  Superconducting resonators, used in astronomy and quantum computation, couple
strongly to microscopic two-level defects. We monitor the microwave response of
superconducting resonators and observe fluctuations in dissipation and
resonance frequency. We present a unified model where the observed dissipative
and dispersive effects can be explained as originating from a bath of
fluctuating two-level systems. From these measurements, we quantify the number
and distribution of the defects.
