  We calculate ab-initio the electronic states and the Born dynamical charge Z*
of the alkaline-earth oxides in the local-density approximation. We investigate
the trend of increasing Z* values through the series, using band-by-band
decompositions and computational experiments performed on fake materials with
artificially-modified covalence. The deviations of Z* from the nominal value 2
are due to the increasing interaction between O 2p orbitals and unoccupied
cation d states. We also explain the variations, along the series, of the
individual contributions to Z* arising from the occupied band manifolds.
