Genetic differentiation
The redundancy analysis (RDA) performed on 74,137 SNPs obtained by RAD-sequencing, including location (Barcelona, Montpellier or Warsaw), habitat (urban vs. forest), and sex as explanatory variables was highly significant (P <  0.001) but explained only a small fraction (i.e. less than 2%) of the total variance (R²  =  0.018, Figure 2A, SI Table S1). All three variables were significant (location: P  =  0.001 ; habitat: P  =  0.004; sex: P  =  0.001) revealing small genetic structuration between groups. Partial RDA revealed that the net variation explained by habitat (R²  =  0.004, P  =  0.001) was inferior to the net variation explained by location (R²  =  0.012, P  =  0.001) but higher than sex (R²  =  0.002, P  =  0.004, Table S1). As expected, when removing the Z chromosome from the data, sex became non-significant (P  =  0.260), whereas the effects of other variables remained significant and of similar magnitude (Table S2).
Genome-wide differentiation between populations was relatively low on average (mean FST  =  0.019), in the order of 1 to 2% between habitats for each location (FSTBarcelone(rur-urb) = 0.018+-0.001 ; FSTMontpellier(rur-urb) =  0.012+-0.001 ; FSTWarsaw(rur-urb) =  0.018+-0.001, Table 1), suggesting relatively high gene flow and limited genetic drift among populations. Mean FST on autosomes was lower (mean FST = 0.014, Table S3) than on the Z chromosome (mean FST = 0.022, Table S4).