Submitted on 24 Oct 2007 (v1), last revised 28 Feb 2008 (this version, v2).-- Version 2 improved exposition and figures.-- 10 pages, 3 figures, revtex4.Quasinormal modes of asymptotically AdS black holes can be interpreted as poles of retarded correlators in the dual gauge theory. To determine the response of the system to small external perturbations it is not enough to know the location of the poles: one also needs to know the residues. We compute them for R-charge currents and find that they are complex except for the hydrodynamic mode, whose residue is purely imaginary. For different quasinormal modes the residue grows with momentum q, whereas for the hydrodynamic mode it behaves as a damped oscillation with distinct zeroes at finite q. Similar to collective excitations at weak coupling the hydrodynamic mode decouples at short wavelengths. Knowledge of the residues allows as well to define the time scale τ_H from when on the system enters the hydrodynamic regime, restricting the validity of hydrodynamic simulations to times t > τ_H.K. L. is supported by a Ramón y Cajal contract. S.M. is supported by an FPI 01/0728/2004 grant from Comunidad de Madrid. I.A. is supported by grant BES-2007-16830. I.A., K. L. and S.M. are supported in part by the Plan Nacional de Altas Energías FPA-2006-05485, FPA-2006-05423 and EC Commission under grant MRTN-CT-2004-005104.Peer reviewed