Since we did not control for male courtship song in this study, it seems likely that a proportion of the remaining variation in male fitness could be explained by differences in courtship songs between males.
Indeed many female Gryllus have preferences for particular calling songs and may use them to identify conspecific males [ 60, 63- 66], while courtship song (the only songs females in this study were exposed to) does not seem to be as important.
Thus, the songs we use in this study contain relatively little power at high spectral modulation rates (i.e., modulation of stimulus power across frequency channels) and little power at high temporal modulation rates (i.e., modulation of stimulus power over time).
This may be explained by an apparent upper limit in song rate, since the highest mean group song rates observed in this study are in the upper range of what has been observed in other studies using male starlings, both in the wild and in captivity [13], [25], [26], [27].
Timidity may also be aggravated by the manner in which services are rendered for instance in this study the songs sung in the antenatal clinic deter men from involvement.
All songs in our study were popular or children's songs and this might have enhanced concentration during the intervention phase, resulting in more focused "attention to life" or self-consciousness.
Interestingly, this class of songs was the least effective natural class of songs in our study.
Thus, while explicit literary interpretations of song lyrics do not appear consistent in this study, other work has suggested that sung lyrics have a greater influence over listeners' mood than the same melody played on an instrument [38].
The 16 male birds used in this study were song isolated as hatchlings from 7 through 20 days post-hatch in a separate room.
However, such an explanation is unlikely in this study system since song convergence rather then divergence occurs in sympatric populations of nightingales [ 43, 47, 71].
