  Given a closed Riemann surface $\Sigma$ equipped with a volume form $\omega$,
we construct a natural probability measure on the space $\mathcal{M}_d(\Sigma)$
of degree $d$ branched coverings from $\Sigma$ to the Riemann sphere
$\mathbb{C}\mathbb{P}^1.$ We prove a large deviations principle for the number
of critical points in a given open set $U\subset \Sigma$: given any sequence
$\epsilon_d$ of positive numbers, the probability that the number of critical
points of a branched covering deviates from $2d\cdot\textrm{Vol}(U)$ more than
$\epsilon_d$ is smaller than $\exp(-C_U\epsilon^3_d d)$, for some positive
constant $C_U$. In particular, the probability that a covering does not have
any critical point in a given open set goes to zero exponential fast with the
degree.
