The authors deduce the generalized Bianchi identities for classical Lagrangian field theories on gauge-natural bundles without the a priori introduction of a connection. Their proof is based on a global decomposition of the variational Lie derivative of the generalized Euler-Lagrange morphism and the representation of the corresponding generalized Jacobi morphism on gauge-natural bundles. Further, the existence of canonical global superpotentials for gauge-natural Noether conserved currents is proved without resorting to additional structures.