Fraction of XXL bright cluster galaxies hosting an X-ray AGN (L_{X [0.5-10 keV]} > 10⁴³ erg s⁻¹) within a 2r₅₀₀ radius, divided by the field fraction. The results are plotted as a function of redshift. We overplot the results of similar analyses of cluster samples (blue) or individual high-redshift proto-clusters (black), as described in Sect. 5. Open symbols mark different luminosity limits used for the X-ray AGNs.