Consider a autonomous Hamiltonian $h:W\rightarrow \mathbb{R}$, where $W$ is a symplectic manifold. Let $\mathrm{sgrad}(h)$ denote the vector field on $W$ that is dual to the differential $Dh$ using the symplectic form.

Then it seems to be often stated that if $h$ is $C^2$ small then the only 1-periodic orbits of $\mathrm{sgrad}(h)$ are constants. Why is this?