Free audio of wild birds can be found at eNature, as well as at the Macaulay Library website. At both sites, you can listen online for free; however, you must pay to download the Library's files, and eNature requires users to request a license to reuse its content.
ENature.com contains a large collection of nature sounds, including the calls and cries of 550 wild bird species found in North America. All audio on the site is free to visitors to listen to. Visitors who wish to use the audio for their own projects must negotiate a licensing agreement with eNature staff.
The Macaulay Library, based at Cornell University, has an even larger collection of nature sounds available, but any downloads must be ordered from the university. Simply listening to the audio tracks is free. The library covers almost 75 percent of all known bird species.