in order to confirm the anti-exhaustion hypothesis, the two factors proposed by lambrechts and dhondt that influenced a low percentage performance time needed to be confirmed. these two factors are the initial level of song output (the greater the initial output, the greater the drift would be), and the number of switches between song types (after a switch in song types, performance output was increased). the results from this study could conclude that a repeated stereotyped song is difficult to maintain over a long period of time, which supported the anti-exhaustive hypothesis.