Fig. 1

Ratios of Z-associated and autosomal effective population sizes. Matched pairs of sexually dichromatic (red dots) and monochromatic (black dots) bird species are plotted together. Short horizontal lines represent species averages across individuals. The top panel shows the phylogenetic relationship among studied species (grey line represents the position of Zebra Finch). Five of the species pairs are illustrated in surrounding plates with upper right dichromatic species (males in front) and lower left monochromatic species. The three pairs not shown are: Williams’ Sapsucker vs. Red-breasted Sapsucker, Red-bellied Woodpecker vs. Red-headed Woodpecker, and Red-breasted Nuthatch vs. Pygmy Nuthatch. Monochromatic species have reduced genetic variation on the Z chromosome compared to its matching dichromatic species (Wilcoxon test p = 0.008; mixed-effect linear regression p < 0.001; PGLS regression p < 0.001; see Additional file 6 for details). Grey dotted line represents the null expectation that Z-associated effective population size is ¾ of autosomal effective population size