Discrete microwave absorption lines in YBa 2Cu 3O 7-δ single crystals are analyzed as a function of microwave power, temperature and prior exposure to higher fields. The intensity of the low field lines shows critical dependence on the microwave power and temperature. The lines broaden and saturate with increasing microwave power. No broadening is observed at higher temperatures, but only a decrease in intensity. Exposure of the sample to progressively higher fields initially introduces a shift in the position of the lines dependent on the direction of the field scan, and then vanishing of the signal.