  We associate with k hermitian N\times N matrices a probability measure on
R^k. It is supported on the joint numerical range of the k-tuple of matrices.
We call this measure the joint numerical shadow of these matrices. Let k=2. A
pair of hermitian N\times N matrices defines a complex N\times N matrix. The
joint numerical range and the joint numerical shadow of the pair of hermitian
matrices coincide with the numerical range and the numerical shadow,
respectively, of this complex matrix. We study relationships between the
dynamics of quantum maps on the set of quantum states, on one hand, and the
numerical ranges, on the other hand. In particular, we show that under the
identity resolution assumption on Kraus operators defining the quantum map, the
dynamics shrinks numerical ranges.
