  We argue that recently observed superconductivity-induced blue shift of the
plasma frequency $\delta \omega_{pl}$ in $Bi_2Sr_2CaCu_2O_{8+\delta}$ is
related to the change in the integrated dynamical structure factor associated
with the development of the spin resonance below $T_c$. We show that the
magnitude of $\delta \omega_{pl}$ is consistent with the small integrated
spectral weight of the resonance, and its temperature dependences closely
follow that of the spin resonance peak.
