Reviewer #1's Review

“This is a very nice study describing and accounting for some of the intraspecific variation in sperm size in birds. I have made a number of suggestions below that I feel will improve the presentation and accessibility of this work to others but, even as it stands it is a landmark study that will generate considerable interest in looking at this pattern in other taxa. As I point out below, the predictions you make about sperm size variation need a more solid footing. Whilst a complete theoretical treatment of this issue is certainly beyond the scope of your paper, I think a little more information on why you would predict this pattern might be useful earlier on in the ms (you do deal with this in the Discussion). As also noted below, I am not convinced that GSI is the best measure of relative testis size in this study, nor that GSI is a better measure of the intensity of sperm competition than EPP rate (and in fact your results argue just the opposite). All in all, this is a very stimulating study that breaks some new ground in the study of sperm evolution.”

n.b. These are the general comments made by the reviewer when reviewing this paper. Specific points addressed during revision of the paper are not shown.