  We study the asymptotic response of polar ordered active fluids ("flocks") to
small external aligning fields $h$. The longitudinal susceptibility
$\chi_{_\parallel}$ diverges, in the thermodynamic limit, like $h^{-\nu}$ as $h
\rightarrow 0$. In finite systems of linear size $L$, $\chi_{_\parallel}$
saturates to a value $\sim L^\gamma$. The universal exponents $\nu$ and
$\gamma$ depend only on the spatial dimensionality $d$, and are related to the
dynamical exponent $z$ and the "roughness exponent" $\alpha$ characterizing the
unperturbed flock dynamics. Using a well supported conjecture for the values of
these two exponents, we obtain $\nu = 2/3$, $\gamma = 4/5$ in $d = 2$ and $\nu
= 1/4$, $\gamma = 2/5$ in $d = 3$. These values are confirmed by our
simulations.
