Top: selection probability function P(m, z) of the z ∼ 5.7 LBG selection introduced in this study. The contours (outside-in) indicate probabilities of selection (including detection) of 0.1, 0.25, 0.5 and 0.75. Bottom: fraction of recovered objects as a function of redshift obtained from the simulated observation of 10 000 LBGs in each field and at each redshift step (Δz = 0.01). The green dot–dashed line corresponds to objects recovered with the colour selection for R_ci′z′-LBGs of Ouchi et al. (2004a). The red dashed line corresponds to i′-dropout colour selected objects and the blue solid line corresponds to objects recovered with the z ∼ 5.7 LBG criteria. In both fields, the blue line peaks at z ∼ 5.7 and the width at half-maximum is ∼0.2. The vertical long-dashed lines show the redshift of the C iv systems known to date at z > 5 in each field. In the left-hand panel, the blue long-dashed lines indicate the redshift of two C iv systems at z_abs = 5.7242 and 5.7428 (Ryan-Weber et al. 2009; Simcoe 2011). The grey long-dashed lines indicate the systems at z_abs = 5.5172 (Simcoe 2011; D'Odorico et al. 2013) and z_abs = 5.9757 (D'Odorico et al. 2013). In the right-hand panel, the blue long-dashed vertical line indicates the redshift of one C iv system at z_abs = 5.7383 (Ryan-Weber et al. 2009). This work focuses on objects in the environment of the z_abs ∼ 5.7 C iv systems only.