Birds used in the various experiments reported herein.

Plus signs (+) indicate data from this bird for this treatment were used in analysis. Minus signs (−) indicate that birds were subject to the experimental treatment, but sang only at a single background noise level, or yielded otherwise unusable data and were not included in analyses. An additional four birds in 2008 and three birds in 2010 were used in either helmet or air sac pressure experiments, but did not sing in any condition and are not included here.