The evidence for divergent sexual selection among closely related barn swallow populations is strong

Lifjeld's comment provides an opportunity to illustrate the intricacies of the regression to the mean (RTM) effect, to clarify the difficulty in teasing apart RTM from allocation bias, and to re-examine our results in relation to RTM and in the context of related evidence. Here, we show that (a) the correlations between paternity change and initial paternity are mathematically expected and can equally be produced when changes are caused by the experimental manipulation itself. (b) The approach taken by Lifjeld to control for RTM is overly conservative because it is based on the unrealistic assumption of zero correlation between individuals' repeated measurements. Yet, even when using this conservative method, the main effects we originally reported are still detectable. (c) The combined effect of color darkening and tail elongation in Israel is additionally supported by an increase in the number of extra-pair young in other nests and by three independent studies of this population. (d) The experimental effect of color darkening in North America has been replicated successfully and is consistent with multiple correlative studies. Thus, divergent sexual selection in barn swallow populations is supported by both a conservative reanalysis and multiple, independent analyses of experimental and observational datasets.

Automated summary

This study uses the regression to the mean (RTM) effect to clarify the experimental results and demonstrates the correlation between changes in paternity and initial paternity through mathematical modeling. The control method for RTM is evaluated and it is found that the main effects reported originally can still be detected. Additionally, the increase in the number of extra-pair young in other nests and multiple independent studies also support the combined effect of color darkening and tail elongation in the barn swallow population in Israel.