song" which was aired on various ABC and NBC stations. They used this video of the Black-capped Chickadee singing in their film clip. Scientists found that when the Black-capped Chickadee's song contained more "dees", they were telling other chickadees the danger was greater.
You can read more about the Black-capped Chickadee in the Backyard Birds section.