  In this work we characterize the unusual nonlinear dynamics of the resonance
response, exhibited by our NbN superconducting microwave resonators, using
different operating conditions. The nonlinear dynamics, occurring at relatively
low input powers (2-4 orders of magnitude lower than Nb), and which include
among others, bifurcations in the resonance curve, hysteresis loops and
resonance frequency shift, are measured herein using varying temperature,
applied magnetic field, white noise and rapid frequency sweeps. Based on these
measurement results, we consider a hypothesis according to which Josephson
junctions forming weak links at the boundaries of the NbN grains are
responsible for the observed behavior, and we show that most of the
experimental results are qualitatively consistent with such hypothesis.
