  In niobium superconducting radio frequency (SRF) accelerating cavities a
decrease of the quality factor at lower fields - a so called \emph{low field Q
slope or LFQS} - has been a long-standing unexplained effect. By extending the
high $Q$ measurement techniques to ultralow fields we discover two previously
unknown features of the effect: i) saturation at rf fields lower than
$E_\mathrm{acc} \sim 0.1$~MV/m; ii) strong degradation enhancement by growing
thicker niobium pentoxide. Our findings suggest that the LFQS may be caused by
the two level systems in the natural niobium oxide on the inner cavity surface,
thereby identifying a new source of residual resistance and providing guidance
for potential non-accelerator low field applications of SRF cavities.
