This data file contains the following information: Supplement S1 Sampling dates and locations for Old World leaf warbler males used in the analyses with accession numbers for Natural History Museum Oslo (NHMO). Supplement S2 Summary of sperm morphology traits (head length, midpiece length, flagellum length and total length) for 21 species of Old World leaf warblers along with co-efficient of variation in total sperm length within and among males, the values for the latter adjusted for small sample sizes are also shown. Supplement S3 Sperm morphology measurements (head length, midpiece length, flagellum length and total length) for all the warbler males included in this study. Supplement S4 Combined testes mass data along with the sources for the data and calculations of the % body mass made accounted for by testes mass for ten of the Old World leaf warbler species included in this study. Supplement S5 Testes dimensions data for seven Old World leaf warbler species from the Field Museum of Natural History and California Academy of Sciences along with the locations and dates of data collection. Supplement S6 Calculations of combined testes mass (CTM, sum of left and right testes mass) from the testes dimensions for seven Old World leaf warbler species. Supplement S7 Genbank accession numbers for the genes used to reconstruct the Old World leaf warbler phylogeny used in our study. Supplement S8 Results of a phylogenetic generalized least squares analysis (PGLS) of standard deviation of (log) total sperm length against mid-latitude and (log) total sperm length, showing results similar to those obtained using CVam as the dependent variable. Supplement S9 Data on combined testes mass and extra-pair paternity for passerine bird species other than the Old World leaf warblers.