1.4 GHz-to-3.6 μm flux density ratios versus 1.4 GHz radio flux density. The diagonal solid line marks the lowest R_3.6 values we can trace due to the combined radio and SERVS detection limits (derived assuming S_{1.4 GHz} > 100 μJy and S_{3.6 μm} > 1 μJy, i.e., the smallest lower limit we measure in our sample). The dotted lines indicate flux density ratios R_3.6 = 100, 200, and 500. Flux ratios for reliably IR-detected IFRSs are reported as crosses (position with errors bars), while flux ratios of IR undetected/unreliable sources are indicated by triangles (lower limits). To each field corresponds a different colour, as explained in the legend.