Reviewer 1's Review (Wiebke Schuett)

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Overall, this manuscript is an important contribution to the growing field of personality research as well as to the field studying extra-pair copulations. As pointed out by the author, a number of studies have investigated the occurrence of extra-pair paternity and how this is linked to male characteristics; however, the possibility that females could differ consistently in their tendency to engage in extra-pair copulations has widely been unconsidered but could have vast ecological and evolutionary consequences. In the present study, Forstmeier addresses the question of whether females vary intrinsically in their readiness to copulate in different contexts and over time. Also, the impact of the phenotype of a female's social partner and that of an extra-pair male on the decision-making by the female to engage in extra-pair copulations is investigated.

The main claims of the study are the following:
1) Differences in sexual responsiveness between individual females exist but these differences could be more conspicuous to detect when socially paired (as opposed to be sexually inexperienced) because other factors like male identity get more important.
2) Natural variation in male attractiveness exists and can be measured in both choice chamber and extra-pair tests but not in encounters of sexually inexperienced zebra finches.
3) Females' tendency to engage in EPCs might - against wide belief - depend more on the quality of the extra-pair male than the quality of the social partner.
4) Against common belief, the degree of male song rate (as long as song rate is present) is not an indication of male attractiveness for extra-pair copulations but the time a female spent with a male during mate choice tests is an indication for the same.

The claims are all supported by the data. The claims are set into context to a number of published studies.

As the experiment is well-designed and analysed, the manuscript clearly written and composed, which enables the reader to follow the complex experiment, I only have one suggestion to make:

I feel it could be interesting to also/alternatively include into the analyses the relative difference in the attractiveness of the social partner and of the partner in an extra-pair trial. This could involve analysing the differences between the relative times spent with the social and extra-pair partner (in experiment 2) and their influence on the occurrence of EPCs (in experiment 3; line 354 following); also, influences of differences in male beak colour on female responsiveness could be investigated (line 395 following). This would allow assessing the importance of the attractiveness of a male for an EPC relative to the attractiveness of the social partner.
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N.B. These are the general comments made by the reviewer when reviewing this paper in light of which the manuscript was revised. Specific points addressed during revision of the paper are not shown.