  We consider the unitary matrix model in the limit where the size of the
matrices become infinite and in the critical situation when a new spectral band
is about to emerge. In previous works the number of expected eigenvalues in a
neighborhood of the band was fixed and finite, a situation that was termed
"birth of a cut" or "first colonization". We now consider the transitional
regime where this microscopic population in the new band grows without bounds
but at a slower rate than the size of the matrix. The local population in the
new band organizes in a "mesoscopic" regime, in between the macroscopic
behavior of the full system and the previously studied microscopic one. The
mesoscopic colony may form a finite number of new bands, with a maximum number
dictated by the degree of criticality of the original potential. We describe
the delicate scaling limit that realizes/controls the mesoscopic colony. The
method we use is the steepest descent analysis of the Riemann-Hilbert problem
that is satisfied by the associated orthogonal polynomials.
