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Because the CBC is monitoring birds in the non-breeding season (and most species are not singing, at least in North America) the use of playback and attractant noises have always been permitted on the CBC, where allowed by law. With the proliferation of iPods and other hand-held devices that can readily play bird vocalizations and elicit responses from otherwise hidden birds, this use is reported to be on the rise.
If prudent use of audio broadcast has been used routinely on a CBC, those methods should be continued to maintain consistent counting data over time.
The use of playback on a CBC should be very judicious, and never done in a fashion that could affect the behavior of target species in any significant way.