  On the basis of the Bogoliubov--de Gennes theory we study the transformation
of the quasiparticle spectrum in the mixed state of a mesoscopic
superconductor, governed by an external magnetic field. We analyze the low
energy part of the excitation spectrum and investigate the field--dependent
behavior of anomalous spectral branches crossing the Fermi level. Generalizing
the Caroli--de Gennes--Matricon approach we present an analytical solution
describing the anomalous branches in a vortex with an arbitrary winding number.
We also study the spectrum transformation caused by the splitting of a
multiquantum vortex into a set of well separated vortices focusing mainly on a
generic example of a two--vortex system. For vortices positioned rather close
to the sample surface we investigate the effect of the quasiparticle reflection
at the boundary on the spectrum and the density of states at the Fermi level.
Considering an arbitrary surface curvature we study the disappearance of an
anomalous spectral branch for a vortex leaving the sample. The changes in the
vortex configuration and resulting transformation of the anomalous branches are
shown to affect strongly the density of states and the heat conductance along
the magnetic field direction.
