  We explore the evolved galaxy population in the proto-clusters around four
high-z radio galaxies at 2<~z<~3 based on wide-field near-infrared imaging.
Three of the four fields are known proto-clusters as demonstrated by
overdensities of line emitting galaxies at the same redshifts as the radio
galaxies. We imaged the fields of three targets (PKS1138-262, USS0943-242 and
MRC0316-257) to a depth of Ks~22 (5sigma) over a 4'x7' area centered on the
radio galaxies with a new wide-field NIR camera, MOIRCS, on the Subaru
Telescope. Another target (USS1558-003) was observed with SOFI on the NTT to a
depth of Ks=20.5 over a 5'x5' area. We apply colour cuts in J-Ks and/or JHKs in
order to exclusively search for galaxies located at high redshifts: z>2. To the
5sigma limiting magnitudes, we see a significant excess of NIR selected
galaxies by a factor of two to three compared to those found in the field of
GOODS-South. The spatial distribution of these NIR selected galaxies is not
uniform and traces structures similar to those of emission line galaxies,
although the samples of NIR selected galaxies and emitters show little overlap.
We focus on the NIR colour-magnitude sequence of the evolved population and
find that the bright-end (M_{stars}>10^{11}Msun) of the red sequence is well
populated by z~2 but much less so in the z~3 proto-clusters. This may imply
that the bright-end of the colour-magnitude sequence first appeared between z=3
and 2, an era coinciding with the appearance of submm galaxies and the peak of
the cosmic star formation rate. Our observations show that during the same
epoch, massive galaxies are forming in high density environments by vigorous
star formation and assembly.
